The CERN-Neutrinos–to–Gran Sasso project (CNGS) is devoted to prove unambiguously the νμ → ντ oscillation channel in the atmospheric sector. The high-energy and high-intensity CNGS muon neutrino beam, generated at CERN, is directed towards the Italian Gran Sasso National Laboratory (LNGS), where the OPERA long-baseline experiment searches for ντ appearance. In this paper, a review of the performance and physics potential of the experiment together with the description of dedicated strategies to observe events with decay topologies in the OPERA target will be given

Pastore, Alessandra

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DOI 10.1393/ncc/i2011-10979-5Communications: SIF Congress 2010IL NUOVO CIMENTO Vol. 34 C, N. 5 Settembre-Ottobre 2011Decay search in the OPERA experimentA. Pastore(∗)Dipartimento di Fisica “M. Merlin”, Universita` di Bari and INFN, Sezione di Bari - I-70126Bari, Italy(ricevuto il 30 Dicembre 2010; revisionato il 25 Luglio 2011; approvato il 10 Agosto 2011;pubblicato online il 22 Settembre 2011)Summary. — The CERN-Neutrinos–to–Gran Sasso project (CNGS) is devoted toprove unambiguously the νμ → ντ oscillation channel in the atmospheric sector. Thehigh-energy and high-intensity CNGS muon neutrino beam, generated at CERN,is directed towards the Italian Gran Sasso National Laboratory (LNGS), wherethe OPERA long-baseline experiment searches for ντ appearance. In this paper, areview of the performance and physics potential of the experiment together withthe description of dedicated strategies to observe events with decay topologies inthe OPERA target will be given.PACS 14.60.Pq – Neutrino mass and mixing.PACS 13.15.+g – Neutrino interactions.PACS 14.60.Lm – Ordinary neutrinos.PACS 29.40.Gx – Tracking and position-sensitive detectors.1. – IntroductionIn the last decades, the mixing of neutrino flavor states and the consequent flavoroscillation hypothesis have been confirmed in the atmospheric sector by several experi-ments performed in disappearance mode [1]. A final proof will be set by studying theνμ → ντ oscillation channel in appearance mode.The Oscillation Project with Emulsion tRacking Apparatus (OPERA) [2] was de-signed to reach this goal through the direct observation of ντ in an almost pure νμ beam.The CERN-Neutrinos–to–Gran Sasso (CNGS) [3] high-energy neutrino beam (〈Eνμ〉 17GeV) is produced at CERN and delivered at LNGS where the OPERA detector islocated. It has been conceived in order to maximize the number of ντ charged-currentinteractions in the target. Contamination of the beam in terms of νμCC interactions isabout 2%, lower than 1% from νeCC and νeCC interactions and of the order of 10−6 in(∗) E-mail: alessandra.pastore@ba.infn.itc© Societa` Italiana di Fisica 139140 A. PASTOREFig. 1. – View of the OPERA detector.case of ντ prompt. A nominal beam intensity of 4.5 × 1019 p.o.t. per year is expectedfor five years of physics runs. Accordingly, we expect about 4700 νμ charged and neutralcurrent events per year.If the νμ → ντ oscillation hypothesis would be confirmed, the number of tau eventsobserved in the OPERA detector after 5 years of data taking is expected to be equal to10.40 with a background of 0.75 events (Δm223 = 2.5× 10−3 eV2 at full mixing).2. – The OPERA experimentOPERA [2] has a massive detector, exploiting the advantages of nuclear emulsionsdetection technique.This is the way to maximize statistics and to rely on a very high spatial accuracy andgranularity that should cope with the peculiar signature of short-lived τ lepton producedin ντCC interactions (in the millimeter range at the CNGS beam energy).The detector construction was completed at the beginning of 2008 and the data takingsuccessfully started in the Summer of 2008.The OPERA hybrid apparatus consists of two identical Super Modules (SM1 andSM2) as shown in fig. 1. Each Super Module is composed of a target section and a muonspectrometer. Each target section (∼ 625 t) is organized in 31 vertical “walls”, transverseto the beam direction. Walls are filled with “bricks” with an overall mass of 1.25 kt. Theyare followed by double layers of scintillator planes acting as Target Trackers (TT) thatare used to locate neutrino interactions occurred within the target.The brick is the basic unit of the detector: it consists of 57 nuclear-emulsion filmsinterleaved with 1mm thick lead plates, with a total length of 7.9 cm along the beamdirection, transverse dimensions of 10.2× 12.8 cm2 and a total mass of 8.3 kg. An addi-tional tightly packed doublet of emulsion films (Changeable Sheets, CS) is glued to thedownstream face of each brick, in order to connect the electronic detectors