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Experimental status of high energy neutrino astronomy
Nuclear Physics B (Proc. Suppl.) 14A (1990) 97-102 North-Holland
97
EXPERIMENTAL STATUS OF HIGH ENERGY NEUTRINO ASTRONOMY
Robert SVOBODA Louisiana State University, Baton Rouge, LA 70803-4001, USA Current status of high energy astronomy is reviewed from an experimental point of view. Present data from the Baksan, Kamiokande, and IMB detectors constrain neutrino flux enhancement factors (A) to be less than about 400, based on extrapolations from reported ultrahigh energy (UHE) "D.C ." gamma ray observations . 1 . INTRODUCTION The field of neutrino astronomy can be divided into three general areas : 1) the study of solar neutrinos [1], 2) the study of stellar collapse [2], and 3) the search for and study of energetic point sources . Only the first two areas have been realized experimentally and can be considered "established" fields of study . Discoveries and measurements have been intriguing (as in the case of the measured deficit of solar neutrinos by the Homestake [3] and Kamiokande [4] detectors) and sometimes spectacular (as in the case of SN1987a) . The effort to further these fields has intensified in recent years as exemplified by the large number of experiments either running or under construction . These include the Homestake Detector [5], the Kamiokande Detector [6], the IMB Detector [7], the Soviet-American Gallium Experiment (SAGE) [8], GALLEX [9], the Sudbury Neutrino Observatory (SNO) [10], Baksan Scintillation Telescope [11], the Large Scintillation Detector (LSD) [12], the Large Volume Detector (LVD) [13], and the Monopole and Cosmic Ray Observatory
0920-5632/90/$03 .50 © Elsevier Science Publishers B.V. (North-Holland)
(MACRO) [14] . In light of the above, it seems reasonable to consider that the development of high energy (>2 GeV) neutrino astronomy might also produce new phenomena and discoveries (to borrow a hack phrase, "open a new window on the universe") . Several experiments are in the proposal stage which hope to do just this : D D-II (a deep ocean experiment off Ha~aii), GRANDE (a air shower and neutrino detector near Little Rock, Arkansas), Lake Baikal Experiment (located in a deep lake near Irkutsk), and a large detector to be built in a covered lake in Italy (as yet unnamed) . The methods, goals, and prospects for success of such experiments are reviewed here . 2.
METHODS 2 .1 THE SIGNAL Muon (and anti-muon) neutrinos are assumed to be produced at an astrophysical source by high energy protons from a compact object colliding with a "target", such as an accretion disk, a gas cloud, or a companion star [15] . These muon neutrinos travel to earth where same of them interact in the medium surround-
98
R. Svoboda/High energy neutrino astronomy
ing a muon detector . Muons produced from charged-current interactions carry away half the energy of the parent neutrino, on the average, and so they have great penetrating power. They also preserve the direction of the parent neutrino to within a few degrees (fig . 1) . The muons that enter the detector are tracked and projected back on the sky . Regions of the sky that have significantly more muons than expected from background (discussed below) can then be identified as sources . 2 .2 THE BACKGROUND The most intense background is the ambient flux of muons resulting from cosmic ray interactions in the atmosphere . This flux is about 170 m 2-s 1 at the surface of the earth, more than 12 orders of magnitude greater than any expected muon flux from astrophysical neutrinos . Fortunately, this flux is all downward-going and so can be discriminated against when the astrophysical source is below the horizon (and therefore producing upward-going muons) . A more serious background is the flux of muon neutrinos from atmospheric cosmic ray interactions on the opposite side of the earth from the detector. These produce a roughly isotropic flux of upward-going muons at a rate of about 2 .2 x 10°9 t-2-S-2-Sr 1 for muons > 2 GeV [16] . This (very roughly) corresponds to about 1/3 upward-going muons per 1 degree solid angle/yr/10,000 m2 of flat detector . This background can be compared with figure 1, the expected angular distribution of muons about the direction of a source (in this case calculated for the GRANDE detector) . It is this background
which eventually limits the sensitivity of all detectors . 2 .3 THE DETECTORS There are currently three large neutrino detectors running in the world, Baksan, IMB, and Kamiokande . Pertinent features of these detectors are given in table 1. TABLE 1 . Operating BE Astrophysical Neutrino Ckotectors Detector Location Effective Area in operation ------ ------------------------------------------------------------Sakean Neutrino Telescope
Hakman Valley, U .S .S .R .
Irvine-MichiganBrookhaven
Cleveland,
Ramickande
Toy-,
100 sq .m.
(est .)
December, 1976
390 sq .m.
August,1982
165 sq .m .
July, 1983
U .S .A .
Japan
3 . GOALS OF H.E . NEUTRINO ASTRONOMY The goals of high energy (HE) neutrino astronomy can be summarized as follows : 1) to determine if there exist HE neutrino sources, and if so, to study them. 2) to provide independent confirmation of UHE and WE gamma-ray results . 3) to look for new phenomenon, such as dark matter and superconducting cosmic strings . 3 .1 CONFIRMATION OF VHE, UHE GAMNA-RAY RESULTS If neutrinos from astrophysical sources are produced in a way similar to the way they are produced in particle beam accelerators then there will be gamma rays produced also . This can be seen from the reactions : p + n - hadrons + *° ®e± + (-Pa .f.
(y)
+
N,
W±
R. Svoboda/High
energy neutrino astronomy
It can be shown [17] that the neutrino flux is usually related to the gamma ra flux by the formula [17] : 2 1v d d0.y -- - X (E) --dE y dEy
where X(E) is an energy-dependent "enhancement factor" which depends upon the geometry and matter density of the production site [18] . As an example, for a model of a massive companion rotating about a neutron star (figure 2) it is assumed that protons are accelerated by the intense magnetic fields near the surface of the neutron star and impinge upon the surface of the companion . Both gamma rays and neutrinos are produced, but when the column density of exceeds 200 g-cm 2 , virtually all the gamma rays interact and do not escape to the other side of the star. This means that gamma rays can be efficiently produced only when the line of sight to the neutron star fr~jm earth lies very near the limb Neutrinos, on the of the companion . other hand, will not interact in the companion (except for energies >100 TeV [19]) and therefore will be greatly enhanced with respect to the gamma rays . For this specific model, enhancement factors of 20 to 300 have been suggested [19,140] . Sometimes included in A is the attenuation of the gamma rays on the microwave background via e+e- pair productien .[21] This is very energy and distance dependent, with a threshold of about 10 1` eV . For most galactic sources this amounts to no more than a factor of 3 (the mean free path at 1015 eV is 10 kpc) .
Confirmation of gamma ray results by a neutrino detection would have the following important consequences : 1) it would confirm that g rays are actually being produced by these sources via meson decay . 2) it would provide a flux (and therefore luminosity) normalization for these sources unaffected by column density, geometry, and microwave attenuation . This would be a great step forward in understanding the underlying acceleration mechanisms and energy budgets of then-sources . 3) it would be strong evidence that the muon rich "gamma ray" signal events observed by some UHE arrays [22] are, in
fact, photons . It should be emphasized that the sensitivity of neutrino detectors is much greater when coupled with simultaneous gamma-ray observations. This is because the typical atmospheric neutrino background rate for a detector like GRANDE is only about 1 event/year . Thus the observation of even a single neutrino event coincident with a gamma-ray burst lasting a few hours would be quite convincing evidence for a positive: detection from which a considerable amount of information could be gleaned . 3 .2 THE SEARCH FOR NEW
SOURCES AND NEW
PHENOMENA
There are many suggested Possible sources for neutrinos other than those reportThese ed by gamma-ray observations . include : 1) sources shrouded in a thick matter blanket such that only low energy photons and neutrinos escape [23] . Such a source would be similar to SN1987a . 2) nearby sources which are obscured due An example of to intervening matter.
