Status of the MINOS experiment and review of long-baseline neutrino oscillation experiments in Europe and North America

ELSEVIER

Status of the MINOS Experiment and Review of Lang-Baseline Neutrino Oscillation Experiments in Eurape and North America Jon Urheim, representing the h,IIXOS Collaboration

iL

“Sclwol of Physics and Astrunomy, University of hIinnesota, I16 Chusch St. SE, MinneqAs, Minnesota 5545.5 USA

1. Introduction

At this conference, we have heard results from non-ac;celeratc~)r experiments (~811 the atmospheric (Super-Karniol~rde [1] ) and solar neutrirlo (SIX0 [‘] and Super-K [Sj) anomalies, as well w rrsults from K2K [4]? rXI accelerator-based ‘long-baseline’ experiment bearing on the former. JVe high all heard status reports on the KamLAND [5] and hliniBoone [G] experiments which are in ea~lp stages of collecting data with reactor ad accelerator neut.rinos, probing oscillations in the ‘solar’ and ‘LSND’ regions of squared mass diflerence AH?, respectively. Tc)gether, these esperirnents are beginning to provide a coherent, though incomplete, picture [7-Q] of neutrino IIEES~Bi*rid mixing. This contribution focuses 011 long-baseline experiments, whi&, within the next several years, will brgin to collect data with intewe high-energy v(~ beams generated by a,ccelerators at Fermiand CERN (CNGS/OPERA lab (I\-uMI/h4IlWS) and potentially CNGS/ICARUS). These experiments aim to study oscillations of Y,,‘s with frequencies dictated by the ‘a~tmosplwr-ic’ A&, (0.0016 < Am;lna < 0.0039 ev2, 9m CL [l]). These experiments will be able to confront, with prw&ion data, critical aspects of the ernesging picture, The oscillation interpretation c~)f atmospheric neutrino data from Super-K [1] is @ 2003 Elsevier Science B.V doi:lO.l016/S0920-5632(02)02088-l

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very strong, leading to the allowed Av&~~~ I+ by data from ICLK [4]. Super-K data akw suggests that the primary oscillation rrmde is vP -+ v, , and that the mixing angle 023 is consistent with rnaxima1 (sir? 2&j > 0.92 at 90% CL [l]). Direct confirmation of the w/J-r I/, hypothesis mill come from OPERA and ICARUS which are capable of identifJ&yg vi- interactions in t.heir detectors. I\ImOS will nieasur-e A762 and sin” 28,~ for this nmk precisely, zlrrd will test CPT aprnetry in the neutrino sector by measuring ufi md PI, osr:illat.ion parameters separately. A key element missing from the current pitrt,ure of neutrino masses and rnixings is the third mixing angle, 81~3. A non-zero value is a, necessary condition for CP rion-~onse~~-\;;rtiori via neutrino mixing. Sinw 02~ is nearly maximal and since & a,ppears to be large (tan” 01:: N 0.34) [%,7,$], not only rnight the W-violating ef?ects be observable [provided 81:~ is not too much smaller tliaJ,n present experiment~al limits), but also t,hey rnightj lra,tl played a significant, role in leptogenesis in the early universe [lo]. Experimentttlly, 4,~ would give rise to a subdominant l/Ah + 11~oscillation at A& ,l~. Clearly, observing such osciMions is an important goal for the long-baseline expeximents: the three experiments discussed here are well-positionec1)ned to do this. @cm given above, and is supported

Rrthermore, new experiments to rneaslure 613 a.nd study W-violating e-ffects are being conceived, The JHF-Kamioka project in Japan was discussed in a separat,e talk [ll]: at the end of this contribution, I will describe some of the ideas for new experiments in Europe and Korth America. 2. The NuMI/MINOS

