source: https://doi.org/10.7892/boris.82793 | downloaded: 12.6.2021

The OPERA experiment

N. Agafonovaa, A. Aleksandrovj, A. Anokhinab, S. Aokic, A. Arigad, T. Arigad, D. Bendere, A. Bertolinf, C. Bozzag,R. Brugneraf,h, A. Buonaurai,j, S. Buontempoj, B. Büttnerk, M. Chernyavskyl, A. Chukanovm, L. Consiglioj,N. D’Ambrosion, G. De Lellisi,j, M. De Serioo,p, P. Del Amo Sanchezq, A. Di Crescenzoj, D. Di Ferdinandor,

N. Di Marcon, S. Dmitrievskim, M. Dracoss, D. Duchesneauq, S. Dusinif, T. Dzhatdoevb, J. Ebertk, A. Ereditatod,R. A. Finip, T. Fukudat, G. Galatii,j, A. Garfagninif,h, G. Giacomellir,u,1, C. Göllnitzk, J. Goldbergv, Y. Gornushkinm,G. Grellag, M. Gulere, C. Gustavinow, C. Hagnerk, T. Harac, A. Hollnagelk, B. Hosseinii,j, H. Ishidat, K. Ishigurox,K. Jakovcicy, C. Jollets, C. Kamiscioglue, M. Kamiscioglue, J. Kawadad, J. H. Kimz, S. H. Kimz,2, N. Kitagawax,B. Kliceky, K. Kodamaaa, M. Komatsux, U. Kosef, I. Kreslod, A. Lauriai,j, J. Lenkeitk, A. Ljubicicy, A. Longhinab,

P. Loverrew,1, A. Malgina, M. Malenicay, G. Mandriolir, T. Matsuot, V. Matveeva, N. Maurir,u, E. Medinacelif,h,A. Meregaglias, S. Mikadoad, P. Monacelliw, M. C. Montesii,j, K. Morishimax, M. T. Muciacciao,p, N. Naganawax,

T. Nakax, M. Nakamurax, T. Nakanox, Y. Nakatsukax, K. Niwax, S. Ogawat, N. Okateval, A. Olshevskym, T. Omurax,K. Ozakic, A. Paoloniab, B. D. Parkz,3, I. G. Parkz, L. Pasqualinir,u, A. Pastorep, L. Patriziir, H. Pessardq, C. Pistillod,

D. Podgrudkovb, N. Polukhinal, M. Pozzator,u, F. Pupillin, M. Rodaf,h, H. Rokujox, T. Roganovab, G. Rosaw,ac,O. Ryazhskayaa, O. Satox, A. Schembrin, I. Shakiryanovaa, T. Shchedrinaj, A. Sheshukovm, H. Shibuyat,T. Shiraishix, G. Shoziyoevb, S. Simoneo,p, M. Siolir,u, C. Sirignanof,h, G. Sirrir, M. Spinettiab, L. Stancof,

N. Starkovl, S. M. Stellaccig, M. Stipcevicy, T. Straussd, P. Strolini,j, S. Takahashic, M. Tentir, F. Terranovaab,ae,V. Tioukovj, S. Tufanlid, P. Vilainaf, M. Vladimirovl, L. Votanoab, J. L. Vuilleumierd, G. Wilquetaf, B. Wonsakk,

C. S. Yoonz, S. Zemskovam, A. Zghicheq

aINR - Institute for Nuclear Research of the Russian Academy of Sciences, RUS-117312 Moscow, RussiabSINP MSU - Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, RUS-119992 Moscow, Russia

cKobe University, J-657-8501 Kobe, JapandAlbert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics (LHEP), University of Bern, CH-3012 Bern, Switzerland

eMETU - Middle East Technical University, TR-06531 Ankara, TurkeyfINFN Sezione di Padova, I-35131 Padova, Italy

gDipartimento di Fisica dell’Università di Salerno and “Gruppo Collegato” INFN, I-84084 Fisciano (Salerno), ItalyhDipartimento di Fisica e Astronomia dell’Università di Padova, I-35131 Padova, Italy

