- Email: [email protected]
Future neutrino telescopes
~H UCLEARPHYSIC~ PROCEEDINGS SUPPLEMENTS t:1 SI VII:V,
Nuclear Physics B (Proc. Suppl.) 38 (1995) 484M-94
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Future Neutrino Telescopes John G. Learned = ~Department of Physics and Astronomy, University of Hawaii, Manoa, 2505 Correa Road, Honolulu, HI 96822, USA We briefly review the four major high energy neutrino telescopes currently under construction and the motivation for their construction. Progress is such that several instruments with muon detection in the 104 m 2 class should be operating in the next several years. Plans are advancing for an international consortium to propose a k m 3 scale instrument. We highlight a newly identified experiment which may be carried out with such an instrument, if there are substantial numbers of neutrinos in the P e V energy range, as are indeed predicted from some AGN models. The idea is to search for what we call "double bang events", which contain a P e V cascade followed by a charged particle track and then a second and larger cascade separated by about 100 m. This appears to be a nearly background free signature for charged current r production and decay, and would constitute strong evidence for v¢ oscillations, and thus mass. We discuss possible sources of re's, backgrounds for these events, both few, and implications of such detetion.
1. I N T R O D U C T I O N The perceived importance of attempting high energy neutrino astronomy appears to be on the rise: the technology is certainly ripe, the community seems to be increasingly interested in this subject, and it seems that the scientific promise has never been more encouraging. Due to space imitations we present here an update on the situation since the last Neutrino Workshop in Granada, rather than a comprehensive tutorial, for which the reader is referred elsewhere[l]. It is true and unfortunate that several objects with calculable fluxes of high energy neutrinos (eg. Cyg X-3 and Her X-l) have faded with time, due to recent non-detections of those objects in T e V P e V g a m m a rays. (It should be noted that if the gammas are from 7r° decay, as most expect for these energies, non-detection of gammas tells us little about neutrinos: there may be neutrinos without gammas but not vice versa). On the other hand, the detection of G e V 7s from distant active galaxies by the E G R E T instrument on the Compton GRO has fueled considerable calculational effort to predict neutrino fluxes from AGNs (see Figure 1). These objects, the most luminous in the universe, may well be emitting a significant fraction (order of one half) of their gravitationally powered radiation in ultra high energy neutrinos[2, 3]. In fact the spectral 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SAD[ 0920-5632(94)00784-5
signature is sufficiently unique that the summed flux from all sources would be discernible over the atmospheric background in the generation of detectors now being built if current models are within an order of magnitude of being correct[4]. Energies of up to perhaps 1016 e V in substantial numbers (up to thousands of events per year in DUMAND II, for example) would permit not only exploration of the central engines of these galaxies, but particle physics explorations for new phenomena. Observation of the spectacular signature of the Glashow resonance may be possible. Attenuation of these ultra high energy neutrinos through the earth's core may permit the first direct measurements of the core density, and ultimately earth tomography with neutrinos. It may even be possible to detect the first of a newly defined class of events, which we designate "double bang", which provide a unique signature for tau neutrino interactions, signalling neutrino mass and mixing (more about this below). Furthermore, the energies are high enough that we may well be able to employ hydrophones to hear as well as see the highest energies. Acoustic detection could then open the long dreamed route to UHE neutrino detection employing multi-kin 3 volumes. Aside from the excitement over AGN neutrinos, another development in astrophysics deserves mention, the developing mystery of the ori-
,LG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484-494
gin of g a m m a ray bursts, now thought to be possibly cosmological in origin. One popular interpretation is that they are due to the final merging of two compact stellar objects, with consequently large energy liberation from gravitational infall. Multi-GeV 7's have been observed by GRO. Though speculative, it seems possible that significant neutrino pulses may also be emitted. If such were observed the implications would obviously have tremendous importance[5].
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log(E/GeV) Figure 1. Predictions of neutrino flux from AGNs, taken from Szabo-94 paper, including several models by various authors.
Other than the above, there continue to be calculations of neutrino emission near cosmic relics[6], such as cosmic strings, or in decays or annihilations of cosmological residue from the Big Bang. The latter, which could account for much of the missing matter of the visible universe, could be in the form of WIMPs which become trapped in the earth's or sun's core and annihilate to energetic neutrinos (GeV to TeV depending upon mass of the WIMP). Calculations indicate that the instruments under construction could make a discovery over a wide range of parameter space in supersymmetric models, going about a factor of ten further than the limits placed by LEP and underground experiments.
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The atmospheric neutrinos provide another opportunity for elementary particle physics investigations, via a search for u~ disappearance as reflected in the zenith angle dependence of the multi-GeV flux. In fact the region in mass (/~m2) and mixing angle (sin2(20)) space to be explored is that favored by the unexpected low energy (100 - 700 MeV) deficit in the ratio of v~,/v, as observed by the IMB, Kamioka and Soudan underground experiments. DUMAND and NESTOR, because of great depth (-~ 4 km may make crucially independent studies in the critical angular region near the horizon, where the flux changes most rapidly with angle. Studies of other aspects of cosmic ray physics and exploration for new phenomena, due to exploring a factor of 100 in sensitivity beyond the underground detectors at high energies, continues to offer a rich program in physics with great potential for significant discovery. One may well ask if the instruments being built now are actually large enough to inititate high energy neutrino astronomy. Of course this is the question most critical to experimentalists, and to funding agencies. The sensible approach, acknowledging that we really do not know, is to build bigger detectors, taking steps on a logarithmic scale, until we get into business. Because of the existence of UHE cosmic rays (up to i020 eV or so), we know that the neutrinos are there at some level, but where? Fortunately, however, there does seem to be a consensus amongst astrophysicists who have thought about the issue[4] that the range needed is somewhere from just beyond the present size of underground detectors (< 1000 m 2) to around 100,000 m 2. The conservative experimentalist would aim for a full 1 km 2. Unfortunately that seems to be too great a step from existing practice and budgets, so the next few years will see detectors in the 104-s m 2 range, and we hope for the best. The net expected point source sensitivity for a 100 GeV neutrino energy threshold, 20,000 m 2 detector deep underearth, is about 10 -1° u/cm2/sec with energy above 1 TeV. This flux is close to the levels claimed by VHE "r detectors.