Project

The iYuM1 project consists of an intense r//,, beam produced by 120~D-GeVprotons ext,ra,crted from the Fermilab Main Injector (MI), directed nortllwest and downward by 37 mrad, and reemerging from the earth in northern MinnesotLa. The MINOS ‘Far’ Detector is a 5.4-kt.on magnetized steel and plastic scintillator tracker and cdorimenter under construction in the Soudan iron mine 2341 ft below ground. With a baseline of 735 km, the first maximum in the oscillation probability occurs at E, ++ 1.5 GeV f’or an-2 = o.iJo25 &‘“. The &gnat we for neutrino oscillations is a dip in the eneru spectrum of v,~‘s detwted at Soudan relative to extrapuliltions from measurements by a l-kton ‘Near Detector located on the Fermilab site. The locution of the dip depends on the value of A&t while its depth gives a measure of sin” 28, Below, I summarize the status of t,he beam and d&e&or construc.*tion, as well as the physics capabilities .and outlook. Beam Line The MI will provide 4 x 101” I20-GeV protons in an 8 pus singl~-turrL-extracted pulse onto a graphite t.arget once every 1.9 seconds, for an average bean1 pcwer of 0.4 MW. Just beyond the target are f,w~ parabolic aluminum magnetic horns, foot:using charged ha,drons of energies that depend on the relative positions of target 3Ald horns, as shown in Fig. I. Downstream of the 2nd horn is a G?ijm vacuum decay pipe, followed by a steel hadron absorber and rock for muon absorpGon. The Near Detector is located 317m beyond t,he end of the decay pipe, The excavation of the beam line and detector hall areas is essentially complete, a,nd the decay pipe has been installed and mcased in concrete. Work will begin on the ou&ting of these areas and the construction of surface buildings above 2.1. The NuMf

Figure 1. Neutrinc:, energy spectra of the NuMI beam for three horn/target configuratians, in unit.s of charged-current (CC) events per kton-yr observed in the MINOS Far Detector? assuming no oscillations.

the target area and the I’&r Detector hall in the coming months. Engineering is comp1et.e for most of the technical components, and IIIZU\II~ eo~nponents are being fabricated.. Overall, the project is on schedule to be completed in l&e 2004, wit,h beam on target expected by the end of the 2004. 2.2.

The MINOS

Fax Detector

The h’IINOS Far D&et:tor is organized as two supermodules, each with 243 planes of steel 2.54 cm thirk and 8111 x 8m wide in an octagonal s&e. Each plane is formed fiorn eight 1.27~cm thick steel plates, cut so a.s to fit in the mine shaft, &V&X; these sheets itre plug-;-n:elded undergrouud. Onto each plane of’ steel is attached eight ~~&X+tor modules as shown in Fig. 2. The resulting steel plus scintillator module assemblies a.re then hung from their ‘ears on 8 steel support structure, much like file folders in a filing cabinet. In successi\re planes, the scintil&or modules dtcrnate between the orientation shown in the figure and one in which they are rotated by !XP. Through the center of’ each supermodule is a hole fw it.s rrmgnet cc.)il: t,his is energized to 15,KlO Amp-turns! generating ir. uniform toroidal rnagn&c field in the steel, A scintillator module consists of 20 or 28 polyst;vrene strips 4.1 cm x 1 cm in cross-section (cc)- rxtruc ._ 1P(I wi t11 a rrflect~ing laxer of Ti02) awl up to 8m long, encased in a light-tight ahm~inum

Stutus

of the MINOS

experiment

and reuiew

of long-baseline

neutrino

oscillation

experiments

39

Run2281. Snarl2621.

’ PMT box

-.-~

Figure 2. La.yout. of scintilla~~or modules for a. plane in the MIKOS F&r Dete&or

sheet metd enclosure. Readout is achieved by wa~~eleng~h-shifting (WLS) fibers glued into a. groove in one face of ea.& scintillator st.rip. The WLS fibers are routed through plastic manifolds to connectors at ea.& end of a module, from which clear optical fiber cables carry the waveleugthshifted scintillation light to Ha~mamatsu M-16 multi-&armel pl-~~~~.omult,i~~lier tubes housed in racks next to the detector. For aormdly-iaddell~ rnininlurrl-ionizing pwticles, the signals from the two erds of the fiber in a @ven strip typically sum to 10 - 15 pliotoelect,rons. We record pulsehright and timing information. Fur beam running, detector readout is triggered by a timing system GPS-synched to the MI beamspill. For cosmics the detector is self-triggering, utilizing dynode signals from the PMT’s. 2.2.1.