iDipartimento di Fisica dell’Università Federico II di Napoli, I-80125 Napoli, ItalyjINFN Sezione di Napoli, 80125 Napoli, Italy

kHamburg University, D-22761 Hamburg, GermanylLPI-Lebedev Physical Institute of the Russian Academy of Sciences, RUS-119991 Moscow, Russia

mJINR - Joint Institute for Nuclear Research, RUS-141980 Dubna, RussianINFN - Laboratori Nazionali del Gran Sasso, I-67010 Assergi (L’Aquila), Italy

oDipartimento di Fisica dell’Università di Bari, I-70126 Bari, ItalypINFN Sezione di Bari, I-70126 Bari, Italy

qLAPP, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, FrancerINFN Sezione di Bologna, I-40127 Bologna, Italy

sIPHC, Université de Strasbourg, CNRS/IN2P3, F-67037 Strasbourg, FrancetToho University, J-274-8510 Funabashi, Japan

uDipartimento di Fisica e Astronomia dell’Università di Bologna, I-40127 Bologna, ItalyvDepartment of Physics, Technion, IL-32000 Haifa, Israel

wINFN Sezione di Roma, I-00185 Roma, ItalyxNagoya University, J-464-8602 Nagoya, Japan

yIRB - Rudjer Boskovic Institute, HR-10002 Zagreb, CroatiazGyeongsang National University, 900 Gazwa-dong, Jinju 660-701, Korea

aaAichi University of Education, J-448-8542 Kariya (Aichi-Ken), JapanabINFN - Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati (Roma), Italy

acDipartimento di Fisica dell’Università di Roma “La Sapienza”, I-00185 Roma, ItalyadNihon University, J-275-8576 Narashino, Chiba, Japan

aeDipartimento di Fisica dell’Università di Milano-Bicocca, I-20126 Milano, ItalyafIIHE, Université Libre de Bruxelles, B-1050 Brussels, Belgium

Available online at www.sciencedirect.com

Nuclear and Particle Physics Proceedings 267–269 (2015) 87–93

2405-6014/© 2015 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/nppp

http://dx.doi.org/10.1016/j.nuclphysbps.2015.10.087

http://www.elsevier.com/locate/nppphttp://dx.doi.org/10.1016/j.nuclphysbps.2015.10.087http://dx.doi.org/10.1016/j.nuclphysbps.2015.10.087http://www.sciencedirect.com

Abstract

The OPERA experiment was designed to study νμ → ντoscillations in appearance mode using the CERN toGran Sasso high energy neutrino beam. From 2008 to2012, 19505 CNGS neutrino interactions were recordedin the OPERA detector. At the present status of theanalysis, 4 ντ candidate events have been observed, es-tablishing the oscillation mechanism in the atmosphericsector with a significance of 4.2 σ. The oscillation anal-ysis will be presented in detail and the candidate eventswill be described. The final measurement of the atmo-spheric muon charge ratio in the TeV region will be alsoreported.

1. Introduction

Over the past decades, strong evidence in favor of thehypothesis of neutrino oscillations has been provided byseveral experiments. In particular, the mixing of neu-trino flavor states in the atmospheric sector has beenmainly investigated in disappearance mode [1, 2, 3, 4].The Oscillation Project with Emulsion-tRacking Appa-ratus (OPERA) [5] is a long baseline neutrino experi-ment designed to provide the first measurement of νμ →ντ oscillations at the atmospheric scale in direct appear-ance mode, i.e. through the detection of the τ leptonsproduced in charged current (CC) interactions of oscil-lated ντ’s. The detector is located in the undergroundGran Sasso Laboratory (LNGS, Italy) and was exposedto the CERN Neutrinos to Gran Sasso (CNGS) [6] νμbeam from 2008 to 2012 collecting an integrated inten-sity of 17.97 × 1019 protons on target. Given the meanbeam energy of 17 GeV and the baseline of 730 km, theaverage L /E ratio of 43 km / GeV is adequate for neu-trino oscillation studies at the atmospheric scale.