R. Svoboda/High energy neutrino astronomy
100
42
ANGLE
VV .
-
~
U
Ici
60
b - Baksan (upper limits) i - IMB (upper limits) k - Kamioka (upper limits) H - Haarale Park H - Buckland Park F - Fl" EBe K - Kiel k b i
.w
i h
C
O
O
Co
36
wn
t~1J
OD O 64
Angular Cut (deg .) Figure 1 - Angle between incident neutrino and detected muon calculated for two possible source spectral indices .
NEUTRON STAR ACCELERATED PROTONS
Figure 2 - The "generic" beam dump model, a neutron star (the particle accelerator) orbiting about a companion (the beam dump) .
k
~r
i
b
_
~.r
_ra
star
11
36
34
/
H
/
F
/ "I
1
1
1
I-Ll_1 j
5
1
10
1
L
1
1
1.1
50
Distance (kpc) Figure 3 - A comparison of reported UHE gamma ray luminosities with upper limits on the neutrino luminosities from operating neutrino detectors . No enhancement factor has been assumed. The dashed line represents the sensitivity expected for the proposed GRANDE detector .
R. Svoboda/High energy neutrino astronomy
such a source is the Cas A supernova remnant [24] . This supernova happened about 300 years ago and was only about 3 kpc away (to be compared to SN1987a, which was 50 kpc away), but remained undiscovered until detected by radio techniques in this century . 3) distant sources which have significant microwave attenuation to be undetectable in UHE gamma-rays [2 1.] . 4) Exotic sources, such as the capture and annihilation of dark matter in the sun [25] or from the decay of superconducting cosmic strings [26] . 4.
CURRENT EXPERIMENTAL RESULTS
It is useful to calculate what X would have to be in order for reported gamma ray sources to have been detected in current neutrino experiments . To do this one can compare published UHE gamma ray "D .C." luminosities with limits on neutrino luminosities . Since luminosity varies linearly with the flux, L = kLY (if h is not too energy dependent) . Figure 3 shows this comparison graphically . Upper limits on the neutrino luminosity as measured by the Baksan [27], IMB [28], and Kamioka [29] detectors are plotted versus the distance of the source . These luminosities are adjusted to a common threshold of 10 GeV and a differential neutrino spectral index of 2 .1 with no cut-off . The points plotted for the Havarah Park [30], Buckland Park [31], Fly's Eye [32], and Kiel [33] UHE air shower detectors are the publâ*ihed luminosities scaled down to a 10 GeV threshold using an assumed gamma ray spectral index of 2 .1 . No neutrino enhancement factor is assumed (i .e ., A=1) . As can be seen the current neutrino limits are about a factor of 400 above these
results, and thus are on the edge of the most optimistic enhancement models . e dashed line indicates the A=1 sensitivity of the proposed GRANDE detector (i .e ., a 3 sigma "D .C ." excess above background after 10 years of operation) .The next generation of neutrino detectors will therefore have the sensitivity to reach lambda factors of about A=10, well within expectations of "reasonable" models . Thus there is good reason to believe that they may be able to confirm the gamma ray results . UHE "D.C.°° gamma ray results are used for this comparison rather than the more numerous VHE burst results due to the difficulty in calculating a time-averaged flux and the uncertainty over how much of the gamma-ray flux at these relatively low energies come from meson decay. Though the gamma ray flux is extrapolated to a 10 GeV threshold from PeV energies, there is experimental evidence to justify the use of a single spectral index (=2 .1) [34] . 5.
CONCLUSIONS Current HE neutrino detectors stop short of being able to constrain models for gamma-ray production from galactic sources or confirm observations. Proposed detectors should be able to conf irm the gamma ray observations source production, and constrain models in which the neutrino to gamma flux ratio is greater than 10 . REFERENCES 1 . J .N . Bahcall and R.K . Ulrich, Rev. of
Modern Physics, Vol . 60 (1988) 297 .