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2G. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484 494
2. H I G H E N E R G Y SCOPES UNDER
NEUTRINO TELECONSTRUCTION
There has been quite an evolution in the group of projects aiming at building a neutrino detector on the 104 rn 2 scale in the last several years. The four telescopes currently in various stages of construction are compared in Table 1. See the individual group reports for more details[7-10]. There have been numerous proposals for combined neutrino and air shower arrays for placement in lakes or specially built ponds. Of these, the best known is the U.S. GRANDE effort, which has been abandoned, as are the other five similar proposals. (MILAGRO, in a 5000 m 2 pond near Los Alamos, is now going ahead, but it is too shallow and too small for neutrinos, and is aimed at doing gamma ray astronomy in the T e V energy range). AMANDA[7], a neutrino detector project in the ice at the South Pole, has developed swiftly. The team placed four strings of twenty 8 inch PMTs down hot-water-drilled holes in the 19934 season, at 800 - 1000 m depth, until the drill ceased operation. Reportedly the optical scattering is sufficiently great (10 - 20 c m scattering length, presumed to be due to bubbles) as to render the array incapable of any significant muon directional determination. Fitting of the calibration pulse diffusing between strings seperated horizontally by 2 0 - 30 rn permits deduction of both scattering and absorption lengths. It appears that the absorption length is much longer than laboratory measurements, in the range of 55 m, comparable to the clearest ocean waters. The group intends to try again in 1995-6 at a depth of 1 3 0 0 - 1500 m, which will apparently mark a go/no go hurdle for this experiment. The Baikal project[8], the oldest of the endeavors, has 36 large photmultipliers installed and operating at 1 km depth, and aims at a full 200 module array, NT-200, over the next few years. With the same number and size of modules as DUMAND II, they are projected to have an effective muon detection area of about 1/10 that of DUMAND II. This is due to the relatively shallow depth of Baikal and consequent necessity to cluster the tubes more closely. This
group deserves great credit because despite difficult times they have had P M T s installed and operating since 1992 (with some problems, now repaired) and have recorded a vast number of downgoing muons, with a 22 e v e n t ~ s e e trigger rate. Angular distributions have been presented, and data in active analysis. It is not clear if the up/down discrimination will be adequate, with this small fraction of the whole array, to extract the few neutrino events which must have been recorded already however. DUMAND II[9] began installation in December 1992 with the installation of a 12 electro-optic port junction box on the 4.76 krn ocean b o t t o m and laying of a 30 krn cable to the shore station at Keahoh Point, Hawaii. Various instrumentation (hydrophones, pingers, TV, lights, and environmental monitoring) was installed and the site was surveyed to geodetic coordinates. The first 434 rn tall instrument string with 24 optical detectors (with 5 more hydrophones and other ancillary instrumentation) was also installed, but failed due to a leak in the main electronics housing after 10 hours of operation. During the operating period some data was collected, and that data is reassuring about function, sensitivity and backgrounds. The string was recalled, brought back to the laboratory and repaired. At present three strings await the use of a deep ocean submarine for placement and connections, now scheduled for June 1995. The planned NESTOR[10] configuration is slightly more dense, with photomultipliers in up/down pairs, as with Baikal. There will be 14 large PMTs per hexagonal "floor", and 12 fioors per "tower". Each floor is deployed as an umbrella-like Titanium structure which opens in the water, with six spines. N E S T O R will also employ fiber optic communications to shore though the digitization and modulation schemes are not chosen as yet. 180 Hamamatsu PMTs and housings have been purchased and are being tested. The N E S T O R Collaboration has recently grown with addition of French collaborators. They are probably two years from an operating array. N E S T O R may also have the opportunity to detect neutrino oscillations via a neutrino beam from CERN, for which it is well situated.
JG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484 494
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Table 1 Comparison of neutrino telscopes under construction. Effective area and solid angle estimates are by the author, for 5 TeV muons. Experiment AMANDA Baikal DUMAND NESTOR 1996 array NT-200 II 1 tower Location South Pole Siberia Hawaii Greece Depth (km) > 1.3 1.0 4.7 3.7 P M T diam (in) 8 15 15 15 P M T raft/type EMI QUASAR Ham./Phil. Ham. Number P M T s 80 200 216 168 Ae/! (m 2 at 1 TeV) 9,000 5,000 20,000 10,000 Solid Angle/21r sr 0.9 0.8 1.2 1.0 P M T Bkgrd Rate (kcs) 1.3 50-100 50 60 Partial Oper. Date '96 '93 '95 '97
I a m often asked about the comparative merits of the experiments and particularly the differences between employing ice and the deep ocean. While there are obvious advantages and disadvantages to b o t h approaches, I regard them as complementary. Statements have been made that the ice approach is far less costly, which does merit some comment. It appears to be easier to get started with a small array at the South Pole, and the A M A N D A group has been able to tap a large existing infrastructure there. Ocean experiments, D U M A N D in particular, have had to absorb a heavy initial engineering overhead, now past. It appears to me that costs for a future large array will be more nearly independent of location and t h a t the decision towards one venue or another will rest upon what is learned from the present endeavors. The four experiments are listed in Table 1. The effective area listed is my estimate of the effective muon collection area for good resolution for 5 TeV muons. I believe the order of sensitivity is representative, based upon number and size of P M T s , depth and background noise rate. There has been some confusion in the community because people quote effective areas based upon various assumptions of input spectra, triggering criteria, noise rate, acceptability criteria for fitted events, etc. For example, in searching for point sources of neutrinos with flat spectra ( I / E ) , the effective area for muons of mean energy of a few TeV, is appropriate with tight cuts
for good angular resolution. For atmospheric neutrinos and W I M P searches, muons of a few tens of GeV are more appropriate, and angular resolution is not very important. In all cases, however, the cuts must be tight enough to prevent the (long and non-gaussian) angular distribution tail of huge downgoing muon rates from overwhelming the neutrino signals. The rigor of these cuts depends upon background count rate and upon depth. DUMAND and N E S T O R have ocean radioactivity, AMANDA has shallow depth, Baikal has both shallow depth and noise with which to contend.
3. T H E N E X T TOR
STEP
- A 1KM
DETEC-
The foregoing, along with other talks at this conference[11-13], present a strong case, in my view and that of m a n y others, for beginning the technology studies and system design needed to make a formal proposal for a next generation neutrino telescope. Such an instrument will go beyond the capabilities of the small university groups now hosting experiments, and will demand the engagement of national laboratories. As a result of discussions in the US and Europe during the past year, two laboratories are under active discussion as possible hosts for this project (perhaps co-hosts), Saclay in France, and JPL in California. The A M A N D A and D U M A N D groups have been cooperating in a study of a k m s in-
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.LG. Learned~Nuclear Physics B (Proc. SuppL) 38 (1995) 484 494
Tau Dectey
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Figure 2. A schematic view of a "double bang" event near a deep ocean detector whose modules are indicated by dots. Such cascades should be visible to detectors from hundreds of meters distance.
strument, with personnel from J P L in Pasadena, and LBL in Berkeley. The agreed-upon strategy is to design competing instruments for the ocean and ice, with one to be chosen based upon the experience to be gotten over the next several years. Further discussions took place at the Snowmass '94 Workshop, and plans were formulated for the beginning of collaboration organization. 3.1. D o u b l e B a n g E v e n t s The interaction of high energy tau neutrinos (v~'s) in DUMAND-like detectors [9] will present a spectacular "double bang" signature. The existence of such events depends upon the presence of 10 is e V neutrinos in adequate numbers, as are in fact predicted from active galactic nuclei (AGN)[4] for example. The interesting signals are from the charged current (cc) quark interactions of v , ' s . Since the 7- mass is about 1.8 G e V , a 7- of 1.8 P e V and with c7- of 91 #m[15] would fly roughly 90 m before decay. The signature, as illustrated i n Figure 2, is: 1. a big hadronic shower from the initial v, interaction, 2. a muon like 7- track, and then
3. a second and even larger particle cascade, To give some scale to this, the ratios of detectable photons from these three segments are roughly 1011 : 2 . 1 0 e : 2 . 1 0 tx. Such large bursts of light would be visible from distances of hundreds of meters by present technology photomultipliers. The charged 7- will be hard to resolve from the bright Cherenkov light of the cascades, and the photon arrival times will not be very different. However, simply connecting the two cascades by the speed of light will suffice to make an unambiguous association (including direction of the cascades) of the two bursts. In fact, as we discuss later, it appears that the double bang signature alone is nearly background free, sufficiently so as not to have to invoke lack of either incoming or outgoing charged particles (muons) to produce a clean sample. The double bang event topology appears to be a unique signal for real 7- production by v,'s, thus permitting "discovery" of the vr, and inferring mixing of neutrino flavors. Finding even one of these events would have significant implications. The rate for such events is of course unknown experimentally now. If we employ optimistic fluxes from the Szabo and Protheroe model[3] for the sum of neutrinos from all AGN, we estimate 1000 events of this type per year in a volume of 1 k m a of water or ice, in a 2 P e V energy band. While the experiments now being constructed (AMANDA, Baikal, DUMAND, and NESTOR) would expect to see a b o u t one such event per year, they should easily determine if the AGN flux is present. We show below that v , ' s are unlikely to originate in commonly considered astrophysical sources, but are likely to appear due to neutrino mass and mixing, over a large range of allowable (and even favored, if the solar neutrino puzzle and the atmospheric neutrino anomaly have anything to do with oscillations) neutrino mixing parameter space. We know of no other way to make a v, a p p e a r a n c e experiment with cosmic rays, no way has been proposed for an accelerator experiment except for the use of emulsions making observations of relatively large 6rn ~ > 1 e V 2, and no way of detecting Vr'S except statistically at
