OPERA: A First ντ Appearance Candidate

Transcription

1 OPERA: A First ντ Appearance Candidate Björn Wonsak On behalf of the OPERA collaboration. 1

2 Overview The OPERA Experiment. ντ Candidate Background & Sensitivity Outlook & Conclusions 2/42

3 Overview The OPERA Experiment. ντ Candidate Background & Sensitivity Outlook & Conclusions 3/42

4 OPERA: Oscillation Project with Emulsion tracking Apparatus Neutrino beam (νμ) from CERN to Gran Sasso underground lab (Italy) Goal: Goal: Observation of ντ appearance 732 km LNGS vµ vτ? Physics runs: 2008, 2009 and 2010 completed. 4/42

5 CERN Neutrinos to Gran Sasso Beam Beam Characteristics: p.o.t./year <Eν> 17 GeV L 732 km (νe+νe)/νµ 0.87% νµ/νµ 2.1% ντ/νµ negligible (~10-7) Total exposure expected: p.o.t. 5/42

6 Detector Concept Goal: Direct observation of ντ in νµ beam νµ.. ντ Oscillation τ- + X CC- interaction τ decay B.R.: µ- ντ νµ 17.7 % h- ντ (neutrals) 49.5 % e- ν τ ν e 17.8 % h+h-h- ντ (neutrals) 15.0 % OPERA has to look for this special topology decay kink 6/42

7 Background Processes Most important background processes: Charm production and decay Hadron re-interactions in lead Large-angle muon scattering in lead Signal ντ CC Background µ, e, hadron τ νµ CC + charm prod. charm µ muon misidentified Use Emulsion Cloud Chambers (ECC) to achieve a high enough spatial resolution and density. 7/42

8 The OPERA Brick Sandwich of 56 Pb sheets 1mm + emulsions High spatial resolution (track: σx 0.05μm, σθ 2mrad, vertex: σx 1μm) Changeable Sheets (CS) with emulsion doublet for first checks 1 mm 10 X0 τ CS ν 8.3kg Pb emulsion layers plastic base 200µm 45µm each 10.2 x 12.7 x 7.5 cm3 ECC = Stand-alone Detector: Momentum measurements for hadrons (multiple scattering) Pion/Muon separation at low energy (de/dx) Electron identification and energy measurements for e,γ reconstructed electromagnetic shower 14 mm 14 mm 8/42

9 Hybrid Detector Emulsion Cloud Chambers 9/42

10 The OPERA Detector 10/42

11 The OPERA Detector Super Module 1 (SM1) Super Module 2 (SM2) 11/42

12 The OPERA Detector Target SM2 Target SM1 Target Region: - Target Tracker (Scintillator) - Lead/Emulsion Bricks ( per SM) Target mass: ~1.25 kton 12/42

13 The OPERA Detector Target μ v X B B Magnetic Spectrometer: Magnet Region: Iron & RPCs Precision Tracker: 6 planes of drift tubes 13/42

14 Expected Performance (Proposal) Assumptions: Maximal mixing, 22.5x1019p.o.t. 4.5x1019p.o.t./year) τ Decay Channel B.R. (%) Signal Background τ µ τ e τ h τ 3h Total Expected Events: ~ νμ CC+NC interactions ~ 520 νμ interactions ~ 205 νe + νe interactions ~ 115 ντ CC interactions For full mixing and m2 = 2.5 x 10-3 ev2 (scales with ( m2)2). 14/42

15 Overview The OPERA Experiment. ντ Candidate Background & Sensitivity Outlook & Conclusions 15/42

16 CNGS Beam Performance & Statistics x1019 p.o.t. no bricks Commissioning x1019 p.o.t. 38 events Commissioning x1019 p.o.t events (scan input) First physics run x1019 p.o.t events (scan input) Physics run x1019 p.o.t. (23.Nov.) 4246 events (scan input) Physics run pot 9637 events (scan input) collected until 23/11/ Days 16/42

17 Event Statistics (June 2010) Analysis of first data: 5391 events collected by run Brick tagging efficiency times vertex location efficiency: ~60% 1617 neutrino vertices located (50% of statistics) 1088 events with decay search completed (187 NC and 901 CC events) Corresponds to 1.89x1019p.o.t. 35% of events Expected Signal: 0.5 ντ events (for full mixing and m2=2.5x10-3 ev2) 17/42

18 Muonless Event Electronic Detector View: (Date: 22 August 2009, 19:27 (UTC)) 18/42

19 From CS to Vertex Localisation CS prediction Scan-back in ECC 19/42

20 From CS to Vertex Localisation CS prediction Scan-back in ECC 20/42

21 From CS to Vertex Localisation Scan-back in ECC 21/42

22 From CS to Vertex Localisation Kink detection Scan-back in ECC 22/42

23 From CS to Vertex Localisation Large-area scan, full reconstruction of vertices and γ 23/42

24 Reconstructed ντ Candidate 24/42

25 Reconstructed ντ Candidate PL17 PL18 PL19 PL20 PL Main questions: Is there a muon? Attachment of γ1/2? Cuts fullfilled? Primary vertex kink point (secondary vertex) τ (4) 7 3 γ2 (further out) γ1 Daughter (8) p (2) 6 1mm lead 25/42

26 Track Analysis Vertex tracks followed down (through several bricks) to assess the muonless nature of the event. Residual probability of νμcc event (due to a possibly undetected large angle muon) ~1%. Nominal value of 5% assumed! 26/42

