The project of a large underground experiment (NOE) devoted to long baseline
neutrino oscillation measurement is presentedComment: 7 pages, 9 figures, Talk at 5th International Workshop on Tau Lepton
  Physics (TAU98), To be published in Nucl. Phys. B, Proc. Supp

Allison

Ambrosio

Apollonio

Barbarino

E. Scapparone

Fukuda

Osteria

English

arXiv

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Nuclear Physics B - Proceedings Supplements

NOE: a neutrino experiment for the CERN-Gran Sasso long base line project

The project of a large underground experiment (NOE) devoted to long baseline neutrino oscillation measurement is presente

Scapparone, E

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1NOE: a neutrino experiment for the CERN-Gran Sasso Long Base line projectE. Scapparone for the NOE CollaborationLaboratori Nazionali del Gran SassoS.S.17 km 18+910, 61070, Assergi (AQ), ItalyTalk at 5th International Workshop on Tau Lepton Physics (TAU98)Santander, Spain, September 14 - 17, 1998To be published in Nucl. Phys. B, Proc. Suppl( writeups available at http: //www.ifca.unican.es/tau98/index writeups.html)2NOE: a neutrino experiment for the CERN-Gran Sasso Long Base lineprojectE. Scapparonea for the NOE CollaborationaINFN, LNGS S.S.17 km 18+910, 61070, Assergi (AQ), ItalyThe project of a large underground experiment (NOE) devoted to long baseline neutrino oscillation measure-ment is presented. The apparatus, consisting of TRD and calorimeter modules, has been optimized to be sensitivein the region of sin2 2θ and ∆m2 suggested by the atmospheric neutrino oscillation signal.1. IntroductionThe scientic goal of the NOE long baseline(LBL) experiment is the measurement of neutrinomasses looking at νµ ! νe and νµ ! ντ oscilla-tions. The philosophy of NOE design is to haveoscillation sensitivity by looking for the τ decay(νµ ! ντ oscillation) or for an electron excess(νµ ! νe oscillation) and by measuring a decitof muons in apparent NC/CC ratio.The main experimental hint for the ν oscilla-tion search in the region of low m2 (10−2 10−3 eV 2) comes from the muon decit observedin atmospheric neutrino flux measurements [6{8].Recent results from LBL reactor neutrino exper-iment (CHOOZ) excluded neutrino oscillation inνe disappearance mode, up to sin2 2θ > 0.18 forlarge m2 [9].Taking into account the conrmations of theatmospheric neutrino anomaly and the negativeCHOOZ result, a LBL experiment has to fulllthe following requirements:1. ντ tagging. The search for ντ appear-ance is mandatory to conrm the oscillationphenomenon. This search requires detectorhigh performances to tag the τ decay.2. Measurement of the ratio NC/CC.This robust and unambiguos test is impor-tant to investigate on the existence of a neu-trino oscillation signal. There is no doubtthat this measurement can be done onlywith a massive detector.∗see [5] for the complete list of the Collaboration3. Atmospheric neutrinos. After the lastresults from Superkamiokande, suggestingsmaller values of m2, the interest for theatmospheric neutrinos is raised up. It wouldbe interesting to test this eect using a mas-sive apparatus based on a dierent tech-nique with respect to the water Cerenkovdetectors.4. Fast response. If a beam from CERNto Gran Sasso will be available in the nextyears, a strong competition with Americanand Japanese LBL programs is foreseen: atpresent, the 7 kton NOE project can ade-quately compete with the 8 kton MINOSdetector and with K2K.According to these remarks the NOE programcan be summarized in this way: Direct ντ appearance by kinematical τ de-cay reconstruction and inclusive (NC/CC)νµ disappearance. Investigation of νµ ! νe oscillation in amixing angle region two orders of magni-tude beyond the CHOOZ limit. Atmospheric neutrino studies.In order to improve the ντ search, the appara-tus has been equipped with Transition RadiationDetector (TRD) interleaved between calorimetricmodules (CAL). The combination of TRD andCAL information strongly enforces e, µ, and piidentication, thus permitting the study of the3τ decay. In particular the τ appearence can bedetected through the τ ! eνν channel, with aclean signature, taking advantage of the low back-ground (residual νe beam). Moreover the goodelectron identication in the TRD and the low piobackground allow to reach high sin2 2θ sensitivitylooking for νµ ! νe oscillation, thus considerablyenlarging the region investigated by CHOOZ.2. The NOE detectorThe detector (Fig. 1) is a typical xed tar-get apparatus consisting of a sequence of 12 ba-sic modules(BM). Each BM is composed by alighter part (TRD target) in which vertex, e-identication and kinematics are dened, followedby a scintillating ber calorimeter devoted to en-ergy measurement and event containement. TheBasic Module (BM) is shown in Fig. 2. Appear-ance measurement is performed using events gen-erated in TRD (2.4 kton), while disappearancemeasurements are performed by looking at eventsgenerated both in the TRD and in the calorimeter(total mass  7 kton).The calorimetric element is an 8 meter long barlled with iron ore and scintillating bers2. Thecalorimetric module is made by alternate planesof crossed bars. The calorimetric bar consists ofmore logical cells with square cross section. Theiron ore is radiopure and practically cost free.A lot of R&D has been performed to improvethe scintillating bers performance. The presentproduction of 2 mm diameter scintillating bersprovide an attenuation length λ=4.5 m and anlight yield L  20 pe/MeV. These gures allow tobuild 8 m long bars. Further investigations to im-prove ber features are in progress: longer berswould allow to increase the NOE cross section(9 9 m2) and therefore the total mass (8 kton).It is worth noting the very high intrinsic granu-larity of the proposed calorimeter : the averagedistance between the bers inside the absorber isof the order of 3 mm. The bers are groupedtogether at each side of the calorimetric bar andsent to single or multipixel photodetector.The energy resolutions for electrons and2Extruded scintillator strips with wavelength shifter fibershave been also studied.Figure 1. Schematic view of the NOE detectorFigure 2. The Basic Module of the NOE detectorhadrons in the calorimeter have been evaluatedby means of a GEANT Montecarlo simulation.They are, respectively, σ(E)/E = 0.01+0.17/pEand σ(E)/E = 0.08 + 0.42/pE. The TRD mod-ule consists of 32 vertical layers of 8  8 m2area, each made by polyethylene foam radiator(ρ  100 mg/cm3) and 256 proportional tubes(33 cm2 cross section), lled with an Ar (60%)- Xe (30%) - CO2 (10%) mixture, already testedin MACRO experiment. Consecutive layers havetubes rotated of 90.A graphite wall of 5 cm thickness is set in front4of each of the rst 24 layers of the TRD moduleacting as a 174 ton target for νe and ντ interac-tions, to be identied in the following layers. Thelast target wall is followed by 8 TRD layers inorder to identify the secondary particles. Eachtarget wall corresponds to 0.25 X0 while the en-tire TRD basic module corresponds to about 7 X0and 3.5 λI . The total length is about 3.76 m.So many layers of proportional tubes permit todetermine the muon energy by means of multiplemeasurements of energy loss dE/dx. Combiningthe informations coming from both subdetectors(TRD and CAL) the discrimination between e,µ and pi is largely enforced allowing the study ofseveral neutrino oscillation channels.3. ντ appearance and requirements aboutν beamThe rate of ντ CC events is given byRτ = A∫στPoscdE, (1)where E is the energy, στ the ντ CC cross section,Posc the oscillation probability,  the muon neu-trino flux and A the number of target nucleons inthe detector. The search for ντ requires that theterm στPosc is large. Therefore a dedicated ν-beam has to provide most of its flux in the energyrange where the factor στPosc is larger.Assuming the mixing of two neutrinos, the os-cillation probability isPosc = sin2 2θ sin2(1.27m2L/E), (2)where L = 731 km is the distance CERN - GranSasso. We have to take into account that the τcross section grows slowly with energy above athreshold of about 3.5 GeV . The factor στPoscis shown in Fig. 3 for dierent values of m2.The optimal energy is about 15 GeV for m2 =0.01 eV 2 and decreases gently with m2 towardsa limiting value of about 10 GeV .In the following, the beam from Ref. [10] and 5years of data taking are assumed.4. τ appearance searchesTau appearance search is performed on the ba-sis of kinematical identication of the τ decay.00.510 10 20 30 40 50 00.20.40.60 10 20 30 40 5000.10.20.30 10 20 30 40 50 00.050.10 10 20 30 40 5000.0050.010.0150 10 20 30 40 50∆m2=10-2Energy (GeV)P*σ (10-38 cm2 )∆m2=7.2 10-3Energy (GeV)P*σ (10-38 cm2 )∆m2=5 10-3Energy (GeV)P*σ (10-38 cm2 )∆m2=3 10-3Energy (GeV)P*σ (10-38 cm2 )∆m2=10-3Energy (GeV)P*σ (10-38 cm2 )∆m2=5 10-4Energy (GeV)P*σ (10-38 cm2 )00 . 