.AG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484-494 proposed long baseline accelerator experiments. In the following we explore the physics implications of the observations of the double bang events in a little more detail, discussing the kinematics, sources of u~'s, the sensitivity to two and three neutrino mixing, and potential (and we conelude small) backgrounds. 4. E s s e n t i a l l y Full K i n e m a t i c s One m a y in principle measure the total energy of the incident neutrino and nearly the full kinematics of the double bang events by adequate sampling of the Cherenkov radiation. The observation of light from the first cascade yields the energy transferred to the quark, El. The magnitude of the light from the tan track plus the flash of light from the second cascade gives the energy (E2) kept by the T except for that carried away by the decay neutrino(s). The sum (JET -= Et + E2) gives the rough incoming neutrino energy, and the ratio of the first cascade energy to the total energy ( E t / E T ) provides the ~ y value. The cross sections and < y > are almost equal for v~ and ~ at this energy[16]. Observing the y distribution is a check on the observations, and departures from expectations could signal new physics. In calculating the v~ flux, the measured y distribution will permit correction for the potentially unobserved events near y = 0 (no initial cascade) and near y = 1 (initial cascade with most of the energy and the tau decays too close to the first cascade for resolution). The near equality of the cross sections for particle and antiparticle permits the total flux to be calculated independently of the mix in the cosmic beam. In a v~ cc interaction, < y > is about 0.25 at these energies[16] and the energy deposited is < Et > ~ 1/4 Ev while the 7" carries about ~ 3/4 of the v~ energy. In the subsequent decay of v, the energy deposited will be about 2/3 E~ ~ 1/2 E~ when the decay is hadronic (which is about 64% of the time). Hence the ratio of the average energy of the second "bang" to the first one is given by E 2 / E t ~ 2. This is characteristic of these t/~ events and making a cut of E 2 / E t > 1 eliminates several kinds of background events as we discuss later. Detailed calculations for signal as
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well as background E1 versus E2 distribution will be presented elsewhere[14]. The threshold energy for discriminating two cascades will be determined by requiring a r that flies far enough so that the two cascades can be distinguished, and so that there are no "punch through" events. This distance will be of the order of some few times the cascade length (order 10 rn), and thus our threshold for r detection via this means would be, as suggested above, about a P e V . Aside from the physics limitation of several tens of meters, there will also be a detector limitation depending upon detector density and response, but probably of similar magnitude. We note that the observation of the double bang events presents the opportunity to measure the P e V v~ cross section via the angular distribution in the lower hemisphere, which decreases towards the nadir due to attenuation through the earth (--~ 90% for straight upwards travelling neutrinos in this energy range). For future studies of earth tomography, the potential of this process is great, since it does not depend upon convolution over the y distribution and muon range, as is necessary to extract information from the upcoming muon flux alone. Also, given the enormous light output of the cascades one would expect that timing from the detectors (at intermediate distances, since nearby detectors of present design will surely be saturated) would give excellent vertex resolution, and thus the initial neutrino direction to a precision of the order of 10 . In principle, of course, having both cascades and almost all energy "visible" one can deduce the initial neutrino direction with arbitrary precision, perhaps making optical precision ultimately possible in some future neutrino telescope. From the ensemble of measurements with a DUMAND-like array we will have: the r rate from double bang events gives the vr + Pr flux; measurents of the UHE muon flux permits calculation the v~+P~, flux; the W - resonant event rate yields the Pc flux at 6.4 P e V ; observation of the cascade rate (as a function of energy) produces the sum of neutral current interactions of all flavors of neutrinos, and charged currents without visible #'s and r ' s (mostly v~s).
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JG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 48~494
In fact if we assume t h a t we know Pe/ve, then we can calculate the flavor ratios without employing the muon measurements (which will have different systematic errors). For the more optimistic AGN fluxes cited[3] the numbers of events collected in a krn 3 detector in one year would permit calculation of the flavor content to a few percent. 4.0.1. A s t r o p h y s i c a l v F l a v o r C o n t e n t In the absence of neutrino oscillations (discussed in the following section), from the seemingly most likely astrophysical high energy neutrino sources we expect nearly equal numbers of particles and anti-particles, half as m a n y ve's as vt,'s , and virtually no v~'s. This comes about simply because the neutrinos are thought to originate in decays of pions (and kaons) and subsequent decays of muons. Most astrophysical targets are tenuous even compared to the earth's atmosphere, and would allow for muon decay in flight. The extreme case of an astrophyscial beam d u m p target of sufficient density to suppress 7r and k decay (as would happen with a nucleon b e a m striking a neutron star), but not so thick as to absorb the directly produced fluxes (a neutron star would absorb almost any but a tangential high energy neutrino beam), seems to be unlikely. Beyond the geometric difficulty, producing a detectable flux of high energy neutrinos from such an inefficient source makes it even more improbable. There are some predictions for flavor independent fluxes from cosmic defects and exotic objects such as evaporating black holes. Observation of tau neutrinos from these would have great importance. Indeed as we show below, for such exotic sources along with the presence of oscillations, we would have a unique flavor signature permitting unravelling the source content. The flux ratio o f v ~ , : v ~ : v ~ = 2 : l : 0 i s c e r tainly valid for AGN models such as the ones due to Stecker, et a/[2] in which the neutrinos come from lr's produced via the process 7 + P --* N + lr. In the calculations of Protheroe and Szabo [2] there are two additional features: the additional v~ flux due to escaping neutrons modifies the v u : ve flux ratio to 1.75 : 1 and about 10% of the v-flux is due to proton-proton (pp) inter-
actions. Some fraction of the pp neutrinos will then give rise to neutrinos from rapidly decaying secondaries containing heavy quarks ( " p r o m p t " production). Depending on the amount of p r o m p t v-flux due to D s and B/~ production (and decay) there could be a non-zero v~ component present. A conservative upper bound on the relative v~ content is < 0.1 for any target situation. Because most of the secondary pions will have ample opportunity to decay the my fraction probably will be nearer 10 -4 . 4.0.2. S e n s i t i v i t y t o N e u t r i n o O s c i l l a t i o n s The ~m 2 sensitivity (from L / E ) for this observation is then fantastic, going down to the order of 10 - i s eV 2 (the distance is out to the AGN, 100 Mpc). To determine the two neutrino mixing angle sensitivity limit requires detector specific simulations. The limitation has to do with the AGN neutrino flux magnitude and effective volume for these events, and will probably be limited by statistics, at least in the near future. We guess it will be no better t h a n 0.01 in the best of situations (as with most oscillation experiments). The parameter space for a combined fit to the atmospheric and solar neutrino d a t a allowing for three flavor mixing has been given by Fogli, et a/.[17], and by Acker, e~ al.[18], respectively. We have evaluated the survival and transition probabilities for the whole range of allowed values of the three and two mixing angles, and the results are indicated in Figure 3. This figure plots the fraction of muon neutrinos versus the fraction of electron neutrinos, and thus each point specifies a fraction of tau neutrinos (it is analogous to the color triangle). The initial expected flux (v~ : v~ : v~:: 2 : 1: 0) i s a t f g = 0.66 a n d f ~ = 0.33. We observe that almost all combinations of acceptable mixing angles result in saturation values to be observed with AGN neutrinos which lie between the lines f~ + f~, = 0.88 and f~ + f~, = 0.66. Hence a substantial number of v~ events are expected (0.34 > f~ > 0.12) for all situations which solve the solar and atmospheric problems with neutrino oscillations. Moreover, since in the case of two flavor mixing it is impossible to obtain f ~ / f t , > 1, observation of the d a t a falling
JG, Learned~Nuclear Physics B (Proc. SuppL) 38 (1995) 484-494 i-4
0
Prom~, N u s ~ ~'Q~ ~ ~x,x" \
Observed Underground
,,<
Expected C oo o.c
0
1.0
0.0
Muon Fraction
Figure 3. The fraction of muon neutrinos versus electron neutrinos, allowing for a fraction of tau neutrinos. Expected initial flux is at 2/3, 1/3. Full mixing would result in 1/3, 1/3. The points represent various solutions to the solar and atmospheric neutrino problems. The point corresponding to pure prompt v-beam is at 0.30, 0.47 & 0.23 taus.
above the diagonal f~ = fg line is clear evidence for three flavor mixing. We would like to stress that a small "impurity" of r,'~s in the initial neutrino fluxes, as discussed in the previous section, has a negligible effect on the v~ signal due to vl, -t,~ oscillations with moderately large mixing angles. Interestingly, the purely prompt flux case, for which we are not aware of any model, gives rise to a very distinct flux pattern on arrival, vg:tJe:t% = 1.0:1.56:0.72---0.3:0.47:0.23. This represents a very singular point in Figure 3, distinguished by the dominance of the ue flux. We thus conclude that almost any astrophysical source of neutrinos will result in significant numbers of v~'s, if neutrinos have mass and they oscillate.
