OPERA is an experiment that aims at detecting the appearance of tau-neutrino
in an almost pure mu-neutrino beam (the CNGS neutrino beam) through
oscillation. OPERA is a hybrid detector that associates nuclear emulsions to
electronic detectors. The nuclear emulsion provides the resolution necessary to
detect tau-neutrino CC interactions. The first physics run started in July and
ended in November 2008. In this presentation, the status of the emulsion
technology and of the analysis of its data is reported.Comment: 6 pages, 16 Postscript figures, To be published in the proceedings of
  DPF-2009, Detroit, MI, July 2009, eConf C09072

Fukuda, Tsutomu

English

arXiv

OPERA is an experiment that aims at detecting the appearance of tau-neutrino in an almost pure mu-neutrino beam (the CNGS neutrino beam) through oscillation. OPERA is a hybrid detector that associates nuclear emulsions to electronic detectors. The nuclear emulsion provides the resolution necessary to detect tau-neutrino CC interactions. The first physics run started in July and ended in November 2008. In this presentation, the status of the emulsion technology and of the analysis of its data is reported.OPERA is an experiment that aims at detecting the appearance of tau-neutrino in an almost pure mu-neutrino beam (the CNGS neutrino beam) through oscillation. OPERA is a hybrid detector that associates nuclear emulsions to electronic detectors. The nuclear emulsion provides the resolution necessary to detect tau-neutrino CC interactions. The first physics run started in July and ended in November 2008. In this presentation, the status of the emulsion technology and of the analysis of its data is reported

