Recent T2K results on CP violation in the lepton sector

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1 Recent T2K results on CP violation in the lepton sector presented by Per Jonsson Imperial College London On Behalf of the T2K Collaboration, December , Fort Lauderdale, USA.

2 Outline Neutrino Oscillations The T2K Experiment Beam Near detectors Far detector Results θ23and Δm232 Electron anti-neutrino appearance δcp and mass hierarchy Future Plans Conclusions 2

3 Three flavour mixing in lepton sector The neutrino flavour eigenstates are mixtures of the mass eigenstates Pontecorvo-Maki-Nakagawa-Sakata Mixing Matrix PMNS 3

4 Neutrino Oscillations Neutrino oscillations discovered in 1998 Neutrinos have non-zero mass and mixing angles Many open questions: What is the neutrino mass hierarchy? Is Δm231 positive or negative? CP symmetry is violated if δcp 0,π Is the θ23 mixing angle maximal? Δm221 = (7.53 ±0.2)x10-5 ev2 Δm232 = (2.44 ±0.06)x10-3 ev2 sin2θ12= ±0.01 (θ12~33 ) sin2θ23= ±0.06 (θ23~45 ) sin22θ13= ±0.005 (θ13~8 ) Is there CP violation in the lepton sector? PDG2015 If not, which quadrant? What are the precise values of the mixing angles θij? Majorana or Dirac? Δm2ij= m2i-m2j 4

5 Neutrino oscillation probabilities at T2K E=0.6 GeV Muon neutrino survival probability: Sensitive to θ23 and Δm232 Electron neutrino appearance probability (expansion by α = Δm221 / Δm231): L=295 km Comparing neutrino and anti-neutrino disappearance: test of CPT symmetry For anti-neutrinos, replace δ and x with -δ and -x Leading term depends on θ13 and θ23, CP-violating phase δ => P(ν ν ) P(ν ν ), μ e μ e Matter effect gives sensitivity to mass hierarchy: sign of x. 5

6 Effect of CP violation at T2K Asymmetric effect: δcp = -π/2 maximizes P(νμ νe), minimizes P(νμ νe) δcp = +π/2 minimizes P(νμ νe), maximizes P(νμ νe) Effect of δcp and Mass Hierarchy on appearance probability is similar. Size of effect: δcp: 0 to (±) 20% effect Mass hierarchy: ±10% 6

7 The T2K Collaboration The T2K collaboration includes about 500 members from 11 countries (Canada, Include France, Germany,info Italy, collaboration herejapan, Poland, Russia, Spain, Switzerland, UK, USA). 7

8 The T2K Experiment In Japan Off-axis ν beam 2.5 μ Two production modes: Neutrino and anti-neutrino Neutrino flux peaks at 0.6 GeV Less than 1% νe under the peak 280 m νμ /νe νμ SK: 50 kt water, 22.5 kt fiducial INGRID ND280 8

9 J-PARC Linac Rapid Cycling Synchrotron Near Detector To Kamioka Neutrino Main Ring Bird s eye view of J-PARC 17 July

10 T2K Beam Line 30 GeV proton beam from J-PARC main ring 90 cm graphite target Neutrinos produced from pion and kaon decays For anti-neutrinos: Invert focusing-horn polarity which selects the charged pions and kaons Dedicated hadron production measurements from NA61/SHINE crucial for T2K (anti)neutrino flux predictions Off-axis beam: centre of beam direction 2.5 off from direction to SK Narrow beam energy peak around oscillation maximum, Fewer high-energy neutrinos, so less problematic background 10

11 Near Detectors: INGRID On-axis detector, Consists of 16 modules, 7 horizontal, 7 vertical and 2 off diagonal, each module is a cube of 1 m3, Each module is a sandwich of 11 scintillator and 10 iron layers, surrounded by 4 veto planes, Neutrino beam centre is obtained with accuracy of ~ 0.1 mrad from horizontal/vertical distribution of neutrino event rate, 10mrad Horizontal 10mrad Vertical Provides cross section measurements and constraining beam flux 11

12 Near Detectors: ND280 Off-axis detector 2 Fine-Grained Detectors (FGDs) of scintillator bars, one has water target, neutrinos 3 gas-filled TPCs to reconstruct and identify charged particle tracks 0.2 T magnetic field used for charged particle momentum determination P0D: π0 detector ECAL: Electromagnetic CALorimeter SMRD: Side Muon Range Detector Information on neutrino flux and interactions from reconstructed tracks ND280 strongly reduces systematic uncertainties of T2K oscillation analysis 12

13 Far Detector: Super Kamiokande Situated 1000 m under ground in Kamioka mine, 295 km from J-PARC 50 kt water Cherenkov detector with a fiducial volume of 22.5 kt Inner Detector is instrumented with inch PMTs Outer Detector has inch PMTs 41.4 m 39.3 m 13

14 Particle ID and Event Selection νμ μ νe e Sharp Cherenkov ring, only direct light from μ Fuzzier Cherenkov ring, due to scatter/em shower Excellent μ/e separation Probability to misidentify a muon as an electron < 1% Select 1 ring events for analysis νμ selection e-like PID p > 100 MeV/c e No Michel electron E < 1.25 GeV rec Pass π0 rejection μ-like PID p > 200 MeV/c μ At most 1 Michel electron νe selection 14