2. The OPERA experiment

2.1. Detector

The OPERA detector [7] consists of two identicalsuper-modules as shown in Fig. 1. Each super-modulehas a target section followed by a muon spectrome-ter. Each target section is arranged in 31 vertical walls,filled with lead/emulsion bricks and followed by pairsof tracker planes made of plastic scintillator strips (Tar-get Tracker, TT). The overall target mass is about 1.2 kt.

1Deceased2Now at Kyungpook National University, Daegu, Korea.3Now at Samsung Changwon Hospital, SKKU, Changwon, Korea.

The brick is the basic target unit and consists of 57 nu-clear emulsion films, interleaved with 1 mm thick leadplates, with a total volume of (7.9 × 10.2 × 12.8) cm3and a mass of 8.3 kg. OPERA emulsion films are madeof two 45 μm-thick sensitive layers deposited on eachside of a 205 μm plastic base. On the downstream faceof the brick, a pair of additional emulsion films in a sep-arate envelope (Changeable Sheets, CS [8]) acts as aninterface between the brick and the TT. The TT iden-tifies the brick where the neutrino interaction occurred.The brick is promptly removed from the target and theemulsion films are developed and analysed by means ofautomated high-speed scanning systems [9, 10]. Eachspectrometer consists of a dipolar magnet instrumentedwith planes of Resistive Plate Chambers (RPC) and drifttube detectors in order to identify muons and measuretheir momentum and charge. The charge sign misiden-tification probability is about 0.3% up to 50 GeV/c; themomentum resolution is about 20% in the same kine-matical range. A veto system, consisting of planes ofglass RPCs, is placed in front of the first super-modulein order to tag the interactions occurring in the upstreamrock.

2.2. ντ detection signature

The challenge of OPERA is the separation, on anevent-by-event basis, of CC interactions induced by os-cillated ντ’s from dominant νμ interactions.

The experimental signature consists in the detectionof the τ lepton decaying in one prong (muon, singlehadron or electron) or in three hadrons over typical dis-tances of a few hundred microns.

The use of nuclear emulsion films, acting as sub-micrometric tracking devices, allows resolving the neu-trino interaction and the τ lepton decay vertices with ad-equate background rejection power. Moreover, the com-bined use of nuclear emulsions and high-density ma-terial plates makes it possible to measure particle mo-menta by multiple Coulomb scattering (MCS) [11] andreconstruct electromagnetic showers initiated by γ’s andπ0’s.

3. Physics results

Table 1 shows the details of the five CNGS runs. Thetotal number of neutrino interactions recorded in theOPERA detector is 19505. For runs 2008 and 2009,all predicted events have been searched for in the twobricks with the highest probability of containing theneutrino interaction. Only the most probable brick hasbeen analysed so far for runs 2010 − 2012. In the

N. Agafonova et al. / Nuclear and Particle Physics Proceedings 267–269 (2015) 87–9388

Figure 1: The OPERA detector in the hall C of Gran Sasso Laboratory.

Table 1: Details of the CNGS runs: beam intensity, efficiency and run duration are reported, as well as the number of recorded neutrino interactionsin OPERA.

Run 2008 2009 2010 2011 2012 Totalp.o.t (× 1019) 1.7 3.53 4.09 4.75 3.86 17.97SPS efficiency 61% 73% 80% 79% 82% 77%Beam days 123 155 187 243 257 965ν interactions 1931 4005 4515 5131 3923 19505

Table 2: Numbers of events used for the present νμ → ντ oscillation analysis.Run 2008 2009 2010 2011 2012 Total0μ events 148 250 209 223 149 9791μ events (pμ ≤ 15 GeV/c) 534 1019 814 749 590 3706Total number of events 682 1269 1023 972 739 4685

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6

71

4

53

2 8 daughter

CS

γ1γ2

2mm

10mm 2000 μm

p0

d

d

d

d

2

1

3−1

3−2

22 44

τV

V10

2V

Figure 2: Sketch of the first (left) and second (right) ντ candidate events detected by OPERA.

present analysis, the whole sample of events withouta reconstructed muon in the electronic detectors (0 μ)have been considered. A cut on the muon momentum(pμ < 15 GeV/c) has been applied for 1 μ events to im-prove the signal to noise ratio. Details on the datasetare summarised in Table 2. The total number of fullyanalysed neutrino interactions is 4685.