0 0 10 . 0 0 20 . 0 0 30 . 0 0 40 10 20 30 40 50Figure 3. ντ CC cross section multiplied by theoscillation probability for different values of m2.Ball et al.Apollonio et al.NGSGeVEvents/1019  pot/kton0510152025303540450 5 10 15 20 25 30 35 40 45 50Figure 4. it νµ CC interaction spectra for dier-ent proposed beams.5The τ ! eνν is the favourite channel for thissearch due to the low background level and thethe good electron identication capability of theTRD. It is worth noting that in the region of at-mospheric anomaly the oscillation probability is50 100 times higher than expected in NOMAD,as a consequence a much lower background rejec-tion power is required.In order to check the overall NOE perfor-mances, a complete chain of event simulation andanalysis has been performed. Event generatorsthat include Fermi motion, τ polarization andnuclear rescattering inside the nucleus have beenused to simulate quasi elastic, resonance and deepinelastic events.Generated events are processed by a GEANTbased MonteCarlo in which calorimeter and TRDgeometrical set-up are described in detail, downto a scale of a few mm. Fiber attenuation length,Birks saturation, photoelectron fluctuations andreadout electronics non linearities for both TRDand calorimeter have been taken into account.DST of processed events (τ ! eνν, νµ NC and νeCC) have been produced and analysed.Electron identication is performed by lookingfor high energy releases in the TRD and in thecalorimeter readout elements in fully containedevents. The electron candidate is the one thatmaximizes collected energy in a 5 cone centeredat the interaction vertex. Electron direction isreconstructed by weighting hits position by col-lected energy. With present algorithms an angu-lar resolution of 0.6 and a 180 MeV/c resolutionon the measurement of transverse momentum areachieved.The remaining part of the event is used to re-construct the hadronic component. The obtainedresolution on the measurement of transverse mo-mentum is 420 MeV/c.Topological cuts on the electromagnetic showerare applied to reject νµ NC events with pio fak-ing electrons. Fig.6 shows a tipical neutral cur-rent event with a pio: the electromagnetic showerdoesn’t start in the event vertex, allowing an easyrejection of the event. Work is in progress to im-prove the reconstruction eciency and νµ NC re-jection. Additional cuts are performed to reducethe background:0204060801001201401601800 20 40 60 80 100 120 140 160 1800204060801001201401601800 20 40 60 80 100 120 140 160 180Figure 5. Effects of φe−h φm−h cut on background(νe) and signal (ντ ). total reconstructed energy < 15 GeV , electron energy > 1.5 GeV , the component of electron momentum per-pendicular to the hadronic jet directionQlep > 0.75 GeV/c, transverse massMT =√4peT pmT sin2(φe−m/2) < 2 GeV , φe−h φm−h correlation as shown in Fig. 5.Resulting eciencies and residual backgroundsare reported in Table 1.5. Neural network for the ratio NC/CCThe measurement of the NC/CC ratio is per-formed looking at the observable Robs= (no µ)/µ,where (no µ) is the number of events not showinga muon track while µ is the number of events witha reconstructed muon. The procedure to perform6Figure 6. The event display of a typical NC eventwith a pio. The hits on the left of the vertical linebelong to the TRD, those on the right belong tothe Calorimeter.this measurement is well known [5]. The oscilla-tion probability can be written as:P =Robs −Rth − (Rth + 1)Robs(1 − ηB) + η(1−B) (3)where  is the ratio of νe to νµ in the beam,B is the branching ratio for τ decay in muon,Rth=σNC/σCC and η=στ/σµ. The last ratio hasto be carefully evaluated. For istance, consider-ing the NGS beam and m2 ’10−2-10−3 eV 2,η ranges between 0.25 and 0.32. In this mea-surement, the systematics associated with Rth,play of course an important role and can be inprinciple reduced by using a near detector. Nev-ertheless, near and far beams are not identical,introducing a possible systematic error, requir-ing a detailed study. From an experimental pointof view the separation of (no µ) events from µevents is usually supposed to be straightforward.On the contrary, this measurements become dif-þ event containementCut Res. NC Res. n e beam  eff. t  fi e n n1 79% 67% 79%þ high released energy track in two views: Ey = EzCut Res. NC Res. n e beam  eff. t  fi e n n2 19% 88% 91%   þ compact shower, starting from the vertex, topology.