2 . V. Trimble, Rev . Vol . 60 (1988) 859 .
of Modern Physics,
3 . R . Davis, B .T . Cleveland, J .K. Rowley, Proc . 20th ICRC Vol . 4, Moscow, August 2-15 (1987) 328 .
102
R. Svoboda/High energy neutrino astronomy
4 . K.S . Hirata, et al ., Phys . Rev. Lett . 63 (1989) 16 .
19 . M .H . Reno and C, Quigg, Phys . Rev . D 37 (1988) 657 .
5 . J . K. Rowley, B . T. Cleveland, and R. Davis, Proc . Conf . on Solar Neutrinos and Neutrino Astronomy, eds . M.L . Cherry, K .Lande, and W.A . Fowler, AIP 126 (1985) 1.
20 . J .G . Learned, P. Gorham, Proc . Japan-U .S . Seminar on Cosmic Ray Muon and Neutrino Physics, Tokyo, June (1986) 64 ; V.S . Berezinsky et al ., Il Nuovo Cimento 8C (1985) 185 .
6 . K.S . Hirata, et al ., Phys . Rev . D 38 (1988) 448 .
21 . M. Honda, Ap .J . 339 (1989) 629 . 22 . M Samorski and W. Stamm, Ap .J . 268 (1983) L17 ; B .L . Dingus et al ., Phys . Rev . Lett . 61 (1988) 1906 .
7 . S .T . Dye, et al ., Phys . Rev . Lett . 52 (1989) 2069 . 8 . Barabanov, I .R ., et al ., Proc . Conf . on Solar Neutrinos and Neutrino Astronomy, eds . M . L. Cherry, K. Lande, and W.A . Fowler,AIP 126 (1985) 175 . 9 . W. Hampel, Proc . Conf . on Solar Neutrinos and Neutrino Astronomy,eds . M .L . Cherry K .Lande, and W .A . Fowler, AIP 126 (1985) 162 . 10 . G . Ewan, Proc . 7th Workshop on Grand Unification/ICOBAN ®86, ed . J . Arafune, Toyama, April 16-18 (1986) 95 . 11 . E .N . Alexeyev, et al ., Proc . 20th ICRC Vol . 4, Moscow, 2-15 August (1987) 351 ; E .N . Alekseev, et al ., Sov. Astron . Lett . 14 (1988) 41 ; N.M . Boliev, et al ., Proc . of 17th ICRC Vol . 7, 13- 25 July, Paris, (1981) 106 .
23 . M.M . Shapiro and R. Silberberg, DUMAND Summer Workshop, Lake Baikal, (1979) 269 ; V.S . Berezinsky and O .F . Prilutsky, Proc . of Neutrino-76, Aachen, (1976) ; T .K . Gaisser and Todor Stanev, Phys . Rev . Lett . 58 (1987) 1695 . 24 . D.H . Clark and F .R . Stephenson, The Historical Supernovae (Pergamon, Oxford, 1977) . 25 . S . Ritz and D . Sekel, CERN -TH .4627, SCIPP-87/99 (1987) ; T.K .Gaisser, G. Steigman, and S . Tilav, Phys . Rev . D 34 (1986)2206 26 . C .T . Hill, D .N . Schramm, and T.P . Walker, Phys . Rev. D 36 (1987) 1007 . 27 . Talk given by A .E . Chudakov, 7th Workshop on Grand Unification /ICOBAN '86, Toyama, April 16-18 (1986) .
12 . G . Badino et al ., Nuovo Cam . C 7 (1984) 573 ; M . Aglietta et al ., Nuovo Cim .C . 9 (1986) 185 .
28 . R. Becker-Szendy et al ., Proc . Workshop on Neutrino Telescope, Venice, March (1989) in publication .