3.1. Oscillation analysis

In the data sample shown in Table 2, four ντ candidateevents have been observed [12, 13, 14, 15].

The first OPERA ντ candidate is shown in Fig. 2left. The primary neutrino interaction consists of seventracks. One of them exhibits a visible kink of 41±2mrad, after a path length of 1335±35 μm. Two elec-tromagnetic showers, initiated by γ-rays and associ-ated with the decay vertex, have been reconstructed.Their invariant mass is (120 ± 20 (stat.) ± 35(syst.))MeV/c2, supporting the hypothesis that they originatefrom a π0 decay. The observed decay topology and kine-matics are compatible with the tau lepton decay modeτ→ h(nπ0)ντ.

Figure 2 right shows a display of the second ντ can-didate event. The primary vertex consists of two tracksforming an angle of (167.8 ± 1.1)◦ in the beam trans-verse plane. In addition, a forward-going nuclear frag-ment has been detected. One of the two primary parti-cles (the τ lepton candidate) decays after 1466 ± 10 μmin three charged hadrons. One of the daughter tracksinteracts in the same brick containing the neutrino ver-tex, about 1.3 cm downstream (V2 in Fig. 2). The finalstate is composed of two charged tracks and four back-scattered nuclear fragments.

The third ντ candidate is depicted in Fig. 3 left. Theneutrino vertex is formed by two tracks: the τ leptonand a hadron. An electromagnetic shower produced by

a γ-ray and pointing to the primary vertex has also beenobserved. The τ lepton decays into a single prong ata distance of (376 ± 10) μm from the neutrino inter-action point. The daughter particle appears as an iso-lated, penetrating track traversing 24 planes of the TTand six RPC planes before stopping in the spectrometer.The track is reconstructed as a muon by the electronicdetectors with a momentum 2.8 ± 0.2 GeV/c, compat-ible with the measurement obtained by MCS in emul-sion of 3.1+0.9−0.5 GeV/c. The sign of its charge has beenmeasured to be negative by the bending of the track inthe magnetized iron, thus rejecting the hypothesis of aneutrino-induced charmed particle production with sub-sequent muonic decay.

Figure 3 right shows a display of the fourth candidateevent as reconstructed in the brick, in the view trans-verse to the neutrino direction. The primary vertex con-sists of four tracks. One of them (track 1) exhibits akink topology after 1090 μm. The measured kink an-gle is 137 mrad. The decay daughter track was anal-ysed in all traversed downstream bricks and was foundto stop in the spectrometer after leaving a signal in threeRPC planes. The track was identified as a hadron. Twoelectromagnetic showers initiated by the conversion ofγ’s and pointing to the neutrino vertex were also recon-structed.

The procedure developed to detect τ lepton decaysin OPERA is described in detail in [16] and has beenvalidated by studying its application to the search forcharmed particle production in a subsample of data col-lected in runs 2008-2010. In this sample, 50 νμ CC in-teractions with a charmed hadron in the final state wereobserved, while 54 ± 4 were expected.

Table 3 reports the estimated numbers of signal andbackground events for the data sample shown in Ta-ble 2, assuming Δm223 = 2.32 eV

2 and maximal mix-

N. Agafonova et al. / Nuclear and Particle Physics Proceedings 267–269 (2015) 87–9390

FILM

38

FILM

41

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57

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PLASTIC

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μ

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v2 1 parent

daughter

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γ1

200 μm

23

4

Figure 3: Sketch of the third (left) and fourth (right) ντ candidate events detected by OPERA.

ing. In total, four candidate events are detected while2.11 ± 0.42 events are expected with an estimated back-ground of 0.233 ± 0.041 events. The observation ofνμ → ντ oscillation in appearance mode is thus estabil-ished at 4.2σ level. The number of observed ντ eventsis also used to compute the 90 % confidence intervalfor Δm223. The estimated interval with the Feldman -Cousins method [17] is [1.8, 5.0] × 10−3 eV2.