Status

and commissioning

A technical support crew of thirty, divided into two shifts, carriea out the welding, assembly and riggiug of the detector planes. atypically, seven pla.nes are erected per week, with electronics and high voltage cunnrcted a.nd tested m t,hey go up. Testing is performed with electronic and optical pulsing systems as well a,s with cosmic ray muons. Construction of the first supermodule was completed in July? and its coil energized for the first time the day before my presentation at ICHEP.

Figure 3. First%upward-going muon candid&e in the MINOS Fs\lrDetector. ‘Top: plota of time (ns) vs. vertical (y) and longitudinal (zj hit positions. Bottom: ~1vs, Z, g vs. T? and strip vs. plane.

C!c-mpletiun of the set:ond supermodule is scheduled for April 2003, C’osrrlic ray (CR) ~nuons haw prowidrd pulseheight and timing calibrations for the first supermodule. Over periods of months, the detector response is stable: to within 1% in energy and to within - 1 ns in timing. The single-&rip timing resolution has been measured t.cj be N 2.6 ns from residuals to fiits of time versus position for CR muons. We are currently evaluating resolutions in rfluon energy (expected t
.1. Urhr ?i?l?

40

Assembly of MIT\rOS Near Detector pla,nes is also under way, being done cm the srxrfaJ:eat Fermilab. These will be lowered into the detector ha,11once outfitting is complete and thy support struc?~~re has been installed there. A challenging aspect of the Near Detector is the large instant.aneous i-&e from rieutrino interactions, which could be as large as 50 events within the +s beam spill. Multi-hit front-end elect~ronics are necessasy so as to avoid pot.ential biases in the comparison bet.ween the busy PITearDetector and t,he low-occugancy Far Detector- event environments. Finally, the MliXOS cl&e&or will be used as a calorimeter for (1.) identifying CC v, interactions and measuring the electron energy (rtsol’n~ 23%/a), (2) identifying neutral-current (XC) interactions, arid (3) measuring the energy of hadronic showers (resol’nN &50//o/&) in both CC and NC events to he11~in the determination of the e~w+gy of the int,eracting neutrino. Stopping CR muons provide an excellent calibration of energy deposited through ionization, however t,here is no natural source for calibrating the respouse to electrorrragnetic arid liadronic showers. To provide this, a W-plane lrnx lm version of t.he MINOS detector has been running in test beams at CERN, Already, the energy resolution on Eh:I showers has been demonstrated with this data. 2.3.

Physics

Capabilities

and

Outlook

As mentioned, the primary goal for MINOS is to precisely measure the parameters for u,~ -5 11~ oscillaGor1s (assuming this is the rnec-hanism behind the observed atmospheric vih deficits). Initial rtuming will be with the Ion; erwrgy- beam configmation (see Fig, I), which, despite yielding a smaller flux, is best matched to the values of Ad favored by Super-K. Fig, 4 gives an idea of what our reconstructed neutrino energy spectrum might look like, relative to eqwtatiorm assuming no oscillations, after a two-year run. The oscillatory behavior is clear in all three cases, as exernplified by the bottom row of plots. The sensitivit>y to oscillation parameters, accounting for expected systematic uncertainties, is shown in Fig. 5. Overall the outlook for MIl”r’OS is quite good: by 2007, oscillation parameters for v,,. --f v, will bP measwed with good precision. In subsequent

CC energy

distributions

- Ph2le,

IO kt.yr.,

sin*(M)=0.9

Figure 4. Top: expected CC I//~ interaction yields after two years of d&a,-taking wit.11 h~IINOS in the rase of no oscillations (solid histograms), and in the case of oscillations (points) with sin’ 28 = 0.9 for three differerit values of Am”, a.s a. function of reconstructed rieut,rino energy. IXottom: ratios of measured’ (oscillated) to ‘expected (not oscillated) yields, as given by the top plots,

running, horn currer&s can be reversed to take data, with anti-neutrinos, allowing seas&es for possible violations of CPT symmetry. Also by 2007, the search for v,? appearance will be able to extend the reach in sin” 2813 relative to the sensit.ivitp of CMOOZ [12] by a fact,or of 2 to 4, depending on &r-l”. In addition, we will by then have had N 20 kton-yrs of exposure to atrnospheric IJ’S. ‘This will provide additional constraints on oscillation parameter measurements, and an early look at CPT asymmetries will be possible thanks to the magnetic field. Finally, unique opportunities exist for conventional neutrino physics with the MIiYOS Near Det.ecto,r and cosmic ray physics with the Fa’asDetector. 3. The