13 . P . Galeotti, Proc . UP ®87 Conf ., Baksan Valley, 17-19 August (1987) 198 .
29 . Y . Oyama (1989) 1481 .
14 . G . Tarle et al ., Proc . 20th ICRC Vol . 6, Moscow, 2-15 August (1987) 500 .
30 . J . Lloyd-Evans (1983) 784 .
15 . see, for example . D . Eichler, Ap .J . 222 (1978) 1109 ; D. Eichler, Nature 275 (1978) 725 ; D. Eichler and D . Schramm, Nature 275 (1978) 704 .
31 . R .J . Protheroe et al ., Ap .J . Lett . 280 (1984) L47 ; R.J . Protheroe and R. Clay, Nature 315 (1985) 205 .
16 . Y . Minorikawa and K . Mitsui, Europhys . Lett . 7 (1988) 377 ; L.V .Volkova, Sov.J .Nucl .Phys . 3 1 (1980) 784 . 17 . E_W . Kolb, M .S . Turner, T .P . Walker, Phys . Rev. D 32 (1985) 1145 . 18 . V.J . Stenger, Ap .J . 284 (184) 810 .
et
al .,
32 . G .L . Cassiday Lett . 62 (1989) .
et
et
Phys . al .,
al .,
33 . M . Samorski and W . Stamm, (1983) L17 .
Rev . Nature
Phys .
D 39 305
Rev .
Ap .J . 268
34 . T .C . Weekes, Physics Reports 160 Vol . 1 (1988) .
97
EXPERIMENTAL STATUS OF HIGH ENERGY NEUTRINO ASTRONOMY
Robert SVOBODA Louisiana State University, Baton Rouge, LA 70803-4001, USA Current status of high energy astronomy is reviewed from an experimental point of view. Present data from the Baksan, Kamiokande, and IMB detectors constrain neutrino flux enhancement factors (A) to be less than about 400, based on extrapolations from reported ultrahigh energy (UHE) "D.C ." gamma ray observations . 1 . INTRODUCTION The field of neutrino astronomy can be divided into three general areas : 1) the study of solar neutrinos [1], 2) the study of stellar collapse [2], and 3) the search for and study of energetic point sources . Only the first two areas have been realized experimentally and can be considered "established" fields of study . Discoveries and measurements have been intriguing (as in the case of the measured deficit of solar neutrinos by the Homestake [3] and Kamiokande [4] detectors) and sometimes spectacular (as in the case of SN1987a) . The effort to further these fields has intensified in recent years as exemplified by the large number of experiments either running or under construction . These include the Homestake Detector [5], the Kamiokande Detector [6], the IMB Detector [7], the Soviet-American Gallium Experiment (SAGE) [8], GALLEX [9], the Sudbury Neutrino Observatory (SNO) [10], Baksan Scintillation Telescope [11], the Large Scintillation Detector (LSD) [12], the Large Volume Detector (LVD) [13], and the Monopole and Cosmic Ray Observatory
0920-5632/90/$03 .50 © Elsevier Science Publishers B.V. (North-Holland)
(MACRO) [14] . In light of the above, it seems reasonable to consider that the development of high energy (>2 GeV) neutrino astronomy might also produce new phenomena and discoveries (to borrow a hack phrase, "open a new window on the universe") . Several experiments are in the proposal stage which hope to do just this : D D-II (a deep ocean experiment off Ha~aii), GRANDE (a air shower and neutrino detector near Little Rock, Arkansas), Lake Baikal Experiment (located in a deep lake near Irkutsk), and a large detector to be built in a covered lake in Italy (as yet unnamed) . The methods, goals, and prospects for success of such experiments are reviewed here . 2.