predictionsto the brick.The spectrometers consist of a dipolar magnet instrumented with active detectors,planes of RPCs (Internal Tracker, IT) and drift tubes (Precision Tracker, PT). Tasks ofthe spectrometers are muon identification and charge measurement in order to minimizethe background. The charge-sign misidentification probability has been estimated tobe of about 0.3% up to 50GeV/c; the momentum resolution is about 20% in the samekinematical range.DECAY SEARCH IN THE OPERA EXPERIMENT 141The apparatus is equipped with an automatic machine (Brick Manipulator System,BMS), allowing the online removal of bricks from the detector, together with some an-cillary facilities for the emulsions’ handling. Specially designed and dedicated Europeanand Japanese Scanning Stations take care of the nuclear emulsions films measurement.3. – The OPERA strategyEvery time a charged particle produced in a neutrino interaction occurred in a brickproduces a signal in the TT, a brick finding algorithm is applied in order to select the brickwhich has the maximum probability to contain the neutrino interaction. The efficiencyof this procedure is as high as 83% in a sub-sample where up to 4 bricks per event wereprocessed.Once the selected brick is removed from the target by the BMS, the corresponding CSdoublet is detached from the brick and developed in a dedicated underground facility. Thetwo emulsion films are then scanned at LNGS or in Japan. The measurement of emulsionfilms is performed through fast automated microscopes with a scanning speed greaterthan 20 cm2/h, a tracking efficiency of about 90%, sub-micrometric position resolutionand angular resolution of the order of one milliradian.The measured residuals between electronic detectors predictions and CS tracks arefound to be of the order of a cm. If any track originating from the interaction is detectedin the CS, the brick is exposed to cosmic rays (for alignment purposes) and then de-packed. The emulsion films are developed and sent to the scanning laboratories of theCollaboration for event location studies and decay search analysis. The CS-to-brickconnection is achieved with less than 100μm position accuracy and with a slope accuracyof the order of 10mrad.All tracks located in the CS are followed upstream through the brick (scan-back) untilthey stop. A general scanning (no angular pre-selection) is then performed in a 2 cm3volume around the stopping point(s) in order to reconstruct the vertex topology withmicrometric precision.In order to detect decay or secondary interaction topologies, all located vertices arestudied more in details by means of a dedicated procedure.4. – Decay search in nuclear emulsionsPhotographic emulsions initiated accidentally their role in particle physics more than100 years ago [4]. An intense R&D activity led to the development of the so called“Emulsion Cloud Chamber”, a detector consisting of alternate layers of photographicemulsion and absorber, characterized by very high sensitivity and grain uniformity andcapable of observing tracks of single particles with sub-micrometric space resolution.This technique is specially suitable for the observation of short-lived particles and led,in the past, to several important discoveries such as the observation of the first charmedparticle [5], the first direct observation of the decay of beauty particles into charmedones [6] and the first detection of a tau neutrino [7].Today, the OPERA sandwich-like structure made of lead plates (passive material) andthin emulsion layers (tracking device) exploits this feature and, in addition, allows theparticles identification and the measurement of their kinematical parameters by means ofa detailed observation of specific ionization, showering and multiple Coulomb scattering.Particle momenta are evaluated in the emulsion films by the multiple Coulomb scat-tering of tracks in the lead plates using an angular method for charged particles up to142 A. PASTOREFig. 