CNGS

Project

The CERN to Gran Sasso project also aims t.o produce an intense source of neut.rinos? in this case using protons from the 400~GeV CERPI’ SF’S, The beam is directed toward the Gran Sasso Laboratory (LKGS), 732 km to the southeast in Italy. Two experiments will use this beam to search for T-lepton signatures from VJA+ vT oscillations and

Stutus

PhZle,

of the MINOS

10 kt. yf.,

90%

experiment

and reuiew

C.L.

Figure 5. MINOS SO’%CL intervals for oscillation parameters after two years of data-taking ai full beam int.ensity for three pxarneter sets. Overlaid is the 90% CL intervaJ from the 1144-day SuperK atmospheric v measurements. (Note that the constraints reported by Super-K at this conference [l] are more restrictive than shown here.)

for u, appearance from the subdominant oscillation mode. The OPERA exper&ent is an qproved CNGS experiment based cm photographic emulsions, to be located in Hall C. The ICARUS c:c.)llabura.tit.)nhas been approved by LNGS to drploy detect.or modules in Hall D. Excellent progress has been made on the CPIITGS beam line. The t.arget chamber a,nd decay turmel have been excavated, and the civil construction is exprt*trd to be complete by May 2003, The present s&&he has the SPS delivering 4.5 X 10lg protons per year to the CNGS t,arget beginning in 2006. Ideas exist for increasiug the proton flux by 30 - 50% with minimal investments, The mean neutdno energy of the CNGS beam is 17 GeV, well above the location of the first oscillation peak for even the most favorable values of A7r?. Howwer, t
of long-baseline

neutrino

oscillation

experiments

41

of dete&or mass-exposwe at nomirid int,ensity, along with 3660 NC and vcz,CC intwactions;, Space constraints allow only brief descriptions of the 0PER.A alld ICARUS experimems, given below. For more information on these experiments, as well as on the COGS project, the reader is referred to the presentation by S, Katsanevas at the PITeutrino 2002 conference [13]. The OPERA dettet:t,or consists of‘ Pb/emulsion “bricks’, stacked into 62 iwallsY, with 52x64 bricla per wall, for a total target IXW of 1.8 kt,on. Each brick contains 56 emulsion films deposited on thin plastic bases, separated by Pb sheets 1 mm thick.. i’v1InTCX%likeplastic scintillator planes are positioned between the walls to help locate neutrino interac%ioii wrtiees in the emulsion. The ICAR.US detector is a. liy.-Argon TPC?, providing bubble-chamber-like imaging of particle trajectories with sub-rnm resolutions in three dimensions. The ICARIX collaboration has recently tested a 300~ton prototype. A &M-ton wrsion is beirlg installed in LlYGS (completion expected in summer 2003). The fill1 proposal calls for a 3-kton detector to be installed by 2L106. With the currently favored oscilla~tion parameters, the t.wo detectors should see of order 25 T events in five years of ruuning, with negligible backgrounds. The prospects for v, a.ppearance are eqwiadly interesting: sensitivities to ST, around 5 x 10-” aIe expected. 4. Ideas for Future

Efforts

With the recent spate of exciting results On solar mtld atmospheric v’s, t.hr topic: of neutrino mass and mixing has become an areil. when experiments have a chance to really see something new. MlX,rOS and the COGS experiments will contribute significantly to this field, but key yuestions will remain - particulwlg on t.he possibility uf CP violation due to neutrino flavor mixing. It is not too early to prepare for the nest%generation of experiments to explore these questions. 4.1.