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4.0.3. B a c k g r o u n d s : A l m o s t N o n e By background we mean events which can fake the double bang signature of two huge cascades spaced roughly 100 m apart, with the second larger than the first (by typically a factor of two), and the sum of the visible energy being in the range of a few P e V . There are two general types of possible backgrounds due to neutrinos: those into which the neutrino transforms (e, p and r), and particles generated in the cascade resulting from the energy transferred to the struck quark. Neutral current events with an outgoing neutrino have interaction lengths so great as to be negligibly likely to interact again at close range ( q 10 -0 at this P e V energy). Electrons will immediately radiate, and be added to the quark cascade, and be indistinguishable from neutral current events. The most serious background is due to v~, cc events in which the muon travels about 100 m and then loses most of its energy in a single catastrophic Bremsstrahlung radiation. Such events can have energy characteristics similar to signal events. The rate for such events as a fraction of cc events can be estimated as
where R is the radiation length in water and ,~ 0(1/3) for the fraction of energy deposited. This number is of the order 3 • 10 - s and is reassuringly small. Detector simulations are required to numerically evaluate the confusion probability. Whatever that probability is, and we expect it to be low at 2 P e V , it decreases rapidly with increasing energy (short range muon radiation goes up while the tau decay length increases). In a large array, one may also demand that there be no continuing track, since the probability of a 2 P e V muon stopping after 100 m is negligible. However, we estimate that this constraint will not be needed. The other potential source of background from neutrino interactions is from the hadronic side of the neutrino interaction vertex. First of all, we do not know of any particle with the ability to penetrate --. 104 g m / c m ~ of matter before decaying or interacting (100 m is about 100 strong interaction lengths in water), except for leprous. Thus the AE/E
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1G. Learned~Nuclear Physics B (Proc. SuppL) 38 (1995) 48~494
only situation with which we must be concerned is when a 7- results from the decays of particles in the cascade, and moreover only when this T has most of the energy, which we show below to be quite unlikely. One source of this type of background is the production (and decay to r) of D, (and B/~ pairs) in neutral current interactions of v, (and v~,) and charged current interactions of re. These could give rise to "double bang" events if D s (or B) should decay within 10 m to produce an energetic I- which then travels about 100 m before decaying. In such events the average energy of the second bang E2 "~ 2 / 3 E r ,,, 1/2 Eh 1/8 z E v where z = E h / E v . Since z is expected to be rather small, we expect E2/E1 < < 1. Hence the E~./E1 > 1 cut will remove most such events. The number of such events will be N ~ { r N ~ + (1 + r ) N e } (fD, BS~+f~B~) where f, are the fractional production of D, and B/~ respectively. For fi ~ 10-2, r ~ 1/3, Nz = Ate; we find N ~ 1.5- 10-aN~,, most of which will be removed by the energy fraction cut. A somewhat more serious background is diffractive production of D, via charm changing cc interaction of v, in the reaction ve + N --* e + D, + x. There is no Cabibbo suppression, the rate can be as high as 6.10 - a times the cc rate and furthermore D, can have average energy of about 1/2 E~ [13]. Thus E2/E1 can be of order 1 and some events m a y pass the E2/E1 cut. However, the rate is given by 6 • 10 - a • B ~ ' ,~ 2 . 5 . 1 0 - 4 N e and is thus also quite small. Other backgrounds pose even less of a problem. Finally, we note that as a check, studies of the decay p a t h length distribution requiring consistency with the known tau lifetime, should confirm the detection of taus and the lack of contamination of the sample. In sum we conclude that the data selection criteria on spacing and energy of the two bangs will remove virtually all backgrounds and leave a verifiably pure sample of UHE v events. 4.0.4. I m p l i c a t i o n s o f D o u b l e B a n g E v e n t s One impact of the consideration of this potential observation gives motivation to fill the volume, somewhat, of a hypothetical 1 k m a array. Indeed this experiment itseff could perhaps jus-
tify construction of that instrument. Also, given that these events are near the anticipated acoustic detection threshold[19], one m a y contemplate hearing the double clicks from such events at few kra ranges and higher energies. The unusual property of acoustic pulse amplitude which results in slow decrease with distance in water may make practical the search for higher energy neutrino interactions from volumes of m a n y cubic kilometers. Additionally, the possibility of detecting neutrino induced cacades via radio pulses of G H z frequency range continues to be explored in deep ice[20]. As for acoustic sensing, the double microwave pulses due to r production and decay should provide distinctive signatures. We remind the reader that all of the above requires the existence of substantial numbers of P e V neutrinos, which m a t t e r should be resolved in the next few years by the AMANDA, Baikal, DUMAND, and N E S T O R experiments now under construction. If those ultra high energy neutrinos are present in hoped for numbers, then we believe that the observation of the double bang events, along with other previously discussed interactions, will lead to i m p o r t a n t particle physics measurements which cannot be carried out in any other way on earth.
5. S U M M A R Y -
ON THE THRESHOLD
In conclusion, we see that there seems to be growing interest in high energy neutrino astrophysics by virtue of the swelling number of people actually engaged in building instruments° A look at older proceedings in this conference series shows that earlier predictions, including this author's were often optimistic, yet now there seems to be real physical progress. There are four experiments that should be able to reveal the presence of the predicted large fluxes of high energy neutrinos from AGNs, if those predictions are within an order of magnitude or so of reality, perhaps by two conferences from now. Perhaps by the third conference from now we shall be discussing double bang events.
£G. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484 494
Acknowledgements I wish to thank the conference organizers for an excellent meeting and support. Thanks also to S. Pakvasa, and to F. Halzen, H. Rubinstein, R. Svoboda, lost in Eilat T. Weiler, K. Winter, and many others for useful discussions at this meeting. This work is supported by DOE grant DE-FG-03 94ER40833.
REFERENCES 1. J . G . Learned, Phil. r~ans. R. Soc. Lond. A 346, 99 (1994), and references therein. 2. F. Stecker et al., Phys. Rev. Lett. 66, 2697 (1991). 3. A. P. Szabo and R. J. Protheroe, Adelaide preprint ADP-AT-94-4, to be published in Astroparticle Physics (1994), Ref. I, p. 24. 4. "Proceedings of the Workshop on High Energy Neutrino Astrophysics", 23-26 March 1992, V. J. Stenger, ef al., eds., Worm Scientific (1992). 5. B. Paczynski and G. Xu. AP. J. 427, 708 (1994). 6. Y. Tomozawa, these Proceedings. 7. A. Goobar, et al., these Proceedings. 8. G. Domogatsky, et al., "Proceedings of the 3rd NESTOR International Workshop, October 1992 in Pylos Greece", ed. L. K. Resvanis, U. Athens (1994). 9. P . K . F . Grieder, et al., these Proceedings. 10. "Proceedings of the 3rd NESTOR International Workshop, October 1992 in Pylos Greece", ed. L. K. Resvanis, U. Athens (1994). 11. F. Halzen, these Proceedings. 12. B. Berezinsky, these Proceedings. 13. B. Barish, these Proceedings. 14. 3. Learned and S. Pakvasa, UH-511-799-94, subm. Astroparticle Phys., 8/94. Much of the double bang material in this paper is abstracted from that publication. 15. M. Aguilar-Benitez, e~ al., "Review of Particle Properties", Phys. Rev. D 45, Part 2 (1992). 16. C. Quigg, M. H. Reno, and T. P. Walker, Phys. Rev. Let. 57, 774 (1986).