15 Data Sample POT=Protons On Target Results with full good quality data up to May 27: ν-mode: 7.48x1020 POT ν-mode: 7.47x1020 POT Continuous rise in beam power:~225 kw (2014) ~420kW (2016) Accumulated almost the same number of POT in ν- and ν-mode 15

16 Analysis Method NND~(σΦ)NDεND NSK ~(σφ)skεskposc ΔNSK/NSK~ 5% θ13, θ23, Δm231/32, δcp (θ12 and Δm221 fixed from other experiments) 16

17 Systematic Uncertainties Expected events at SK as a function of reconstructed neutrino energy ND280 reduces the systematic uncertainties on expected neutrino events at SK from 1214% to 5-6% νμ νe νμ νe 17

18 Results The results presented in this talk are from the analysis of the combined T2K neutrino and antineutrino data samples (νμ,νμ,νe,νe) with four parameters (δcp, θ13, θ23, Δm232) fitted simultaneously. 18

19 νμand νμdisappearance Neutrino mode Anti-neutrino mode νμ νμ νμand νμ results are consistent Disappearance and distortion of energy spectrum for both ν and ν μ Preliminary μ Previous publications: PRL (2014) PRD (2015) PRL 116, (2016) No evidence of CPT violation 19

20 Constraints on θ23and Δm

21 νe and νe appearance Anti-neutrino mode Neutrino mode νe Clear appearance signal for νe More statistics needed for νe νe Previous publications: PRL 112, (2014) PRD 91, (2015) 21

22 Constraints on θ13 and δcp Number of νe and νe candidates compared with predictions : Number of observed events shows larger asymmetry than expected for δcp = -π/2 and NH T2K-only data fit θ13in agreement with reactor experiments T2K begins to probe δcp Reactor experiment constraint from PDG 2015 (sin2 2θ13= ±0.005) shown δcp ~ -π/2 and NH preferred T2K disfavors region of δcp = +π/2 22

23 Constraints on δcp 90% CL constraints on δcp from Feldman-Cousins method Reactor constraint: sin2 2θ13= ±0.005 (PDG 2015) Best fit gives δcp= , Normal Hierarchy Allowed 90% CL The allowed 90% CL intervals are: < dcp < (NH) and < dcp < (IH) CP conserving values δcp= 0 and p are excluded at 90% C.L. CP conservation hypothesis excluded at 90% CL 23

24 Future Plans 24

25 T2K-II T2K: ends when 7.8x1021 POT is reached (expected around 2021) T2K-II: proposed extension up to 2026 for 20x1021 POT Stage 1 status at this summer's J-PARC PAC (Program Advisory Committe) Plan to gradually increase the beam intensity (currently ~ 400 kw) up to 1 MW in 2021 J-PARC accelerator and beam-line upgrade: Beam power up to 1.3 MW in ~2026 Upgrade of near detectors to improve systematic uncertainties Possible increase of SK fiducial volume T2K-II targeted scenario A step towards Hyper-Kamiokande [arxiv: ] 25

26 Physics Potential of T2K-II Unknown Mass Hierarchy Sensitivity for sin2θ23=0.43 Mass Hierarchy Known With full T2K-II statistics able to: Exclude CP conservation hypothesis at more than 3σ if δcp ~-π/2 Measure θ23 with resolution ~1.7 26

27 Conclusions Since 2010 T2K has accumulated ~ 1.5x1021 POT, 19% of expected total, split equally in neutrino and anti-neutrino mode First search for CP violation in the lepton sector with analysis using both neutrino and anti-neutrino data (νμ/νμ disappearance, νe/νe appearance) Leading results for θ23 and Δm232 CP conservation hypothesis excluded at 90% CL Data prefer maximal (θ23= 45 ) νμ/νμ disappearance δcp = [-3.13,-0.39] NH, [-2.09,-0.74] IH Expect to double neutrino data by summer 2017 and to reach 7.8x1021 POT around 2021 Proposed extension to T2K: Accumulate 20x1021 POT by 2026, Reach > 3σ sensitivity to δcp ~-π/2 27

28 BACKUP 28

29 Flux Prediction Neutrino and anti-neutrino flux prediction tuned with hadron spectra from NA61/SHINE Flux uncertainty reduced from ~30% to ~10% (thin target data) Less than 1% electron (anti)neutrino component at the peak, <10% wrong-sign background, Using predictions of flux correlations between near/far, neutrino/anti-neutrino beam,νμ /νe 29

30 NA61/SHINE experiment at CERN SPS Large-acceptance detector with very good capabilities of charge and mass measurements Located in the CERN North Area Cover almost full T2K {p,θ} phase space Measure pion, proton and kaon production with 31 GeV/c proton beam on carbon target Thin 2cm target (Eur. Phys. J. C 76, 84 (2016)) T2K replica target (published π± yields: Eur. Phys. J. C 76, 617 (2016)) 30

31 Systematics Improvements given by measurements with ND280 data 31

32 Near detector neutrino sample for systematics constraint - 1 μ candidates Neutrino samples, best-fit distributions 32

33 Near detector anti-neutrino sample for systematics constraint Anti-neutrino samples, best-fit distributions 1 μ+ candidates 33

34 SK νμ /νμselection No magnetic field: same selection for neutrino and anti-neutrino beam 34

35 SK νe /νe selection No magnetic field: same selection for neutrino and anti-neutrino beam 35

36 SK spectra Predicted spectra at SK compared to candidate events Energy reconstructed assuming 2-body kinematics 36