METHODS 2 .1 THE SIGNAL Muon (and anti-muon) neutrinos are assumed to be produced at an astrophysical source by high energy protons from a compact object colliding with a "target", such as an accretion disk, a gas cloud, or a companion star [15] . These muon neutrinos travel to earth where same of them interact in the medium surround-
98
R. Svoboda/High energy neutrino astronomy
ing a muon detector . Muons produced from charged-current interactions carry away half the energy of the parent neutrino, on the average, and so they have great penetrating power. They also preserve the direction of the parent neutrino to within a few degrees (fig . 1) . The muons that enter the detector are tracked and projected back on the sky . Regions of the sky that have significantly more muons than expected from background (discussed below) can then be identified as sources . 2 .2 THE BACKGROUND The most intense background is the ambient flux of muons resulting from cosmic ray interactions in the atmosphere . This flux is about 170 m 2-s 1 at the surface of the earth, more than 12 orders of magnitude greater than any expected muon flux from astrophysical neutrinos . Fortunately, this flux is all downward-going and so can be discriminated against when the astrophysical source is below the horizon (and therefore producing upward-going muons) . A more serious background is the flux of muon neutrinos from atmospheric cosmic ray interactions on the opposite side of the earth from the detector. These produce a roughly isotropic flux of upward-going muons at a rate of about 2 .2 x 10°9 t-2-S-2-Sr 1 for muons > 2 GeV [16] . This (very roughly) corresponds to about 1/3 upward-going muons per 1 degree solid angle/yr/10,000 m2 of flat detector . This background can be compared with figure 1, the expected angular distribution of muons about the direction of a source (in this case calculated for the GRANDE detector) . It is this background
which eventually limits the sensitivity of all detectors . 2 .3 THE DETECTORS There are currently three large neutrino detectors running in the world, Baksan, IMB, and Kamiokande . Pertinent features of these detectors are given in table 1. TABLE 1 . Operating BE Astrophysical Neutrino Ckotectors Detector Location Effective Area in operation ------ ------------------------------------------------------------Sakean Neutrino Telescope
Hakman Valley, U .S .S .R .
Irvine-MichiganBrookhaven
Cleveland,
Ramickande
Toy-,
100 sq .m.
(est .)
December, 1976
390 sq .m.
August,1982
165 sq .m .
July, 1983
U .S .A .
Japan
3 . GOALS OF H.E . NEUTRINO ASTRONOMY The goals of high energy (HE) neutrino astronomy can be summarized as follows : 1) to determine if there exist HE neutrino sources, and if so, to study them. 2) to provide independent confirmation of UHE and WE gamma-ray results . 3) to look for new phenomenon, such as dark matter and superconducting cosmic strings . 3 .1 CONFIRMATION OF VHE, UHE GAMNA-RAY RESULTS If neutrinos from astrophysical sources are produced in a way similar to the way they are produced in particle beam accelerators then there will be gamma rays produced also . This can be seen from the reactions : p + n - hadrons + *° ®e± + (-Pa .f.
(y)
+
N,
W±
R. Svoboda/High
energy neutrino astronomy
It can be shown [17] that the neutrino flux is usually related to the gamma ra flux by the formula [17] : 2 1v d d0.y -- - X (E) --dE y dEy
where X(E) is an energy-dependent "enhancement factor" which depends upon the geometry and matter density of the production site [18] . As an example, for a model of a massive companion rotating about a neutron star (figure 2) it is assumed that protons are accelerated by the intense magnetic fields near the surface of the neutron star and impinge upon the surface of the companion . Both gamma rays and neutrinos are produced, but when the column density of exceeds 200 g-cm 2 , virtually all the gamma rays interact and do not escape to the other side of the star. This means that gamma rays can be efficiently produced only when the line of sight to the neutron star fr~jm earth lies very near the limb Neutrinos, on the of the companion . other hand, will not interact in the companion (except for energies >100 TeV [19]) and therefore will be greatly enhanced with respect to the gamma rays . For this specific model, enhancement factors of 20 to 300 have been suggested [19,140] . Sometimes included in A is the attenuation of the gamma rays on the microwave background via e+e- pair productien .[21] This is very energy and distance dependent, with a threshold of about 10 1` eV . For most galactic sources this amounts to no more than a factor of 3 (the mean free path at 1015 eV is 10 kpc) .