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17. G. L. Fogli, E. Lisi, and Do Montanino, Phys. Rev. D49, 3626 (1994). 18. A. Acker, A. B. Balantekin, and F. Loreti, Phys. Rev. D, 49, 328 (1994). 19. J. G. Learned and R. J. Wilkes, 23 ICRC, Calgary (1993), and B. Price, Nuc. Insto and Meth. A325, 346 (1993). 20. New calculations and efforts at measuring background noise and attenuation were reported by John Ralston and by George Smoot and collaborators at the Snowmass '94 Workshop. These efforts may lead to radio detection with an energy threshold lower than a PeV.
Nuclear Physics B (Proc. Suppl.) 38 (1995) 484M-94
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Future Neutrino Telescopes John G. Learned = ~Department of Physics and Astronomy, University of Hawaii, Manoa, 2505 Correa Road, Honolulu, HI 96822, USA We briefly review the four major high energy neutrino telescopes currently under construction and the motivation for their construction. Progress is such that several instruments with muon detection in the 104 m 2 class should be operating in the next several years. Plans are advancing for an international consortium to propose a k m 3 scale instrument. We highlight a newly identified experiment which may be carried out with such an instrument, if there are substantial numbers of neutrinos in the P e V energy range, as are indeed predicted from some AGN models. The idea is to search for what we call "double bang events", which contain a P e V cascade followed by a charged particle track and then a second and larger cascade separated by about 100 m. This appears to be a nearly background free signature for charged current r production and decay, and would constitute strong evidence for v¢ oscillations, and thus mass. We discuss possible sources of re's, backgrounds for these events, both few, and implications of such detetion.
1. I N T R O D U C T I O N The perceived importance of attempting high energy neutrino astronomy appears to be on the rise: the technology is certainly ripe, the community seems to be increasingly interested in this subject, and it seems that the scientific promise has never been more encouraging. Due to space imitations we present here an update on the situation since the last Neutrino Workshop in Granada, rather than a comprehensive tutorial, for which the reader is referred elsewhere[l]. It is true and unfortunate that several objects with calculable fluxes of high energy neutrinos (eg. Cyg X-3 and Her X-l) have faded with time, due to recent non-detections of those objects in T e V P e V g a m m a rays. (It should be noted that if the gammas are from 7r° decay, as most expect for these energies, non-detection of gammas tells us little about neutrinos: there may be neutrinos without gammas but not vice versa). On the other hand, the detection of G e V 7s from distant active galaxies by the E G R E T instrument on the Compton GRO has fueled considerable calculational effort to predict neutrino fluxes from AGNs (see Figure 1). These objects, the most luminous in the universe, may well be emitting a significant fraction (order of one half) of their gravitationally powered radiation in ultra high energy neutrinos[2, 3]. In fact the spectral 0920-5632/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SAD[ 0920-5632(94)00784-5
signature is sufficiently unique that the summed flux from all sources would be discernible over the atmospheric background in the generation of detectors now being built if current models are within an order of magnitude of being correct[4]. Energies of up to perhaps 1016 e V in substantial numbers (up to thousands of events per year in DUMAND II, for example) would permit not only exploration of the central engines of these galaxies, but particle physics explorations for new phenomena. Observation of the spectacular signature of the Glashow resonance may be possible. Attenuation of these ultra high energy neutrinos through the earth's core may permit the first direct measurements of the core density, and ultimately earth tomography with neutrinos. It may even be possible to detect the first of a newly defined class of events, which we designate "double bang", which provide a unique signature for tau neutrino interactions, signalling neutrino mass and mixing (more about this below). Furthermore, the energies are high enough that we may well be able to employ hydrophones to hear as well as see the highest energies. Acoustic detection could then open the long dreamed route to UHE neutrino detection employing multi-kin 3 volumes. Aside from the excitement over AGN neutrinos, another development in astrophysics deserves mention, the developing mystery of the ori-
,LG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484-494
gin of g a m m a ray bursts, now thought to be possibly cosmological in origin. One popular interpretation is that they are due to the final merging of two compact stellar objects, with consequently large energy liberation from gravitational infall. Multi-GeV 7's have been observed by GRO. Though speculative, it seems possible that significant neutrino pulses may also be emitted. If such were observed the implications would obviously have tremendous importance[5].
o
--5
- 6
_
-~ . -~.
- " 4~
_
_. ~ ,, z_' : ; _
,,
-7 ..........
3
? . . . . . .
4
5
6
7
log(E/GeV) Figure 1. Predictions of neutrino flux from AGNs, taken from Szabo-94 paper, including several models by various authors.
Other than the above, there continue to be calculations of neutrino emission near cosmic relics[6], such as cosmic strings, or in decays or annihilations of cosmological residue from the Big Bang. The latter, which could account for much of the missing matter of the visible universe, could be in the form of WIMPs which become trapped in the earth's or sun's core and annihilate to energetic neutrinos (GeV to TeV depending upon mass of the WIMP). Calculations indicate that the instruments under construction could make a discovery over a wide range of parameter space in supersymmetric models, going about a factor of ten further than the limits placed by LEP and underground experiments.
485
The atmospheric neutrinos provide another opportunity for elementary particle physics investigations, via a search for u~ disappearance as reflected in the zenith angle dependence of the multi-GeV flux. In fact the region in mass (/~m2) and mixing angle (sin2(20)) space to be explored is that favored by the unexpected low energy (100 - 700 MeV) deficit in the ratio of v~,/v, as observed by the IMB, Kamioka and Soudan underground experiments. DUMAND and NESTOR, because of great depth (-~ 4 km may make crucially independent studies in the critical angular region near the horizon, where the flux changes most rapidly with angle. Studies of other aspects of cosmic ray physics and exploration for new phenomena, due to exploring a factor of 100 in sensitivity beyond the underground detectors at high energies, continues to offer a rich program in physics with great potential for significant discovery. One may well ask if the instruments being built now are actually large enough to inititate high energy neutrino astronomy. Of course this is the question most critical to experimentalists, and to funding agencies. The sensible approach, acknowledging that we really do not know, is to build bigger detectors, taking steps on a logarithmic scale, until we get into business. Because of the existence of UHE cosmic rays (up to i020 eV or so), we know that the neutrinos are there at some level, but where? Fortunately, however, there does seem to be a consensus amongst astrophysicists who have thought about the issue[4] that the range needed is somewhere from just beyond the present size of underground detectors (< 1000 m 2) to around 100,000 m 2. The conservative experimentalist would aim for a full 1 km 2. Unfortunately that seems to be too great a step from existing practice and budgets, so the next few years will see detectors in the 104-s m 2 range, and we hope for the best. The net expected point source sensitivity for a 100 GeV neutrino energy threshold, 20,000 m 2 detector deep underearth, is about 10 -1° u/cm2/sec with energy above 1 TeV. This flux is close to the levels claimed by VHE "r detectors.