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arXiv:0910.3274v1  [hep-ex]  17 Oct 2009Proceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 1The Nuclear Emulsion Technology and the Analysis of the OPERAExperiment DataTsutomu FUKUDA on behalf of the OPERA collaborationF-lab, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, JAPANO PER A is an experim ent that aim sat detecting the appearance of in an alm ostpure  beam (the CN G Sneutrino beam )through oscillation.O PER A isa hybrid detectorthatassociatesnuclearem ulsionsto electronicdetectors.Thenuclearem ulsion providestheresolution necessary to detect CC interactions.Therstphysicsrun started in July and ended in N ovem ber 2008. In this presentation,the status ofthe em ulsion technologyand ofthe analysis ofitsdata isreported.1. Nuclear EmulsionNuclearEm ulsion isa specialtype ofphotographicem ulsion m ade ofAgBrm icrocrystalsinterspersed ina gelm atrix.A charged particlepassing through suchm edium ionizes the crystals along its path and pro-duces a latent im age. Upon a chem icalprocess,thedevelopm ent,theparticletrajectoryism aterialized bya line ofgrainsofm etallic Ag (0.5 1 m diam eter);typically thegrain density isabout30grains/ 100m(Fig.1).Nuclearem ulsion isa sub-m icron 3D track-ing detector with a resolution of0.3m . In particlephysics, the nuclear em ulsion technology is notablyreputed for the discovery ofthe pion [1]and ofthecharm particle in cosm ic-ray [2]and for the  rstob-servation ofthe tau-neutrino [3].Figure 1: A charged particle track in nuclearem ulsion.2. The OPERA ExperimentIn 1962,M aki,Nakagawaand Sakataproposed thatoscillation m ay existbetween m assiveneutrinosofdif-ferent avours[4].In 1998,theSuper-K am iokandeex-perim entestablished thede citin atm ospheric dueto theirdisappearancethrough theoscillation m echa-nism [5].Thishasbeen con rm ed laterbytheK 2K [6]and then theM INO S [7]acceleratorexperim ents.Thegoalof the O PERA experim ent [8]is to detect forthe  rsttim e the  appearance in a  beam in theatm ospheric sector. The path length of the  lep-tonsproduced in CC interactionsisvery short(c=87m )thusrequiring very high spatialresolution.In2001,The DO NuT experim ent succeeded in detect-ing such interactionsin Em ulsion Cloud Cham bersorECC (Fig.2).The detector is exposed to the CERN CNG S beam with an average energy of17 G eV,wellabovethe  lepton production threshold in  CC interac-tions[9]. The rate ofprom pt isnegligible atsuchenergy. Itis located in the G ran Sasso undergroundlaboratory (LNG S) ata distance of730km from theneutrino source. For the m ost probable m easuredvalues ofthe oscillation param eters, m 2 = 2.4 10 3 eV 2,sin22 = 1.0,thefraction ofneutrinoshav-ing oscillated isabout1.7% .Theexpected num berofdetected  CC eventsisabout2.5 fora nom inalyearof run corresponding to 4.5 1019 protons on target(pot).See also these proceedings[10].Figure 2: A tau-neutrino event detected in the D O NuTexperim ent.3. The OPERA DetectorO PERA isa hybrid detectorcom posed oftwo iden-ticalsuper-m odules. The targets,1.25 kton in total,areeach form ed by about75000unitscalled hereafterbricksbased on theECC technology.They areassem -bled into walls interleaved by two orthogonalplanesofscintillatorstripstargettrackers(TT)[11]used to2 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009identify the bricks in which the interactions occur.Each targetiscom plem ented by a spectrom eterthatidenti es m uons and m easures their charge and m o-m entum . An overallpicture ofthe detectorisshownin Fig. 4 and its detailed description is available in[12].Figure 3: ECC,CS,TT.Figure4: TheO PERA D etector@ G ran Sasso (1400m un-derground).The size ofa brick is 12.8cm  10.2cm  7.9cm .Itiscom posed of57 0.3m m -thick em ulsion  lm s[13]interleaved with 561m m -thick lead plates[14].A  lmhasa 44m em ulsion layerdeposited on each sideofa205m plasticbase(Fig.5).A separatebox contain-ingapairof lm shereaftercalled changeablesheetsorCS isglued on thedownstream faceofeach ECC brickin frontofthe nextTT plane(Fig.3).They serveasinterface between the brick and the TT,bringing thecentim etre spatialresolution ofthe TT down to them level.4. Flow of the event analysisThe eventanalysisisperform ed in two m ain steps:location ofthe neutrino interaction and search for asecondaryvertextopologyofwhich kinem aticsiscom -patible with thatof decay.Figure 5: O PERA lm .4.1. Neutrino interactions locationThesignalrecorded by theelectronicdetectors,TTand spectrom eters,isused toidentify thebricks,m ostoften one and up to 3,in which the neutrino interac-tion is likely to have occurred. Those bricks are ex-tracted from the target by an autom aton. The CSarerem oved from theirbox,developed and analysed,starting with the m ost probable brick. The levelofbackground in the CS is negligible; essentially onlytracksfrom theneutrinointeraction arerecorded.Theidenti cation ofthebrick isthuscon rm ed by  ndingtracks in the CS that are com patible with the elec-tronic data,in which case the brick is disassem bledand its  lm s developed. The tracksfound in the CSare extrapolated to the m ost downstream em ulsion lm where they are searched for. They are then fol-lowed back from  lm to  lm up to thelocation oftheneutrino vertex where they disappear. Finally em ul-sion data istaken around thispointand theneutrinointeraction vertex isreconstructed.4.2. Decay SearchThe four m ain decay channels ofthe  lepton aregiven in Table I.Topologically,they are classi ed as"kink" or "trident" if they have one or 3 chargeddaughters.Charm particleshave sim ilarlifetim e anddecay topologiesasthe lepton.Understanding theirdetection rate istherefore a directveri cation oftheexpected detection e ciency ofthe  lepton.Table I  decay m odes.topology decay m ode ratiokink ! e  18%kink !   