Study of Neutrino Oscillation in the OPERA Experiment

1. Study of Neutrino Oscillation in the OPERA Experiment Tomokazu Matsuo (Toho University) on behalf of the OPERA collaboration* *) 140 physicists, 28 institutions in 11 countries

2. ~1 mm Motivation and Concept First detection of neutrino oscillation in appearance modeusing νμ→ντby tau identification to confirm disappearance results of SK1998. - • Event‐by‐event identification of tau neutrino interaction • Detection of τdecay topology to identify ντ • High spatial resolution required    -, h-, e- decay “kink” SK 1998  oscillation   • Requirements: • long baseline • high energy neutrino • large mass • τidentificationcapability plus 3-prong decay modes Tau ID by DONuT 2001

3. The OPERA experiment A νμ beam was generated at CERN CNGS beam line and detection of neutrino oscillation was performed at LNGS in Italy, at a distance of 730 km. beam compositions

4. OPERA detector Total 1.25 kton, ~150000 ECC bricks SM1 SM2 Brick walls + Target Tracker(TT) Muon Spectrometer RPC+ Drift Tubes (HPT) 730km νμbeam ~20m Target Tracker(TT) Target area Muon Spectrometer Target area Muon Spectrometer Muon ID, momentum and charge measurement Lead plate Emulsion film

5. Location of neutrino interactions in ECC brick Event viewer of OPERA target area 3m 4m 5m CS • Find the brick containing the neutrino interaction using TT hits • Check CS to confirm the brick • Follow uptracks to interaction vertex in the ECC brick • Scan 10 films around interaction vertex (blue box) TT

6. Detection of decay topologies Scan 1cm×1cm around vertex 1cm IP distribution Search track which have large Impact Parameter (IP) τ daughter from ντ(MC) NC+CC νμevents (MC), NC+CC νμevents (Data) Pick up track chains Search vertex topology IP IP > 10μm + 0.01Δz Δz: distance to the vertex track segment

7. Beam exposure and analysis status Run 2008 → 2012 p.o.t. event ‘08 ‘09 ‘10 ‘11 ‘12 CS scanned 14737 CS found 10585 ECC scanned 9629 Located 6067 DS 4969 date date Beam: 5year (965days) 17.97×1019 p.o.t. Overall 80% of the proposal value (22.5×1019p.o.t.) Completed: 2008, 2009 2010-12 on goingwith optimisedstrategy Located: 6067, Decay search: 4969 ~60% of expected value done

8. 1st / 2ndντcandidate events 1stντcandidate event Reported in May 2010 Decay channel: τ→ 1h (τ→ ρντ, ρ→π0π, π0→ 2γ) 2ndντcandidate event Reported in June 2012 Decay channel: τ→ 3h

9. The 3rd candidate event A new ντcandidate event(3rd) Reported first in March 2013 Decay channel: τ→μ

10. The 3rd candidate event Topology of the τ→μ decay event 2 primary track 376 μm pτ pri. h 3 e-pair ph total 4 τ candidate φ=154.5° 1 e-pair muon film 39 film 40 film 41 film 38 film 42 • All variables passed the kinematical cuts of OPERA • γis attached to the primary vertex

11. The 3rd candidate event TT hits of new ντcandidate event(3rd) TOP SIDE zoom

12. The 3rd candidate event Charge sign of μ from curvature First charge measurable event Target Tracker hits P2<0 → negative charge 5.6 σ significance R ~ 85 cm X B Muon momentum by range in the electronic detector: 2.8±0.2 GeV/c MCS in the brick consistent 3.1 [2.6,4.0] GeV/c RPC hits P-value = 0.063% (probability to reconstruct a m+ stopping in the 7th iron layer with p2 < -0.00389 cm-1) Negative μ: 5.6σsignificance The oscillated neutrino is ντ(not ντ)

13. Background sources • Charm: • Charm hadron decaysin νμCC interactions • μis not identified • Hadron re-interaction: • Hadron interacts in a lead plate or an emulsion film close to the primary vertex • nuclear fragments are not observed μ- → ←μ+ Real charm events in OPERA Test experiment • Muon scattering: • Muon interacts near the vertex and exhibits a kink topology with a large angle π- π-

14. Background estimation (hadron) Error bars : Experimental data Histogram : Simulated data 2GeV/c 4GeV/c 10GeV/c We confirmed MC simulation estimation of hadron backgrounds by using the data analysis of ECC bricks exposed to 2GeV/c , 4 GeV/c and 10GeV/c pion beams and reduced systematic uncertainty. 2GeV/c 4GeV/c 10GeV/c Multiplicity Multiplicity Secondary track emission: 30% nuclear fragments:10% MC: b < 0.7 0 5 0 5 0 8 0 15 0 15 0 15 Well produced by MC They argee well Kink angle (1-prong) Emission angle(cosq) 0 0.6 0 0.6 0 0.6 0° 0° 0° 180° 180° 180° Backward Forward Backward Forward Backward Forward New technique for background reduction Nuclear fragment associated probability 100% ・ data - MC nuclear fragments in emulsion 10GeV/c 2 8 4 0% Hadronic background was reduced by “40%”by requiring no association of large angle nuclear fragments.

15. Statistical considerationsExtended sample to muonic interactions 3 observed events in the τ → h(1st), τ → 3h(2nd)andτ → μ(3rd)channels Pvalue = P0 = 1.125 x 10-4 Probability to be explained by background = 7.29×10-4 This corresponds to 3.2 σ significance of non-null observation by simple counting method. At a first likelihood approach, a 3.5σ level significance has been calculated.

16. New developments Large angle scan High speed scan 90% 80% |tanθ|=3.0 Angle limit of normal scan Extended limit more than 6x Speed: 0.6m2/h (under construction) → speed up, increase location efficiency Scan angle: |tan θ| < 4.0 → reduce background efficiency of large angle MIP particle Future prospect: νfrom the bottom 1 2 3 possibility of off-timing cosmic ray event analysis, such as atmospheric neutrino oscillation

17. Conclusions • 3 ντcandidate events have been found. • The 3rd event is τ-→μ- decay channel. • Background have been evaluated more precisely with test experiment analysis. • With a first likelihood approach, a 3.5σ level has been calculated. • 4σ observation is within reach.