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2G. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484 494
2. H I G H E N E R G Y SCOPES UNDER
NEUTRINO TELECONSTRUCTION
There has been quite an evolution in the group of projects aiming at building a neutrino detector on the 104 rn 2 scale in the last several years. The four telescopes currently in various stages of construction are compared in Table 1. See the individual group reports for more details[7-10]. There have been numerous proposals for combined neutrino and air shower arrays for placement in lakes or specially built ponds. Of these, the best known is the U.S. GRANDE effort, which has been abandoned, as are the other five similar proposals. (MILAGRO, in a 5000 m 2 pond near Los Alamos, is now going ahead, but it is too shallow and too small for neutrinos, and is aimed at doing gamma ray astronomy in the T e V energy range). AMANDA[7], a neutrino detector project in the ice at the South Pole, has developed swiftly. The team placed four strings of twenty 8 inch PMTs down hot-water-drilled holes in the 19934 season, at 800 - 1000 m depth, until the drill ceased operation. Reportedly the optical scattering is sufficiently great (10 - 20 c m scattering length, presumed to be due to bubbles) as to render the array incapable of any significant muon directional determination. Fitting of the calibration pulse diffusing between strings seperated horizontally by 2 0 - 30 rn permits deduction of both scattering and absorption lengths. It appears that the absorption length is much longer than laboratory measurements, in the range of 55 m, comparable to the clearest ocean waters. The group intends to try again in 1995-6 at a depth of 1 3 0 0 - 1500 m, which will apparently mark a go/no go hurdle for this experiment. The Baikal project[8], the oldest of the endeavors, has 36 large photmultipliers installed and operating at 1 km depth, and aims at a full 200 module array, NT-200, over the next few years. With the same number and size of modules as DUMAND II, they are projected to have an effective muon detection area of about 1/10 that of DUMAND II. This is due to the relatively shallow depth of Baikal and consequent necessity to cluster the tubes more closely. This
group deserves great credit because despite difficult times they have had P M T s installed and operating since 1992 (with some problems, now repaired) and have recorded a vast number of downgoing muons, with a 22 e v e n t ~ s e e trigger rate. Angular distributions have been presented, and data in active analysis. It is not clear if the up/down discrimination will be adequate, with this small fraction of the whole array, to extract the few neutrino events which must have been recorded already however. DUMAND II[9] began installation in December 1992 with the installation of a 12 electro-optic port junction box on the 4.76 krn ocean b o t t o m and laying of a 30 krn cable to the shore station at Keahoh Point, Hawaii. Various instrumentation (hydrophones, pingers, TV, lights, and environmental monitoring) was installed and the site was surveyed to geodetic coordinates. The first 434 rn tall instrument string with 24 optical detectors (with 5 more hydrophones and other ancillary instrumentation) was also installed, but failed due to a leak in the main electronics housing after 10 hours of operation. During the operating period some data was collected, and that data is reassuring about function, sensitivity and backgrounds. The string was recalled, brought back to the laboratory and repaired. At present three strings await the use of a deep ocean submarine for placement and connections, now scheduled for June 1995. The planned NESTOR[10] configuration is slightly more dense, with photomultipliers in up/down pairs, as with Baikal. There will be 14 large PMTs per hexagonal "floor", and 12 fioors per "tower". Each floor is deployed as an umbrella-like Titanium structure which opens in the water, with six spines. N E S T O R will also employ fiber optic communications to shore though the digitization and modulation schemes are not chosen as yet. 180 Hamamatsu PMTs and housings have been purchased and are being tested. The N E S T O R Collaboration has recently grown with addition of French collaborators. They are probably two years from an operating array. N E S T O R may also have the opportunity to detect neutrino oscillations via a neutrino beam from CERN, for which it is well situated.
JG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484 494
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Table 1 Comparison of neutrino telscopes under construction. Effective area and solid angle estimates are by the author, for 5 TeV muons. Experiment AMANDA Baikal DUMAND NESTOR 1996 array NT-200 II 1 tower Location South Pole Siberia Hawaii Greece Depth (km) > 1.3 1.0 4.7 3.7 P M T diam (in) 8 15 15 15 P M T raft/type EMI QUASAR Ham./Phil. Ham. Number P M T s 80 200 216 168 Ae/! (m 2 at 1 TeV) 9,000 5,000 20,000 10,000 Solid Angle/21r sr 0.9 0.8 1.2 1.0 P M T Bkgrd Rate (kcs) 1.3 50-100 50 60 Partial Oper. Date '96 '93 '95 '97
I a m often asked about the comparative merits of the experiments and particularly the differences between employing ice and the deep ocean. While there are obvious advantages and disadvantages to b o t h approaches, I regard them as complementary. Statements have been made that the ice approach is far less costly, which does merit some comment. It appears to be easier to get started with a small array at the South Pole, and the A M A N D A group has been able to tap a large existing infrastructure there. Ocean experiments, D U M A N D in particular, have had to absorb a heavy initial engineering overhead, now past. It appears to me that costs for a future large array will be more nearly independent of location and t h a t the decision towards one venue or another will rest upon what is learned from the present endeavors. The four experiments are listed in Table 1. The effective area listed is my estimate of the effective muon collection area for good resolution for 5 TeV muons. I believe the order of sensitivity is representative, based upon number and size of P M T s , depth and background noise rate. There has been some confusion in the community because people quote effective areas based upon various assumptions of input spectra, triggering criteria, noise rate, acceptability criteria for fitted events, etc. For example, in searching for point sources of neutrinos with flat spectra ( I / E ) , the effective area for muons of mean energy of a few TeV, is appropriate with tight cuts
for good angular resolution. For atmospheric neutrinos and W I M P searches, muons of a few tens of GeV are more appropriate, and angular resolution is not very important. In all cases, however, the cuts must be tight enough to prevent the (long and non-gaussian) angular distribution tail of huge downgoing muon rates from overwhelming the neutrino signals. The rigor of these cuts depends upon background count rate and upon depth. DUMAND and N E S T O R have ocean radioactivity, AMANDA has shallow depth, Baikal has both shallow depth and noise with which to contend.
3. T H E N E X T TOR
STEP
- A 1KM
DETEC-
The foregoing, along with other talks at this conference[11-13], present a strong case, in my view and that of m a n y others, for beginning the technology studies and system design needed to make a formal proposal for a next generation neutrino telescope. Such an instrument will go beyond the capabilities of the small university groups now hosting experiments, and will demand the engagement of national laboratories. As a result of discussions in the US and Europe during the past year, two laboratories are under active discussion as possible hosts for this project (perhaps co-hosts), Saclay in France, and JPL in California. The A M A N D A and D U M A N D groups have been cooperating in a study of a k m s in-
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.LG. Learned~Nuclear Physics B (Proc. SuppL) 38 (1995) 484 494
Tau Dectey
)
) )
~t__ Interaction Ne~rino
Figure 2. A schematic view of a "double bang" event near a deep ocean detector whose modules are indicated by dots. Such cascades should be visible to detectors from hundreds of meters distance.