2. – Sketches of the so-called long decay (left) and short decay (right) for the short-livedtau lepton inside an OPERA brick.6GeV/c with a resolution better than 22% and a position deviation method for higher-energy particles with a resolution better than 33% on 1/p up to 12GeV/c. Muon mo-mentum is measured in spectrometers, as already mentioned in sect. 2.Gamma-ray search is performed in the measured volume by means of electromagneticshower detection or visual inspection of e-pair at its starting point and its energy isestimated by a dedicated Neural Network algorithm.As mentioned above, ντ appearance search at LNGS is made possible by means of τdecay topologies detection inside the OPERA target.Tau decays inside a brick may be classified into two categories shown in fig. 2: shortand long ones. In the first case the ντCC interaction and the subsequent τ decay occur inthe same lead plate: no track associated to τ lepton can exist in the brick. Long decaysare characterized by longer τ decay length that means there is a chance to reconstructits passing through the emulsion film(s) and therefore the peculiar kink topology.Depending on the decay channel, different strategy to look for τ decays are referred to.At first, a dedicated procedure has been applied to all located vertices in order to selectinteresting topologies by means of topological criteria. This study is realized including10 nuclear emulsion films downstream with respect to the primary neutrino interactionand 5 films upstream, for an overall volume of 2 cm3.In order to detect interesting topologies, a check on impact parameters (ip) of tracksthat have been reconstructed as interaction daughters is mandatory. Only tracks withimpact parameter with respect to the vertex less than 10μm are confirmed to be primarytracks. Tracks not verifying the selection, have to be further studied to investigate thereason for such anomalous value (e.g. scattering in the traversed lead thickness), andpossibly reconsidered in the determination of the vertex position. A so-called “in-track”decay search is then applied to confirmed primary tracks: for each track, the largestslope difference in the first 4 films downstream with respect to the vertex is computed asshown in fig. 3 and compared with a mean reference value estimated taking into accountthe particle multiple Coulomb scattering in lead. This compatibility check is performedto identify possible small kink angles (θkink > 15mrad), signatures of decaying particlesto be analyzed more in detail.Extra tracks originated from neutral decays, interactions and gamma ray conversionsare searched for inside the measured volume, too. Different Monte Carlo optimizedselection criteria are applied for events with the primary neutrino interaction clearlyDECAY SEARCH IN THE OPERA EXPERIMENT 143Fig. 3. – In-track decay search: a schematic view.identified(1) and events where an ambiguity exists on the main vertex definition. Theimpact parameter of each extra-track with respect to the main vertex, together with thelongitudinal distance between the vertex and the most upstream segment of the track,has to be considered as the relevant observable, in both cases.Selected tracks are visually inspected in order to reject electron-positron pairs fromgamma-conversion and then investigated more in details as short decays’ daughter can-didates or long decays’ daughter/parent candidates, according as they stop in the vertexemulsion film or in any other one.Once an interesting secondary vertex topology is found in the event, it is analyzedthrough kinematical criteria depending on the decay channel to be investigated. Selectioncriteria are based on the evaluation of the missing transverse momentum and the φ anglebetween the parent track and the rest of the hadronic shower in the transverse planeat the primary vertex, on the daughter particle momentum, on the missing transversemomentum and on the kink angle at secondary vertex. By way of example, selectioncriteria to be applied to interesting single-hadron long-decay topologies will be listed inthe following.For the primary vertex it is required that– there are no tracks compatible with that of a muon or an electron;– the missing transverse momentum is smaller than 1GeV/c;– the angle φ in the transverse plane between the τ candidate track and the hadronicshower direction is larger than π/2.For the secondary vertex, it is required that– the kink angle is larger than 20mrad;– the secondary vertex is within two lead plates immediately downstream of theprimary vertex;– the momentum of the charged secondary particles is larger than 2GeV/c;(1) For example, muon neutrinos charged current interactions in case of reconstructed muon atthe primary vertex.144 A. PASTOREFig. 4. – Display of the ντCC candidate event.