Search for ν τ interactions with the nuclear emulsions of the OPERA experiment

1. Search for ντ interactions with the nuclear emulsions of the OPERA experiment Fabio Pupilli (INFN – Laboratori Nazionali del Gran Sasso) on behalfof the OPERA Collaboration 22/08/2011 – XV LomonosovConference - Moscow

2. Belgium ULB Brussels Italy Bari Bologna LNF Frascati L’Aquila, LNGS Naples Padova Rome Salerno Russia INR RAS Moscow LPI RAS Moscow ITEP Moscow SINP MSU Moscow JINR Dubna Croatia IRB Zagreb France LAPP Annecy IPNL Lyon IPHC Strasbourg Switzerland Bern ETH Zurich Japan Aichi Toho Kobe Nagoya Utsunomiya Germany Hamburg Turkey METU Ankara Israel Technion Haifa The OPERA Collaboration 160 physicists, 30 institutions, 11 countries Korea Jinju Pupilli Fabio - XV Lomonosov Conference http://operaweb.lngs.infn.it/scientists/?lang=en

3. - nm n Detection principle for nt appearance in OPERA: the importance of nuclear emulsions The challenge is to identify ντ interactions from νµ interactions n Decay “kink” -, h-, e- nm oscillation t- ct ~ 87 mm nt gct Topological and kinematicalreconstruction Pupilli Fabio - XV Lomonosov Conference ● Target mass O(kton) (low  interaction cross-section) ● High granularity detector ( decay detection, background rejection) lead – nuclear emulsion target segmented into basic units called bricks

4. 10 X0 Plastic base 205 mm t n 8,3 Kg Changeable Sheet Doublet (CS) Emulsion layers 44 mm The OPERA core: ECC bricks Emulsion Cloud Chamber: series of emulsion films interspaced with lead plates: - Provides high resolution and large mass in a modular way BRICK: • 57 emulsionsheets (300 mm) + 56 leadlayers (1 mm) • 10.0 x 12.5 x 7.5 cm3, 8.3 Kg, 10 X0 • Changeable Sheets (CS): tightly packed removable doublets of emulsion films glued to the downstream face of each brick • 150000 bricksproduced and installedinto the target •  target mass ~ 1.25 kton [111000 m2 emulsion surface] 1 mm … Pb Pupilli Fabio - XV Lomonosov Conference

5. Mechanical structure Changeable Sheet Doublet(CS) in separate envelope Brick: 57 nuclear emulsion films interleaved with 56 lead plates (1mm thick) Muon spectrometer: Target area: ~150,000 bricks (total target mass ~ 1.25 kt) arranged in 54 walls Walls of ECC brick interleaved plastic scintillator tracker walls Instrumented dipolar magnet for muon spectrometry 10.0cm 12.5cm The OPERA detector: where the bricksare… SM1 SM2 nm Pupilli Fabio - XV Lomonosov Conference Veto

6. OPERA emulsionfilms Industrial production @ Fuji Film A minimal detector AgBr Cristal, Size = 0.2 µm Detection efficiency = 0.16/crystal Emulsion Layer (44micron) Plastic Base (205micron) Emulsion Layer Cross-sectional view (SEM) Intrinsic resolution50 nm sensitivity36 grains/100 µm Pupilli Fabio - XV Lomonosov Conference Deviation from linear-fit line. (2D) A m.i.p. track in emulsion Low energy electrons 20μm

7. Emulsionautomatic scanning ~20 bricks extracted/day  O(1000 cm2)/day of emulsion surface have to be scanned Development of high-speed automatic scanning systems • Europe: ESS (European Scanning System) • Scanning speed: 20 cm2/h • Customized commercial optics and mechanics • “Stop and go” emulsion tomographic DAQ • Micro-track: Track recognised by fitting aligned sequences of grains through an emulsion layer • Japan: S-UTS (Super Ultra TrackSelector) • Scanning speed: 72 cm2/h • Hard-coded algorithms, custom electronic • Synchronization of piezo-controlled objective lens and stage for tomographic emulsion DAQ • Track recognition by summing up tomographic images Pupilli Fabio - XV Lomonosov Conference 44 µm 44 µm Similarperformances

8. Field of view Frommicrotracksto volume tracks 44 µm 3) Film to film aligment computing the 6 parameters of an affine transformation, by an alignment pattern  Volume tracks: fit of aligned base-tracks reconstructed through the volume 1) Tomographic emulsion DAQ and Micro-track reconstruction 2) Base-track formation: geometric joining line between the nearest grains to the platic base of each micro-track (avoid distortion) Performances Evaluatedby a 10 GeVpionexposureofan OPERA emulsionstackwithoutlead, tohaveintrinsicresolutions and efficiencies Long. ang. Resolution (vertical tracks) σ ~ 1.7 mrad Long. pos. Resolution 0.2 µm < σ < 0.7 µm Pupilli Fabio - XV Lomonosov Conference Base-track finding efficiency Long. ang. Resolution (0.6 rad tracks) σ ~ 6.3 mrad Transv. pos. Resolution σ ~ 0.3 µm

9. nm Operation mode: from electronic detector predictiontoVertexreconstruction • Brick finding: • An algorithm selects the brick that most probably contains the interaction (according to Electronic Detectors data) • The impact point on CS of the muon (CC events) or of the barycenter of the shower (NC events) is predicted • CSd general scan: • Large area analysis on CSd to search for tracks compatible with an interaction in the brick and with EleDet data. If not, the next probable brick is extracted • Scan back: • Track found in the CS are searched for and followed in the brick • Stopping point definition • Volume Scan: • 1 cm2, 16 plates (5 upstream, 10 downstream) • interaction reconstruction • Decay Search: • Search for decay, interaction and γ-conversion topologies; if any, full kinematical reconstruction Develop this brick CSd Pupilli Fabio - XV Lomonosov Conference 2 1 1 … 53 57 2 48 49 50 51 52 54 55 56