strument, with personnel from J P L in Pasadena, and LBL in Berkeley. The agreed-upon strategy is to design competing instruments for the ocean and ice, with one to be chosen based upon the experience to be gotten over the next several years. Further discussions took place at the Snowmass '94 Workshop, and plans were formulated for the beginning of collaboration organization. 3.1. D o u b l e B a n g E v e n t s The interaction of high energy tau neutrinos (v~'s) in DUMAND-like detectors [9] will present a spectacular "double bang" signature. The existence of such events depends upon the presence of 10 is e V neutrinos in adequate numbers, as are in fact predicted from active galactic nuclei (AGN)[4] for example. The interesting signals are from the charged current (cc) quark interactions of v , ' s . Since the 7- mass is about 1.8 G e V , a 7- of 1.8 P e V and with c7- of 91 #m[15] would fly roughly 90 m before decay. The signature, as illustrated i n Figure 2, is: 1. a big hadronic shower from the initial v, interaction, 2. a muon like 7- track, and then
3. a second and even larger particle cascade, To give some scale to this, the ratios of detectable photons from these three segments are roughly 1011 : 2 . 1 0 e : 2 . 1 0 tx. Such large bursts of light would be visible from distances of hundreds of meters by present technology photomultipliers. The charged 7- will be hard to resolve from the bright Cherenkov light of the cascades, and the photon arrival times will not be very different. However, simply connecting the two cascades by the speed of light will suffice to make an unambiguous association (including direction of the cascades) of the two bursts. In fact, as we discuss later, it appears that the double bang signature alone is nearly background free, sufficiently so as not to have to invoke lack of either incoming or outgoing charged particles (muons) to produce a clean sample. The double bang event topology appears to be a unique signal for real 7- production by v,'s, thus permitting "discovery" of the vr, and inferring mixing of neutrino flavors. Finding even one of these events would have significant implications. The rate for such events is of course unknown experimentally now. If we employ optimistic fluxes from the Szabo and Protheroe model[3] for the sum of neutrinos from all AGN, we estimate 1000 events of this type per year in a volume of 1 k m a of water or ice, in a 2 P e V energy band. While the experiments now being constructed (AMANDA, Baikal, DUMAND, and NESTOR) would expect to see a b o u t one such event per year, they should easily determine if the AGN flux is present. We show below that v , ' s are unlikely to originate in commonly considered astrophysical sources, but are likely to appear due to neutrino mass and mixing, over a large range of allowable (and even favored, if the solar neutrino puzzle and the atmospheric neutrino anomaly have anything to do with oscillations) neutrino mixing parameter space. We know of no other way to make a v, a p p e a r a n c e experiment with cosmic rays, no way has been proposed for an accelerator experiment except for the use of emulsions making observations of relatively large 6rn ~ > 1 e V 2, and no way of detecting Vr'S except statistically at
.AG. Learned~Nuclear Physics B (Proc. Suppl.) 38 (1995) 484-494 proposed long baseline accelerator experiments. In the following we explore the physics implications of the observations of the double bang events in a little more detail, discussing the kinematics, sources of u~'s, the sensitivity to two and three neutrino mixing, and potential (and we conelude small) backgrounds. 4. E s s e n t i a l l y Full K i n e m a t i c s One m a y in principle measure the total energy of the incident neutrino and nearly the full kinematics of the double bang events by adequate sampling of the Cherenkov radiation. The observation of light from the first cascade yields the energy transferred to the quark, El. The magnitude of the light from the tan track plus the flash of light from the second cascade gives the energy (E2) kept by the T except for that carried away by the decay neutrino(s). The sum (JET -= Et + E2) gives the rough incoming neutrino energy, and the ratio of the first cascade energy to the total energy ( E t / E T ) provides the ~ y value. The cross sections and < y > are almost equal for v~ and ~ at this energy[16]. Observing the y distribution is a check on the observations, and departures from expectations could signal new physics. In calculating the v~ flux, the measured y distribution will permit correction for the potentially unobserved events near y = 0 (no initial cascade) and near y = 1 (initial cascade with most of the energy and the tau decays too close to the first cascade for resolution). The near equality of the cross sections for particle and antiparticle permits the total flux to be calculated independently of the mix in the cosmic beam. In a v~ cc interaction, < y > is about 0.25 at these energies[16] and the energy deposited is < Et > ~ 1/4 Ev while the 7" carries about ~ 3/4 of the v~ energy. In the subsequent decay of v, the energy deposited will be about 2/3 E~ ~ 1/2 E~ when the decay is hadronic (which is about 64% of the time). Hence the ratio of the average energy of the second "bang" to the first one is given by E 2 / E t ~ 2. This is characteristic of these t/~ events and making a cut of E 2 / E t > 1 eliminates several kinds of background events as we discuss later. Detailed calculations for signal as
489
well as background E1 versus E2 distribution will be presented elsewhere[14]. The threshold energy for discriminating two cascades will be determined by requiring a r that flies far enough so that the two cascades can be distinguished, and so that there are no "punch through" events. This distance will be of the order of some few times the cascade length (order 10 rn), and thus our threshold for r detection via this means would be, as suggested above, about a P e V . Aside from the physics limitation of several tens of meters, there will also be a detector limitation depending upon detector density and response, but probably of similar magnitude. We note that the observation of the double bang events presents the opportunity to measure the P e V v~ cross section via the angular distribution in the lower hemisphere, which decreases towards the nadir due to attenuation through the earth (--~ 90% for straight upwards travelling neutrinos in this energy range). For future studies of earth tomography, the potential of this process is great, since it does not depend upon convolution over the y distribution and muon range, as is necessary to extract information from the upcoming muon flux alone. Also, given the enormous light output of the cascades one would expect that timing from the detectors (at intermediate distances, since nearby detectors of present design will surely be saturated) would give excellent vertex resolution, and thus the initial neutrino direction to a precision of the order of 10 . In principle, of course, having both cascades and almost all energy "visible" one can deduce the initial neutrino direction with arbitrary precision, perhaps making optical precision ultimately possible in some future neutrino telescope. From the ensemble of measurements with a DUMAND-like array we will have: the r rate from double bang events gives the vr + Pr flux; measurents of the UHE muon flux permits calculation the v~+P~, flux; the W - resonant event rate yields the Pc flux at 6.4 P e V ; observation of the cascade rate (as a function of energy) produces the sum of neutral current interactions of all flavors of neutrinos, and charged currents without visible #'s and r ' s (mostly v~s).
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In fact if we assume t h a t we know Pe/ve, then we can calculate the flavor ratios without employing the muon measurements (which will have different systematic errors). For the more optimistic AGN fluxes cited[3] the numbers of events collected in a krn 3 detector in one year would permit calculation of the flavor content to a few percent. 4.0.1. A s t r o p h y s i c a l v F l a v o r C o n t e n t In the absence of neutrino oscillations (discussed in the following section), from the seemingly most likely astrophysical high energy neutrino sources we expect nearly equal numbers of particles and anti-particles, half as m a n y ve's as vt,'s , and virtually no v~'s. This comes about simply because the neutrinos are thought to originate in decays of pions (and kaons) and subsequent decays of muons. Most astrophysical targets are tenuous even compared to the earth's atmosphere, and would allow for muon decay in flight. The extreme case of an astrophyscial beam d u m p target of sufficient density to suppress 7r and k decay (as would happen with a nucleon b e a m striking a neutron star), but not so thick as to absorb the directly produced fluxes (a neutron star would absorb almost any but a tangential high energy neutrino beam), seems to be unlikely. Beyond the geometric difficulty, producing a detectable flux of high energy neutrinos from such an inefficient source makes it even more improbable. There are some predictions for flavor independent fluxes from cosmic defects and exotic objects such as evaporating black holes. Observation of tau neutrinos from these would have great importance. Indeed as we show below, for such exotic sources along with the presence of oscillations, we would have a unique flavor signature permitting unravelling the source content. The flux ratio o f v ~ , : v ~ : v ~ = 2 : l : 0 i s c e r tainly valid for AGN models such as the ones due to Stecker, et a/[2] in which the neutrinos come from lr's produced via the process 7 + P --* N + lr. In the calculations of Protheroe and Szabo [2] there are two additional features: the additional v~ flux due to escaping neutrons modifies the v u : ve flux ratio to 1.75 : 1 and about 10% of the v-flux is due to proton-proton (pp) inter-
actions. Some fraction of the pp neutrinos will then give rise to neutrinos from rapidly decaying secondaries containing heavy quarks ( " p r o m p t " production). Depending on the amount of p r o m p t v-flux due to D s and B/~ production (and decay) there could be a non-zero v~ component present. A conservative upper bound on the relative v~ content is < 0.1 for any target situation. Because most of the secondary pions will have ample opportunity to decay the my fraction probably will be nearer 10 -4 . 4.0.2. S e n s i t i v i t y t o N e u t r i n o O s c i l l a t i o n s The ~m 2 sensitivity (from L / E ) for this observation is then fantastic, going down to the order of 10 - i s eV 2 (the distance is out to the AGN, 100 Mpc). To determine the two neutrino mixing angle sensitivity limit requires detector specific simulations. The limitation has to do with the AGN neutrino flux magnitude and effective volume for these events, and will probably be limited by statistics, at least in the near future. We guess it will be no better t h a n 0.01 in the best of situations (as with most oscillation experiments). The parameter space for a combined fit to the atmospheric and solar neutrino d a t a allowing for three flavor mixing has been given by Fogli, et a/.[17], and by Acker, e~ al.[18], respectively. We have evaluated the survival and transition probabilities for the whole range of allowed values of the three and two mixing angles, and the results are indicated in Figure 3. This figure plots the fraction of muon neutrinos versus the fraction of electron neutrinos, and thus each point specifies a fraction of tau neutrinos (it is analogous to the color triangle). The initial expected flux (v~ : v~ : v~:: 2 : 1: 0) i s a t f g = 0.66 a n d f ~ = 0.33. We observe that almost all combinations of acceptable mixing angles result in saturation values to be observed with AGN neutrinos which lie between the lines f~ + f~, = 0.88 and f~ + f~, = 0.66. Hence a substantial number of v~ events are expected (0.34 > f~ > 0.12) for all situations which solve the solar and atmospheric problems with neutrino oscillations. Moreover, since in the case of two flavor mixing it is impossible to obtain f ~ / f t , > 1, observation of the d a t a falling
JG, Learned~Nuclear Physics B (Proc. SuppL) 38 (1995) 484-494 i-4
0
Prom~, N u s ~ ~'Q~ ~ ~x,x" \
Observed Underground
,,<
Expected C oo o.c
0
1.0
0.0
Muon Fraction
Figure 3. The fraction of muon neutrinos versus electron neutrinos, allowing for a fraction of tau neutrinos. Expected initial flux is at 2/3, 1/3. Full mixing would result in 1/3, 1/3. The points represent various solutions to the solar and atmospheric neutrino problems. The point corresponding to pure prompt v-beam is at 0.30, 0.47 & 0.23 taus.