17%kink ! hadron  49%trident ! 3hadrons  15%Single prong events fallinto three categories. For60% ofthe events(Fig. 6-bottom ),the decay occursinside the vertex lead plate and the parenttraversesno em ulsion layer. The decay productsare identi edby theirlarge im pactparam eter(IP)with respecttoProceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 3the prim ary vertex. For 30% ofthe events (Fig. 6-top),the parent traverses at least one  lm . In thiscase,the trajectory ofboth the parent and the de-cay productm ay be reconstructed inside atleastoneplastic base from data registered in the two em ulsionlayers,thetwom icro-tracks,toform abase-track.Thecandidatesareidenti ed by the observation ofa kinkbetween both trajectories.For10% oftheevents,theparent traverses only one em ulsion layer and decaysin the  lm base. There is no base track on the par-enttrajectory buta single m icro-track.W hetherthepresence ofthis m icro-track willissu cientto iden-tify thecandidatesby theobservation ofakink orwillitbe identi ed by the IP m ethod isunderstudy.Figure 6: D ecay Search.5. Emulsion Technology developed forthe OPERA experiment5.1. The OPERA filmThe m assproduction ofthe nuclearem ulsion  lm sused forO PERA isthe  rstexam ple ofan industrialprocess. In the past history, nuclear em ulsion  lmorplateswere poured by hand. M achine productionguarantees hom ogeneity in thickness and sensitivityunreachable before. An added protective coatallowshand m anipulation ofthe  lm . A new feature oftheO PERA  lm is thattracksalready recorded m ay beerased.Thisprocessiscalled "refreshing".5.2. Film refreshingThe "refreshing" process was developed to allowerasing the large background of cosm ic ray tracksrecorded in the  lm s since their tim e offabrication.The signalrecorded in nuclear em ulsion was knownto fade with tim e at a speed depending on the en-vironm ent,causing the sensitivity to track detectionto progressively decrease. In the O PERA  lm ,thisfading e ectiscontrollable.By keeping  lm sathighrelativehum idity (98% )and high tem perature(30C)for3 days,m orethan 99% oftherecorded trackswereerased (Fig.7)while the sensitivity to new recordingis not a ected. Allthe  lm s were refreshed under-ground in the Tono m ine in Japan and then trans-ported to G ran Sasso atsee level.The  lm sused forthe CS require very low background; they were re-freshed a second tim e underground atG ran Sasso.Figure 7: Film refreshing.5.3. High speed scanning systemsThe scanning area of the O PERA CS m easuresin cm 2;this is m ore than 100 tim es larger than forDO NuT,the last experim ent having used the ECCtechnique. Therefore high speed scanning system swere developed in both Japan and Europe. The newJapanese scanning system ,the S-UTS (Super UltraTrack Selector)isshown on Fig.8-left.Foursystem swith a scanning speed of75cm 2 per hourand one of20cm 2 perhourareoperationalin Japan.The previ-ousgeneration operated atspeedsofabout1cm 2 perhour[15];5 such system sare used form anualveri -cation and subsidiary tasks. The European scanningm achine [16],the ESS (European Scanning System )is shown on Fig. 8-right. It operates at a speed of20cm 2 perhour. A totalof33 such system sare cur-rently activeforO PERA.Figure 8: New Scanning M achines.4 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,20095.4. Momentum measurement in ECCbricksThe m om entum of charged particles can be esti-m ated in ECC bricks by m easuring their M ultipleCoulom b Scattering [17]. W hen a particle ofchargez,m om entum p and velocity c traversesa m aterialofdepth x and radiation length X 0,the distributionofthescatteringangleisexpressed by a G aussian,theRM S ofwhich isapproxim ately given by Eq.1.0 =13:6M eVcpzrxX 0(1)The m easurem ent of the angle di erences in twoconsecutive  lm sprovidesthe m om entum estim ation(Fig.9).Theresultsofa testexperim entatK EK areshown in Fig. 10. A brick wasexposed to 0.8G eV/cand 1.5G eV/cpion beam s.Thevaluesofthem om en-tum m easured by M CS arerespectively of0.79G eV/c(p/p = 11% )and 1.53G eV/c(p/p = 16% ).Figure 9: M om entum m easurem entin ECC bricks.Figure 10: Resultofa testexperim entatK EK .5.5. Electron energy measurement inECC bricksFig. 