10. CS analysis Search for tracks coincidences in two consecutive emulsion films (background reduction), in a large area around EleDet prediction, to find neutrino interaction hints a) X-raymarksalignment Doublebasetracksearch Position resolution Δy σ~ 9 μm Δx σ~ 8 μm • Alignment pattern givenby 4 X-raymarksprintedbefore CS development • Global alignment , insensibletolocaldistortions b) Compton alignment Pupilli Fabio - XV Lomonosov Conference Position resolution Δx σ~ 2.5 μm Δy σ~ 2.2μm • Samplingof the scanning area in smallzones, localalignment sensitive tolocalemulsiondistortions • Micrometricprecisionof the alignment, open the possibilitytosearchforcoincidencebetween a micro-track and a base-track Inefficiencyrecovery • Alignmentbased on low energyelectronsemitted in the decayofradioactiveisotopes

11. Residuals between tracks reconstructed in the CS and in the electronic trackers CC events σ~ 9 mm σ~ 7.5 mm NC events σ~ 17 mrad σ~ 20 mrad σ~ 13 mm σ~ 10 mm Pupilli Fabio - XV Lomonosov Conference Connection between the cm world of electronic detectors and the μm world ofemulsionfilms

12. CS-to-Brick connection and Scan-Back σ~ 74 μm • Tracksfound in CS are lookedfor in the first downstream emulsion film of the brick • (CS-to-Brick connection) σ~ 8 mrad Pupilli Fabio - XV Lomonosov Conference • Connectedtracks are followed back in the filmsof the brick, alignedwithlateralX-raymarks, tiltheirstoppingpoint • (Scan-Back) σ~ 12 μm

13. Volume scan and vertexreconstruction • Scan 1 cm2 x (5 up + 10 down) platesaround the stoppingpoint, tosearchforallbase-tracks • Fine alignmentofemulsionfilmsbycosmicraytracksaccumulatedbeforebrickdismounting and volume tracksreconstruction • Interactionpointidentificationas the minimum distancepointoftwo or more tracks; vertexreconstruction Pupilli Fabio - XV Lomonosov Conference Good agreement with Monte Carlo simulationof the reconstructionchain Location of the primary vertex of NC events within the brick Angular distribution of tracks attached to the vertex θy θx ν

14. DecaySearch • Searchfordecaytopologiesexploiting the verygoodangular and position resolutions: • In-trackdecaysearch: searchforkinkalongtracksattachedto the vertex • Extra-trackdecaysearch: searchfortrackswithlarge Impact Parameter (IP) wrttovertex • IP distribution for: • - nt events (MC) • NC+ nmCC events (MC) • NC+ nmCC events (Data) The Impact Parameter has an average value of 2.3 µm (real events) Pupilli Fabio - XV Lomonosov Conference

15. Kinematical info fromemulsionanalysis • Momentummeasurementby Multiple Coulomb Scattering in lead/emulsion sandwich [see arXiv:1106.6211] Comparison with electronic detector measurement • Gamma detection and π0 mass reconstruction: • EM shower energy measured by NN shower shape analysis and Multiple Scattering method Pupilli Fabio - XV Lomonosov Conference 2 EM showers give a reconstructed mass ~ 160 MeV 35 gamma pairs E = 8.1 GeV E = 0.5 GeV 1 σ mass resolution: ~ 45% E = 0.5 GeV

16. Info fromemulsionanalysis:discrimination of interaction/decay kinks by search for highly ionising fragment tracks • Hadronic re-interaction background identification by the detection of highly ionising fragments produced in hadron interactions • 24 justaxposed 2.5 mm × 2.1 mm layers of high resolution microscope tomographic images analysed in the upstream and downstream films of an interaction vertex Nuclear Fragments h • Procedure tested studying a sample of 64 interactions in a OPERA brick exposed to an 8 GeVπ-beam h ν Fragments angle distribution Pupilli Fabio - XV Lomonosov Conference Data/MC (Fluka) comparison NC Fraction of events with fragments:Data (56 ± 7)%, MC 53%

17. A veryinterestingexample: the first ντ candidate a kink is detected Large area scanning Full reconstruction of vertices and gammas Scan-back in ECC CS predictions CS predictions Pupilli Fabio - XV Lomonosov Conference Futher details can be found on the talk of Antonio Ereditato [see also Phys. Lett. B691 (2010) 138]

18. Resultsummary • Analysisof 2008/2009 run (92% completed) [seearXiv:107.2594] • The global analysispicture[updated @ 29/06/2011] 2011 Pupilli Fabio - XV Lomonosov Conference 2010 2009 2008

19. Conclusions • Thanks to their high resolution, nuclear emulsions play a key role in the OPERA hybrid detector, which aims at the direct observation of ντappearance in CNGS beam. • High speed automatic scanning systems have been developed within the collaboration to guarantee a quasi on-line analysis of neutrino events. • Dedicated procedures have been setup for each analysis step (from CS to vertex), to ensure a micrometric resolution in position and an angular resolution of the order of mrad, needed fot the τdecaytopologicalidentification. • Exploiting their high resolution, nuclear emulsions are used for kinematical analysis and for hadronic background reduction, through the identification of nuclear fragments. • The OPERA nuclear emulsion analysis chain has proven to be successful, leading to the reconstruction of more than 3000 neutrino interactions and of the first ντ candidate. Pupilli Fabio - XV Lomonosov Conference