In OPERA, the use of nuclear emulsion films astracking devices allows the identification of electronsproduced in νe CC interactions. Therefore, OPERAis suited to the investigation of νe appearance fromνμ → νe oscillations. A systematic search for νe in-teractions was applied to the full data sample collectedin 2008 and 2009, consisting of 505 0 μ neutrino events.The number of reconstructed νe candidate events is 19,in agreement with the expectation of 19.8±2.8 (syst).To increase the signal to background ratio, a cut on theneutrino energy can be applied. If only events with anenergy value lower than 20 GeV are selected, 4 eventsare detected while 4.6 are expected. The number of ob-served events is compatible with the non-oscillation hy-pothesis and yields an upper limit of sin2 2θ13 < 0.44(90% C.L.) [18]. The analysis is being extended to allruns and 40 νe candidate events have been observed sofar.

OPERA has also set an upper limit for non-standardνe appearance in the parameter space suggested by theLSND [19] and MiniBooNE [20] experiments. In theapproximation of one mass scale dominance, 9.4±1.3(syst) events with a neutrino energy lower than 30 GeVare expected at large mass squared difference, while 6events are found in the data. The corresponding exclu-

sion plot in the oscillation parameters θnew and Δm2new isshown in Fig.4. For large Δm2new values, the 90% C.L.upper limit on sin2 2θnew reaches 7.2 ×10−3. For com-parison, results from other experiments with differentL/E ranges are also reported in this figure.

3.2. Atmospheric muons

The underground Gran Sasso laboratory is a privi-leged location to study TeV-scale cosmic rays. Between2008 and 2012, OPERA collected more than 3 millionatmospheric muon events, out of them about 110000multiple muon bundles were detected. Profiting of theinversion of the magnet polarity that was performedon purpose during 2012 run, the atmospheric muoncharge ratio Rμ = Nμ+/Nμ− in the TeV region wasmeasured both for single and for multiple muon events.The combination of the two data sets collected withopposite magnet polarities allowed minimizing thesystematic uncertainties, thus reaching the most accu-rate measurement in the high energy region to date [21]:

Rμ (nμ = 1) = 1.377 ± 0.006(stat.)+0.007−0.001(syst.)

Rμ (nμ > 1) = 1.098 ± 0.023(stat.)+0.015−0.013(syst.)

As shown in Fig. 5, in the surface energy range be-tween 1 and 20 TeV investigated by OPERA, Rμ is welldescribed by a simple parametric model including onlypion and kaon contributions to the muon flux, show-ing no significant contribution of the prompt compo-nent. The observed behaviour supports the validity ofFeynman scaling in the fragmentation region up to 200TeV/nucleon primary energy.

N. Agafonova et al. / Nuclear and Particle Physics Proceedings 267–269 (2015) 87–93 91

Table 3: Estimated signal and background events for the analysed sample. Numbers are reported separately for each of the four τ decay channels.Observed events in each channel are also reported.

Decay Expected signal Total Observedchannel (Δm223 = 2.32 eV

2) background eventsτ→ h 0.41 ± 0.08 0.033 ± 0.006 2τ→ 3h 0.57 ± 0.11 0.155 ± 0.030 1τ→ μ 0.52 ± 0.10 0.018 ± 0.007 1τ→ e 0.62 ± 0.12 0.027 ± 0.005 0Total 2.11 ± 0.42 0.233 ± 0.041 4

)newθ(22sin

-310 -210 -110 1

)2

(eV

new

2m

Δ

-210

-110

1

10

210LSND 90% C.L.

LSND 99% C.L.