Use of Existing

Facilities

One possible step is to extend the capabilities of existing neutrino beams and/or detectors. The CNGS and NuMI beams are obvious re-

sources, and ideas to use them for dedicated z/, appeararwr experiments a,re being advanced. In both cases, new detectors, optimized for electron ID and rejection of NC backgrounds, would be sited between 0.5” and 2” off-axis relative to the beam eenterliire (this is also the plan for JHFKamioka [ll]) By going off-axis, one gains an a,dvantage due to kii~ematics: the resultant 11energy spectrum is domin&ed by a Jwobean peak, effectively giving a. rrarrow=barrd beam Fur-therrnore, U’S with energies above the peak are naturally suppressed at these angles, thereby reducing otherwise severe NC ba&grounds coming from higher energy v’s, A 20-kt;orr off-ads KuMI detector [13] could be located on the surface either in Minnesota neu the Soudan mine, or fUrther north along the trzns-Canada highway (where the bmrn cmterline is werhead a.t N 15 km). The sites in Miimesota are att,ract,ive because of existing infrastructure at Souda.n, while the Canada sites benefit from t,he longer baseline (- 950 km), allowing ma$t.er effects to become mow prominent. Detector technologies being considered inelude RPf:‘s, liquid scirrt,illator with fiber readout7 water Cherenkoy, and ICARUS-like liquidargon TPC’s. The prc>jccted sertsit.ivitp to S& is at the level of 1.5 x IO-“. The idea, being floated for a new CNGS experiment involves an off-shore w&er Cherenkov detector in the Gulf of Taranto [15], with a baseline of 1200 km. The CNGS beam would be tuned to a lower energy7 so tha‘t for a detector Z0 off axis the mean neutrino energy would be 0.8 GeV - at the ~Wdi of the sectmcl oscillation maximum. 4.2. New Facilities

The cor1st.ruction of new neut.rino beams for long-baseline experimerlts is also under consideration. Most of these ideas make use of existing accek37tors or components to capitalize on infrastructure and keep costs low. One possibility [lG] being explored is that, of upgrading the BNL AGS to a beam power of 0.5 - 1.3 MW? and building one or more neutrinu beam lines directed t.oward sites, such as Soudan, Home&&e, or t.he Waste Isolation Pilot Plant (WIPP) in New h,Iexiro, where multi-function WI-

derground detectors could .be built. In Europe, several int~westing ideas involve beams directed from CERN to the Frejus tunnel. One involves a new 2- GeV supercorrductirg proton linac, based on LEP RF cavities, which would be used to generate a neutrino beam of 300 hielf, appropriate for t,he 13Okm baseline. Another noye idea. is that of ‘beta beams’ in which Us and g7,,beams are generated from a~ccelerated ‘?Xe and “He ions which decay in fliiht. Finally I ha~e not mentioned projects that might be realized on a. longer t.irnescale, such a.s neutrino fa&ories from muon &ors.ge rings [17]. 5, sllnmnary The summary is that. we% at, a very exciting stage in the study of neutrino mass and mixing. Bring cm the neutrino beams! 6. Acknowledgements

I thank S, I~atsanevus for providing me with information and figures on t.hr COGS program. REFERENCES 1. 2. 3, 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17.

C. hhger, these proceedings. S. Oser, these proceedings. h4.. T’q$ns? these proceedings, U. Hay&o, these proceedings. T. Mitsui, these proceedings. A. Bazarko, these proceedings. C. Giunti, these proceedings. D. Wark, these proceedings. C, Gonz&z-Garcia., these proceedings. Z. Xing, these proceedings. Y. Itow, these proceedings. M. Apollonio et al. (CH00Z Collab.), Phys. Lett. B 466 (.1999) 415. Mt~g://neut.rir~o2O0;!.pli.turri.tle A letter of intent was subrnit,ted recently to the Fermilab PAC: ht,tp://wwwriurrii.fria.l.~o~~/frial-rr~rios/riewiriiti;ttives/ See the expression of interest posted at: lit~t.t~>://lir.~iiie.cer-ri,cli/dyd~foscexp~~~s D. Beavis et al. (BNL Neutrino Workiag: Group), hep-ex/o205040, (2002). G. Hanson, these yroceedirrgs.