Confirmation of gamma ray results by a neutrino detection would have the following important consequences : 1) it would confirm that g rays are actually being produced by these sources via meson decay . 2) it would provide a flux (and therefore luminosity) normalization for these sources unaffected by column density, geometry, and microwave attenuation . This would be a great step forward in understanding the underlying acceleration mechanisms and energy budgets of then-sources . 3) it would be strong evidence that the muon rich "gamma ray" signal events observed by some UHE arrays [22] are, in
fact, photons . It should be emphasized that the sensitivity of neutrino detectors is much greater when coupled with simultaneous gamma-ray observations. This is because the typical atmospheric neutrino background rate for a detector like GRANDE is only about 1 event/year . Thus the observation of even a single neutrino event coincident with a gamma-ray burst lasting a few hours would be quite convincing evidence for a positive: detection from which a considerable amount of information could be gleaned . 3 .2 THE SEARCH FOR NEW
SOURCES AND NEW
PHENOMENA
There are many suggested Possible sources for neutrinos other than those reportThese ed by gamma-ray observations . include : 1) sources shrouded in a thick matter blanket such that only low energy photons and neutrinos escape [23] . Such a source would be similar to SN1987a . 2) nearby sources which are obscured due An example of to intervening matter.
R. Svoboda/High energy neutrino astronomy
100
42
ANGLE
VV .
-
~
U
Ici
60
b - Baksan (upper limits) i - IMB (upper limits) k - Kamioka (upper limits) H - Haarale Park H - Buckland Park F - Fl" EBe K - Kiel k b i
.w
i h
C
O
O
Co
36
wn
t~1J
OD O 64
Angular Cut (deg .) Figure 1 - Angle between incident neutrino and detected muon calculated for two possible source spectral indices .
NEUTRON STAR ACCELERATED PROTONS
Figure 2 - The "generic" beam dump model, a neutron star (the particle accelerator) orbiting about a companion (the beam dump) .
k
~r
i
b
_
~.r
_ra
star
11
36
34
/
H
/
F
/ "I
1
1
1
I-Ll_1 j
5
1
10
1
L
1
1
1.1
50
Distance (kpc) Figure 3 - A comparison of reported UHE gamma ray luminosities with upper limits on the neutrino luminosities from operating neutrino detectors . No enhancement factor has been assumed. The dashed line represents the sensitivity expected for the proposed GRANDE detector .
R. Svoboda/High energy neutrino astronomy
such a source is the Cas A supernova remnant [24] . This supernova happened about 300 years ago and was only about 3 kpc away (to be compared to SN1987a, which was 50 kpc away), but remained undiscovered until detected by radio techniques in this century . 3) distant sources which have significant microwave attenuation to be undetectable in UHE gamma-rays [2 1.] . 4) Exotic sources, such as the capture and annihilation of dark matter in the sun [25] or from the decay of superconducting cosmic strings [26] . 4.