KARMEN 90% C.L.

NOMAD 90% C.L.

BUGEY 90% C.L.

CHOOZ 90% C.L.

MiniBooNE 90% C.L.

MiniBooNE 99% C.L.

ICARUS 90% C.L.

OPERA 90% C.L. (Bayesian)

Figure 4: Exclusion plot in the parameter space (sin2 2θnew, Δm2new) for the non-standard νμ → νe oscillation analysis.

* (GeV)θ cos με210 310 410 510

on

su

rfac

eμ

R

1

1.1

1.2

1.3

1.4

1.5

1.6OPERA

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Utah

MINOS

CMS

K modelπK + RQPM modelπK + QGSM modelπK + VFGS modelπ

Figure 5: Muon charge ratio measured by OPERA as a function of the vertical surface energy. The fit result is shown as a continuous line.

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4. Conclusions

The OPERA experiment has collected data in theCNGS neutrino beam from 2008 to 2012, integratingabout 18 × 1019 protons on target. Four ντ candidateevents have been detected so far, leading to the obser-vation of νμ → ντ oscillation at the atmospheric scalein appearance mode. The non-oscillation hypothesis isexcluded at 4.2 σ level.

From the systematic search for sub-leading νμ → νeoscillations in the 2008-2009 data sample, 19 electronneutrino candidates were detected while 19.8 ± 2.8(syst) were expected.

The analysis of the full data sample of atmosphericmuon events provided the most accurate measurementof the muon charge ratio Rμ in the high energy region,confirming the validity of a simple parametric modelwhich includes only pion and kaon contributions to themuon flux.

References

[1] Y. Fukuda et al. (Super-Kamiokande), Phys. Rev. Lett. 81, 1562(1998);J. Hosaka et al. (Super-Kamiokande), Phys. Rev. D 74, 032002(2006).

[2] M. Ambrosio et al. (MACRO), Phys. Lett. B 434, 451 (1998).[3] M. H. Ahn, et al. (K2K), Phys. Rev. D 74, 072003 (2006).[4] P. Adamson et al. (MINOS), Phys. Rev. Lett. 108, 191801

(2012).[5] M. Guler et al. (OPERA), CERN-SPSC-2000-028.[6] CNGS project, http://proj-cngs.web.cern.ch/proj-cngs.[7] R. Acquafredda et al. (OPERA), JINST 3, P04018 (2009).[8] A. Anokhina et al. (OPERA), JINST 3, P07005 (2008).[9] K. Morishima, T. Nakano, J. Instrum. 5, P04011 (2010).

[10] N. Armenise et al., Nucl. Instr. Meth. A 551, 261 (2005);L. Arrabito et al., Nucl. Instr. Meth. A 568, 578 (2006).

[11] N. Agafonova et al. (OPERA), New J. Phys. 14, 013026 (2012).[12] N. Agafonova et al. (OPERA), Phys. Lett. B 691, 138 (2010).[13] N. Agafonova et al. (OPERA), J. High Energy Phys. 11, 036

(2013).[14] N. Agafonova et al. (OPERA), Phys. Rev. D 89, 051102(R)

(2014).[15] N. Agafonova et al. (OPERA), Prog. Theor. Exp. Phys. 101C01

(2014).[16] N. Agafonova et al. (OPERA), Eur. Phys. J. C 74, 2986 (2014).[17] G. J. Feldman and R. D. Cousins, Phys. Rev. D 57, 3873 (1998).[18] N. Agafonova et al. (OPERA), J. High Energy Phys. 1307, 004

(2013).[19] A. Aguilar-Arevalo et al. (LSND), Phys. Rev. D 64, 112007

(2001).[20] A. Aguilar-Arevalo et al. (MiniBooNE), Phys. Rev. Lett. 110,

161801 (2013).[21] N. Agafonova et al. (OPERA), Eur. Phys. J. C 74, 2933 (2014).

N. Agafonova et al. / Nuclear and Particle Physics Proceedings 267–269 (2015) 87–93 93

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