– the total transverse momentum of the decay products is larger than 0.6GeV/c ifthere are no photons emitted at the decay vertex, and 0.3GeV/c otherwise.Charmed particles have similar lifetimes as τ lepton and, if charged, share the samedecay topologies. At the time of this writing, taking into account the sample of analyzedevents from 2008 and 2009 runs [8], the OPERA Collaboration has observed 18 charmdecay candidates and 6 charged-current electron-neutrino interactions in good agreementwith expectations. This demonstrates that the efficiency of the search for short-liveddecay topologies is understood.Moreover, the first charged-current tau neutrino candidate has been detected (0.5 ντexpected with respect to the analyzed statistics) [8].5. – The first tau neutrino candidateThe first ντCC candidate in the OPERA detector is shown in fig. 4. The primaryneutrino interaction is a seven-prong vertex occurred in a brick situated in wall 11 of thefirst SM and well inside the target, allowing a deep analysis of the event.One daughter track exhibits a visible kink. All kinematical variables are satisfying allselection criteria established for hadronic kinks (see table I). None of the charged particlesTable I. – First tau neutrino candidate: kinematical variables.Variablekink (mrad) 41± 2decay length (μm) 1335± 35Pdaugh (GeV/c) 12+6−3Pt decay (MeV/c) 470+230−120missing Pt (MeV/c) 570+320−170φ (◦) 173± 2DECAY SEARCH IN THE OPERA EXPERIMENT 145at the primary and secondary vertex is compatible with being an electron. For tracksthat are not hadrons (no interaction seen), the probability that are left by a muon is lessthan 10−3. The residual probability of νμCC event (possibly undetected large angle μ)is about 1%; the nominal value of 5% is assumed. Two electromagnetic showers causedby γ-rays, associated with the event, have been located and studied. Their emission atthe secondary vertex is the favored hypothesis and the evaluation of their invariant mass((120± 20 (stat.)± 35 (syst.))MeV/c2) supports the fact that they originate from a π0decay.The secondary vertex is compatible with the tau one-prong hadronic decay mode:τ− → h(nπ0) ντThe main sources of background for this channel are hadronic re-interactions and decaysof charmed particles produced in νμ interactions.The charm background produced in muon neutrinos interactions amounts to 0.007±0.004 (syst.) events in the reference data sample; it is less than 10−3 events if comingfrom νe interactions. The dominant background from hadron re-interactions has beenevaluated with a FLUKA based Monte Carlo code to be (1.9 ± 0.1) · 10−4 kinks/NC.When misclassified CC events are included, a total of 0.011 ± 0.006 (syst.) backgroundevents are foreseen. Consequently, the total background in the tau decay channel to asingle charged hadron is 0.018 ± 0.007 (syst.) and its probability to fluctuate into oneevent is 1.8% (2.36 σ). If the search for tau decays is extended to all decay modes, thebackground expectation is equal to 0.045 ± 0.020 (syst.) events which correspond to afluctuation probability to one event of 4.5% (2.01 σ).6. – ConclusionsThe OPERA experiment was designed to explore in appearance mode νμ → ντ os-cillations in the almost pure CNGS νμ beam. The detection of decay topologies in theOPERA target is therefore mandatory to reach the goal of the experiment.Topological and kinematical selection criteria were defined and optimized in order tolocate short-lived particles decays in an OPERA brick. They have been systematicallyapplied to located neutrino interactions that means, at the time of this talk, on a sub-sample of neutrino data taken in the CNGS beam 2008–2009 runs. Several interestingtopologies such as charm decays candidates have been reconstructed in agreement withthe expectations. One muonless event showing a τ → 1-prong hadronic decay topologyhas been detected and studied in detail. It passes all kinematical cuts required to reducethe physics background, being the first ντ candidate event in OPERA with statisticalsignificance of 2.36 σ (for one-prong hadronic decay mode) and 2.01 σ (for all decaymodes).REFERENCES[1] Pontecorvo B., Sov. Phys. JETP, 6 (1957) 429; Pontecorvo B., Sov. Phys. JETP, 7(1958) 172; Maki Z., Nakagawa M. and Sakata S., Prog. Theor. Phys., 28 (1962) 870.[2] Guler M. et al. (The OPERA Collaboration), Experimental Proposal, CERN 98-02, INFN/AE-98/05 (1998); Guler M. et al., Status Report on the OPERA experiment,CERN/SPSC 2001-025, LNGS-EXP 30/2001 add. 1/01.146 A. PASTORE[3] Acquispace G. et al., CERN-98-02 (1998); Bailey R. et al., CERN-SL/99-034 (1999);Ball A. E. et al., CERNSL/ Note-2000-063 (2000).[4] Becquerel H., C.R. Acad. Sci., 122 (1896) 501 and C.R. Acad. Sci., 122 (1896) 1086.[5] Niu K. et al., Prog. Theor. Phys., 46 (1971) 1644.[6] The WA75 Collaboration, Phys. Lett. B, 158 (1985) 186.[7] Kodama K. et al. (The DONUT Collaboration), Phys. Lett. B, 504 (2001) 218.[8] Agafonova A. et al. (The OPERA Collaboration), Phys. Lett. B, 691 (2010) 138.

Decay search in the OPERA experiment

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Decay search in the OPERA experiment

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