( p 0 without initial conversion)Cut Res. NC Res. n e beam  eff. t  fi e n n3 11% 88% 89%þ ETot < 15 GeVCut Res. NC Res. n e beam  eff. t  fi e n n4 94% 45% 97%þ Ee.m. ‡  1.5 GeVCut Res. NC Res. n e beam  eff. t  fi e n n5 55% 90% 84%þ QLep ‡  0.75 GeV/cCut Res. NC Res. n e beam  eff. t  fi e n n6 25% 75% 70%þ MT t  < 2 GeVCut Res. NC Res. n e beam  eff. t  fi e n n7 57% 89% 87%þ f e,h f ptm,h  correlationCut Res. NC Res. n e beam  eff. t  fi e n n8 35% 20% 70%  þ for 1.5x1020 pot and 14000 nmCC (TRD) 5 years : D m2= 5x10-3 eV2   2.7 n eCC + 1.6NC = 4.3 evs.   19 t  evs. D m2= 3x10-3 eV2   2.7 n eCC + 1.9NC = 4.6 evs.   7 t   evs. no oscillation   2.7 n eCC + 2.0NC = 4.7 evs.   0 t   evs.Table 1Details of analysis cuts, efficiency and rejectionpower for main background sources.cult at low energies, having the muons a shorterpath length. In NOE analysis the identication ofcharged and neutral current events has been im-proved by means of a neural network. The algo-rithm uses 24 topological, geometrical and calori-metric event parameters as input. The networkhas been trained with νµ CC and NC MonteCarloevents with a energy uniformly distributed in therange 0  50 GeV . The results of this analysisare shown in Fig. 7. The µ event recognition ismainly dependent on the track length. For neu-trino energy Eν5GeV a good selection/rejectioneciency is obtained. As expected, the eciencydecreases with the energy, becoming the muontrack shorter.7charged current efficiencyneutral current contaminationneutrino energy (GeV)percentage00.10.20.30.40.50.60.70.80.910 10 20 30 40 50neutral current efficiencycharged current contaminationneutrino energy (GeV)percentage00.10.20.30.40.50.60.70.80.910 10 20 30 40 50Figure 7. CC and NC signal. The contaminationis also shown.We are presently working to improve NC andCC separation at low energy. This measurement( Fig. 8) allows a search for νµ! ντ oscillationdown to m2=1.10−3eV 2.6. ConclusionsThe combined use of two subdetectors (TRDand CAL) allows to search for τ appearance signalfor events generated in the 2.4 kton TRD targetwhere electron identication, vertex and kinemat-ics reconstruction are performed at best. Never-theless the whole 8 kton mass can be exploitedfor disappearance oscillation tests.Such measurements can be carried out at thesame time by using an appropriate neutrinobeam. A full analysis has recently been per-formed to demonstrate the feasibility of bothmeasurements. In Fig. 8 the sensitivity of NOEexperiment to ν oscillations is shown.REFERENCES1. G. Barbarino et al., INFN/AE-96/1110-510-410-310-210-111010 -3 10 -2 10 -1 1 νµ → νe                      νµ → ντsin22θ∆m2  (eV2 )sin22θ∆m2  (eV2 )10-510-410-310-210-111010 -3 10 -2 10 -1 1Figure 8. NOE sensitivity to ν oscillations.2. G. Barbarino et al., Nucl. Phys. B (Proc.Suppl.) 48 (1996) 2043. G. Osteria et al., Proceedings of the 7thPisa Meeting on Advanced Detectors, Isolad’ Elba, (1997)4. G. Barbarino et al., Proceedings of TAUP97,Gran Sasso Laboratory, (1997) 2235. G. Barbarino et al., INFN/AE-98/096. W.W.M. Allison et al. (Soudan Collabora-tion), Phys. Lett. B 391 (1997) 4917. M. Ambrosio et al. (MACRO Collaboration),hep-ex/98070058. Y. Fukuda et al. (SuperKamiokande Collabo-ration), hep-ex/98070039. Chooz Collaboration, Phys. Lett. B 420(1998) 39710. A.E. Ball, S. Katsanevas and V. Vassilopouls,"Design Studies for a Long Baseline Neutrinobeam" CERN/ECP 95-1311. K. Elsener et al. "The CERN neutrino beamto Gran Sasso (NGS) - Conceptual technicaldesign", CERN 98/02 and INFN/AE-98/0512. M. Apollonio et al., Addendum to the Ex-pression of Interest to the Gran Sasso Scien-tic Committee "Studies for a long baselineaccelerator and atmospheric neutrino oscilla-tion experiment at Gran Sasso" (1998)

NOE: a neutrino experiment for the CERN-Gran Sasso Long Base line project

NOE: a neutrino experiment for the CERN-Gran Sasso Long Base line
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NOE: a neutrino experiment for the CERN-Gran Sasso Long Base line project

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