CURRENT EXPERIMENTAL RESULTS
It is useful to calculate what X would have to be in order for reported gamma ray sources to have been detected in current neutrino experiments . To do this one can compare published UHE gamma ray "D .C." luminosities with limits on neutrino luminosities . Since luminosity varies linearly with the flux, L = kLY (if h is not too energy dependent) . Figure 3 shows this comparison graphically . Upper limits on the neutrino luminosity as measured by the Baksan [27], IMB [28], and Kamioka [29] detectors are plotted versus the distance of the source . These luminosities are adjusted to a common threshold of 10 GeV and a differential neutrino spectral index of 2 .1 with no cut-off . The points plotted for the Havarah Park [30], Buckland Park [31], Fly's Eye [32], and Kiel [33] UHE air shower detectors are the publâ*ihed luminosities scaled down to a 10 GeV threshold using an assumed gamma ray spectral index of 2 .1 . No neutrino enhancement factor is assumed (i .e ., A=1) . As can be seen the current neutrino limits are about a factor of 400 above these
results, and thus are on the edge of the most optimistic enhancement models . e dashed line indicates the A=1 sensitivity of the proposed GRANDE detector (i .e ., a 3 sigma "D .C ." excess above background after 10 years of operation) .The next generation of neutrino detectors will therefore have the sensitivity to reach lambda factors of about A=10, well within expectations of "reasonable" models . Thus there is good reason to believe that they may be able to confirm the gamma ray results . UHE "D.C.°° gamma ray results are used for this comparison rather than the more numerous VHE burst results due to the difficulty in calculating a time-averaged flux and the uncertainty over how much of the gamma-ray flux at these relatively low energies come from meson decay. Though the gamma ray flux is extrapolated to a 10 GeV threshold from PeV energies, there is experimental evidence to justify the use of a single spectral index (=2 .1) [34] . 5.
CONCLUSIONS Current HE neutrino detectors stop short of being able to constrain models for gamma-ray production from galactic sources or confirm observations. Proposed detectors should be able to conf irm the gamma ray observations source production, and constrain models in which the neutrino to gamma flux ratio is greater than 10 . REFERENCES 1 . J .N . Bahcall and R.K . Ulrich, Rev. of
Modern Physics, Vol . 60 (1988) 297 .
2 . V. Trimble, Rev . Vol . 60 (1988) 859 .
of Modern Physics,
3 . R . Davis, B .T . Cleveland, J .K. Rowley, Proc . 20th ICRC Vol . 4, Moscow, August 2-15 (1987) 328 .
102
R. Svoboda/High energy neutrino astronomy
4 . K.S . Hirata, et al ., Phys . Rev. Lett . 63 (1989) 16 .
19 . M .H . Reno and C, Quigg, Phys . Rev . D 37 (1988) 657 .
5 . J . K. Rowley, B . T. Cleveland, and R. Davis, Proc . Conf . on Solar Neutrinos and Neutrino Astronomy, eds . M.L . Cherry, K .Lande, and W.A . Fowler, AIP 126 (1985) 1.
20 . J .G . Learned, P. Gorham, Proc . Japan-U .S . Seminar on Cosmic Ray Muon and Neutrino Physics, Tokyo, June (1986) 64 ; V.S . Berezinsky et al ., Il Nuovo Cimento 8C (1985) 185 .
6 . K.S . Hirata, et al ., Phys . Rev . D 38 (1988) 448 .
21 . M. Honda, Ap .J . 339 (1989) 629 . 22 . M Samorski and W. Stamm, Ap .J . 268 (1983) L17 ; B .L . Dingus et al ., Phys . Rev . Lett . 61 (1988) 1906 .
7 . S .T . Dye, et al ., Phys . Rev . Lett . 52 (1989) 2069 . 8 . Barabanov, I .R ., et al ., Proc . Conf . on Solar Neutrinos and Neutrino Astronomy, eds . M . L. Cherry, K. Lande, and W.A . Fowler,AIP 126 (1985) 175 . 9 . W. Hampel, Proc . Conf . on Solar Neutrinos and Neutrino Astronomy,eds . M .L . Cherry K .Lande, and W .A . Fowler, AIP 126 (1985) 162 . 10 . G . Ewan, Proc . 7th Workshop on Grand Unification/ICOBAN ®86, ed . J . Arafune, Toyama, April 16-18 (1986) 95 . 11 . E .N . Alexeyev, et al ., Proc . 20th ICRC Vol . 4, Moscow, 2-15 August (1987) 351 ; E .N . Alekseev, et al ., Sov. Astron . Lett . 14 (1988) 41 ; N.M . Boliev, et al ., Proc . of 17th ICRC Vol . 7, 13- 25 July, Paris, (1981) 106 .
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