above the diagonal f~ = fg line is clear evidence for three flavor mixing. We would like to stress that a small "impurity" of r,'~s in the initial neutrino fluxes, as discussed in the previous section, has a negligible effect on the v~ signal due to vl, -t,~ oscillations with moderately large mixing angles. Interestingly, the purely prompt flux case, for which we are not aware of any model, gives rise to a very distinct flux pattern on arrival, vg:tJe:t% = 1.0:1.56:0.72---0.3:0.47:0.23. This represents a very singular point in Figure 3, distinguished by the dominance of the ue flux. We thus conclude that almost any astrophysical source of neutrinos will result in significant numbers of v~'s, if neutrinos have mass and they oscillate.
491
4.0.3. B a c k g r o u n d s : A l m o s t N o n e By background we mean events which can fake the double bang signature of two huge cascades spaced roughly 100 m apart, with the second larger than the first (by typically a factor of two), and the sum of the visible energy being in the range of a few P e V . There are two general types of possible backgrounds due to neutrinos: those into which the neutrino transforms (e, p and r), and particles generated in the cascade resulting from the energy transferred to the struck quark. Neutral current events with an outgoing neutrino have interaction lengths so great as to be negligibly likely to interact again at close range ( q 10 -0 at this P e V energy). Electrons will immediately radiate, and be added to the quark cascade, and be indistinguishable from neutral current events. The most serious background is due to v~, cc events in which the muon travels about 100 m and then loses most of its energy in a single catastrophic Bremsstrahlung radiation. Such events can have energy characteristics similar to signal events. The rate for such events as a fraction of cc events can be estimated as
where R is the radiation length in water and ,~ 0(1/3) for the fraction of energy deposited. This number is of the order 3 • 10 - s and is reassuringly small. Detector simulations are required to numerically evaluate the confusion probability. Whatever that probability is, and we expect it to be low at 2 P e V , it decreases rapidly with increasing energy (short range muon radiation goes up while the tau decay length increases). In a large array, one may also demand that there be no continuing track, since the probability of a 2 P e V muon stopping after 100 m is negligible. However, we estimate that this constraint will not be needed. The other potential source of background from neutrino interactions is from the hadronic side of the neutrino interaction vertex. First of all, we do not know of any particle with the ability to penetrate --. 104 g m / c m ~ of matter before decaying or interacting (100 m is about 100 strong interaction lengths in water), except for leprous. Thus the AE/E
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only situation with which we must be concerned is when a 7- results from the decays of particles in the cascade, and moreover only when this T has most of the energy, which we show below to be quite unlikely. One source of this type of background is the production (and decay to r) of D, (and B/~ pairs) in neutral current interactions of v, (and v~,) and charged current interactions of re. These could give rise to "double bang" events if D s (or B) should decay within 10 m to produce an energetic I- which then travels about 100 m before decaying. In such events the average energy of the second bang E2 "~ 2 / 3 E r ,,, 1/2 Eh 1/8 z E v where z = E h / E v . Since z is expected to be rather small, we expect E2/E1 < < 1. Hence the E~./E1 > 1 cut will remove most such events. The number of such events will be N ~ { r N ~ + (1 + r ) N e } (fD, BS~+f~B~) where f, are the fractional production of D, and B/~ respectively. For fi ~ 10-2, r ~ 1/3, Nz = Ate; we find N ~ 1.5- 10-aN~,, most of which will be removed by the energy fraction cut. A somewhat more serious background is diffractive production of D, via charm changing cc interaction of v, in the reaction ve + N --* e + D, + x. There is no Cabibbo suppression, the rate can be as high as 6.10 - a times the cc rate and furthermore D, can have average energy of about 1/2 E~ [13]. Thus E2/E1 can be of order 1 and some events m a y pass the E2/E1 cut. However, the rate is given by 6 • 10 - a • B ~ ' ,~ 2 . 5 . 1 0 - 4 N e and is thus also quite small. Other backgrounds pose even less of a problem. Finally, we note that as a check, studies of the decay p a t h length distribution requiring consistency with the known tau lifetime, should confirm the detection of taus and the lack of contamination of the sample. In sum we conclude that the data selection criteria on spacing and energy of the two bangs will remove virtually all backgrounds and leave a verifiably pure sample of UHE v events. 4.0.4. I m p l i c a t i o n s o f D o u b l e B a n g E v e n t s One impact of the consideration of this potential observation gives motivation to fill the volume, somewhat, of a hypothetical 1 k m a array. Indeed this experiment itseff could perhaps jus-
tify construction of that instrument. Also, given that these events are near the anticipated acoustic detection threshold[19], one m a y contemplate hearing the double clicks from such events at few kra ranges and higher energies. The unusual property of acoustic pulse amplitude which results in slow decrease with distance in water may make practical the search for higher energy neutrino interactions from volumes of m a n y cubic kilometers. Additionally, the possibility of detecting neutrino induced cacades via radio pulses of G H z frequency range continues to be explored in deep ice[20]. As for acoustic sensing, the double microwave pulses due to r production and decay should provide distinctive signatures. We remind the reader that all of the above requires the existence of substantial numbers of P e V neutrinos, which m a t t e r should be resolved in the next few years by the AMANDA, Baikal, DUMAND, and N E S T O R experiments now under construction. If those ultra high energy neutrinos are present in hoped for numbers, then we believe that the observation of the double bang events, along with other previously discussed interactions, will lead to i m p o r t a n t particle physics measurements which cannot be carried out in any other way on earth.
5. S U M M A R Y -
ON THE THRESHOLD
In conclusion, we see that there seems to be growing interest in high energy neutrino astrophysics by virtue of the swelling number of people actually engaged in building instruments° A look at older proceedings in this conference series shows that earlier predictions, including this author's were often optimistic, yet now there seems to be real physical progress. There are four experiments that should be able to reveal the presence of the predicted large fluxes of high energy neutrinos from AGNs, if those predictions are within an order of magnitude or so of reality, perhaps by two conferences from now. Perhaps by the third conference from now we shall be discussing double bang events.
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Acknowledgements I wish to thank the conference organizers for an excellent meeting and support. Thanks also to S. Pakvasa, and to F. Halzen, H. Rubinstein, R. Svoboda, lost in Eilat T. Weiler, K. Winter, and many others for useful discussions at this meeting. This work is supported by DOE grant DE-FG-03 94ER40833.
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17. G. L. Fogli, E. Lisi, and Do Montanino, Phys. Rev. D49, 3626 (1994). 18. A. Acker, A. B. Balantekin, and F. Loreti, Phys. Rev. D, 49, 328 (1994). 19. J. G. Learned and R. J. Wilkes, 23 ICRC, Calgary (1993), and B. Price, Nuc. Insto and Meth. A325, 346 (1993). 20. New calculations and efforts at measuring background noise and attenuation were reported by John Ralston and by George Smoot and collaborators at the Snowmass '94 Workshop. These efforts may lead to radio detection with an energy threshold lower than a PeV.