11 shows the developm ent of electrom ag-neticshowersgenerated by electronsin an ECC brick.Countingtracksegm entsin theshowerprovidesan es-tim ation ofthe incidentelectron energy [18].Fig.12showsthe resultofa testexperim entatCERN with2G eV and 4G eV electron beam s.There isagreem entbetween them easured and M onteCarlosim ulated dis-tributions ofthe num bers ofsegm ents though errorson data arelarge.Figure 11: Electron energy m easurem entin ECC bricks.Figure 12: Resultofa testexperim entatCERN.5.6. dE/dX measurement in ECC bricksIn nuclear em ulsion, the grain density recordedalongthetrack ofa charged particleisalm ostpropor-tionalto itsenergy lossdE/dX (Fig. 13,[19]). ECCbricks have been exposed to 0.4G eV/c, 0.5G eV/c,0.6G eV/c, 0.74G eV/c, 0.87G eV/c, 1.14G eV/c and2.0G eV/c proton and pion beam s at K EK .Fig. 14-leftillustratesthe particle identi cation capability ofthe m ethod (pion, proton and deuteron contam ina-tion) at 0.87G eV/c. The relation between the grainProceedings ofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009 5density and the m om entum ofthe particlesisshownFig.14-right.Figure 13: A pion track of0.6G eV/c(top) and a protontrack of0.6G eV/c(bottom ).Figure 14: Resultofa testexperim entatK EK .6. Status of the 2008 OPERA run6.1. Location statusTriggersforatotalof1690eventsin thetargetweregenerated by the TT in tim e with the CNG S beamarrival.TablesIIand IIIsum m arizethecurrentstatusoftheeventslocation in theCS and in thebricksaftertheanalysisofthem ostprobableCS indicated by theTT is welladvanced. 797 events have been locatedalready in thebricks.Foreventsnotfound in theCS,theanalysisofthenextm ostprobableCS hasstarted.Table II Currentstatusofthe eventlocation in CS.num berofevents 1473Scanning done 1440Found in CS 1110NextCS :now in progress 330Table III Current status of the event location in ECCbricks.NC CC TO TALECCsreceived in laboratories 218 959 1177ECCsm easured 195 895 1090CS to ECC connection successful178 849 1027Neutrino interactionslocated 119 678 797in the ECCNeutrino interactionsin the 12 46 58upstream wallNeutrino interactionsin the 4 17 21dead m aterial6.2. Decay search statusThe search fordecay verticesin the eventsalreadylocated isin progress.Fig.15 showsin red the m ini-m um distancebetween pairsoftrackswith P> 1G eV/cem itted in realneutrinointeractions.In blue,itshowstheM onteCarlodistribution oftheIP with respecttothe prim ary vertex ofthe daughterparticle of lep-tonsdecaying in thelead plateoftheprim ary vertex.The m inim um distance between allpairs ofprim arytracks is within 10m . This dem onstrates that theO PERA ECC brickshave enough resolution to iden-tify  decay topologiesbased on the IP.Figure 15: D istribution of m inim um distances betweenpairsoftrackswith p > 1G eV/c em itted in realneutrinointeractions(red) and M onte Carlo distribution ofthe IPw.r.t. the prim ary vertex ofthe daughter particle oftauleptons(blue).Som echarm decaycandidateshavealreadybeen de-tected as exem pli ed in Fig. 16. There are 6 tracksincluding a   at the prim ary vertex. O ne ofthemdecaysinto a + aftera path length of1330m .Thekink angleis209m rad and thePtofthedaughterpar-ticle isabout460M eV/c. The beginning ofa photonelectrom agneticshowerisalso seen.6 Proceedingsofthe DPF-2009 Conference,Detroit,M I,July 27-31,2009Figure 16: A charm candidate event.7. The 2009 OPERA RunTheO PERA detectorsu ered no dam agefollowingthe very strong L’Aquila earthquake of April2009.The beginning ofthe run started on June 1 with adelay ofa coupleofweeksonly.A nom inalnum berof4.5 1019 potisrequested from CERN,in which caseabout4800 neutrino eventswould be collected in theO PERA targetsby the end ofthe run and about2.5CC interactionswould be expected to be detected.8. ConclusionsThegoalofO PERA istodetect !  oscillationin the appearance m ode in the CERN CNG S beam .An innovativenuclearem ulsion technology wasdevel-oped forO PERA.In 2008,triggersfor1690 neutrinoevents in the targets were recorded. So far,the pri-m ary verticesof797 oftheseeventswerelocated andanalysed in theECC bricksby anew generation ofau-tom aticscanning system sand thework isin progress.Firstexam plesofcharm decay candidateswerefoundand their kinem aticalanalysis perform ed. The 2009run hasstarted on June1 2009.Itshould lead to thepotentialdetection ofa coupleofCC interactions.References[1]C.Powelletal.,Nature159,694 (1947).[2]K .Niu et al.,Conf.Paper,12th Int.Cos.RayConf.(Hobart)2792{2798 (1971).[3]K .K odam a etal.,Phys.Lett.B 504,218 (2001).[4]Z. M aki, N. Nakagawa and S. Sakata, Prog.Theor.Phys.28,870 (1962).[5]Y.Fukudaetal.,Phys.Rev.Lett.81(1998)1562.[6]M .H.Ahn etal.,Phys.Rev.D 74(2006)072003.[7]P.Adam son et al.,Phys.Rev.Lett.101 (2008)131802.[8]M .G uleretal.,CERN-SPSC-2000-028 (2000).[9]http://proj-cngs.web.cern.ch/proj-cngs/[10]M .De Serio,"The O PERA experim ent: on theway to the directobservation of !  oscilla-tion" these proceedings.[11]M .Dracosetal.,Nucl.Instrum .M ethodsA 581(2007).[12]"An appearanceexperim enttosearchfor ! oscillation in CNG S beam "Experim entProposal,July 10 (2000).[13]T.Nakam ura et al.,Nucl.Instrum .M ethods A556 80 (2006).[14]A.Anokhina etal.,JINST 3 P07002 (2008).[15]T. Nakano, Ph.D. thesis, Nagoya University,(1998).[16]C.Bozza et al.,Nucl.Instrum .M ethods A 551(2005).[17]B.D.Park etal.,Nucl.Instrum .M ethodsA 574(2007).[18]T. Toshito et al., Rev. Sci. Instrum . 74:53-56(2003).[19]T. Toshito et al., Nucl. Instrum . M ethods A516:436-439 (2004).

The Nuclear Emulsion Technology and the Analysis of the OPERA Experiment Data

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