27 Event Track Features Muonless event (favored hypothesis) 27/42

28 γ-attachment to Vertices Distance from IP to 1ry 2ry vertex (mm) vertex (µm) <resolution> IP to 2ry vertex Prob. of Prob. of Attachment hypothesis attach. to 1ry attach. to 2ry (µm) vtx* vtx* <resolution> 1st γ <11> 7.5 <7> < ry vertex 2nd γ <56> 22 <50> ry vertex (favored) * Probability to find an IP larger than the observed one γ2 2ry vertex 1ry vertex γ1 Pointing resolution (1σ) for a given γ: Function of scattering and distance 28/42

29 Kinematical Varianbles of Candidate Satisfying all selection criteria for hadronic kink first ντ candidate! γ1 and γ2 are both assumed as attached to 2ry vertex The uncertainty on Pt due to the alternative γ2 attachment is < 50 MeV 10 years old criteria Blind analysis 29/42

30 Kinematical Varianbles of Candidate Satisfying all selection criteria for hadronic kink first ντ candidate! γ1 and γ2 are both assumed as attached to 2ry vertex The uncertainty on Pt due to the alternative γ2 attachment is < 50 MeV >600 if no γ 10 years old criteria Blind analysis 30/42

31 Characteristics of Decay Topology Decay length Red regions: Measured values for ντ candidate cut mm Daughter momentum Kink angle reject < 20 mrad cut GeV/c rad 31/42

32 Kinematical Cuts to be Passed NC (MC) τ (MC) Missing Pt at primary Missingvertex Pt at primary vertex Reject NC events with larger missing Pt (neutrino) cut cut GeV/c Pt at decay vertex Reject hadron re-interactions cut GeV/c /42

33 Pt Characteristics X (hadron shower) Signal : small φ x φ =180o τ-decay ντn BG: τντ φ φ ππ- νµ τ-x kink πνµn νµπ-x NC (MC) φ τ (MC) cut rad 33 33/42

34 Interpretation of the Event Invariant mass of γγ-system: Compatible with π0 mass value Invariant mass of the πγγ-system: Compatible with ρ (770) πo mass 120 ± 20 ± 35 MeV ρ mass MeV ρ is created in about 25% of the τ decays: τ ρ (π π0) ντ OPERA collaboration: Observation of a first ντ candidate event in the OPERA experiment, Phys. Lett. B 691 (2010) /42

35 Overview The OPERA Experiment. ντ Candidate Background & Sensitivity Outlook & Conclusions 35/42

36 Expected Background Prompt ντ ~10-7/CC Decay of charmed particles produced in νe interactions ~ 10-6/CC Double charm production ~ 10-6/CC Decay of charmed particles produced in νµ interactions ~ 10-5/CC Hadronic reinteractions ~ 10-5/CC 36/42

37 Significance of ντ Observation We observe 1 event in the 1-prong hadronic τ decay channel Background expectation for 1-prong hadron decay: ± (syst) events Probability that the observed event is due to background: 1.8 % Significance of ντ observation in OPERA: 2.36 σ Total background from all decay modes: ± (syst) events Probability that the observed event is due to background: 4.5 % Significance of ντ observation in OPERA: 2.01 σ 37/42

38 Overview The OPERA Experiment. ντ Candidate Background & Sensitivity Outlook & Conclusions 38/42

39 Conclusions OPERA searches for νµ ντ appearance. A complete analysis of a subsample has been done. One muonless event with a τ 1-prong hadron decay topology has been detected. 39/42

40 Conclusions The event passes all kinematical cuts. It is our first candidate event for νµ ντ appearance. m223 values > 7.5 x 10-3 ev2 can be excluded at 90% CL The probability for the event to be background induced is 1.8% σ significance (If all decay modes are included: 4.5% 2.01 σ significance) 40/42

41 Outlook 2010: Getting close to nominal 4.5x1019p.o.t. 2011: Partial compensation expected for the 2012 break 2012: LHC stop? no SPS, no p.o.t. We need enough p.o.t. (22.5x1019) to obtain a significant (4σ) result with high probability we need a run 2013, if SPS is stopped 2012 All events of 2008 and 2009 scanned by end of Waiting for more ντ candidates 41/42

42 Thank you for your attention! 42/42

43 Backup Slides: 43/42

44 Emulsion Scanning 12 scanning laboratories in Europe and Japan Field of view 300µm 2 emulsion sides (45 µm) 300µm 16 tomographic images = 45µm 2D images processing 3D reconstruction of particle tracks Speed : 20cm²/hour/emulsion side 1 plastic base (200 µm) (in Japan >60cm²/hour) 1 emulsion sheet (2 sides) 12h scanning ~5GB 44/42

45 Emulsion Scanning The frames correspond to the scanning area: Yellow short lines: Measured tracks Other colored lines: Interpolation or extrapolation 1 cm Vertex reconstruction & kinematical analysis 3D track segments found in consecutive plates 45/42

46 Emulsion Scanning The frames correspond to the scanning area: Yellow short lines: Measured tracks Other colored lines: Interpolation or extrapolation 1 cm Vertex reconstruction & kinematical analysis Passing-through and low energy tracks rejected 46/42

47 Emulsion Scanning The frames correspond to the scanning area: Yellow short lines: Measured tracks Other colored lines: Interpolation or extrapolation 1 cm Vertex reconstruction & kinematical analysis 47/42

48 4-σ evidence OPERA Discovery probability (%) 3-σ evidence MINOS 2008 m2 (10-2 ev2) 48/42