1. Recent results from the T2K long baseline neutrino experiment K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Kendall Mahn For the T2K collaboration 2012/02/271

2. Canada TRIUMF U of Alberta U of B Columbia U of Regina U of Toronto U of Victoria U Winnipeg York U France CEA Saclay IPN Lyon LLR E Poly LPNHE-Paris Germany RWTH Aachen U Italy INFN Bari INFN Roma Napoli U Padova U Japan ICRR Kamioka ICRR RCCN KEK Kobe U Kyoto U Miyagi U of Ed Osaka City U U of Tokyo Korea Chonnam Nat’l U Dongshin U Seoul Nat’l U Poland NCBJ IFJ PAN T U Warsaw U of Silesia Warsaw U Wroclaw U Russia INR Switzerland Bern ETH Zurich U of Geneva UK U of Oxford Imperial C London Lancaster U Queen Mary U of L Sheffield U STFC/RAL STFC/Daresbury U of Liverpool U of Warwick USA Boston U Colorado State U Duke U Louisiana State U Stony Brook U U of California, Irvine U of Colorado U of Pittsburgh U of Rochester U of Washington The T2K Collaboration Spain IFIC, Valencia IFAE, Barcelona K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Italy INFN Bari INFN Roma Napoli U Padova U 2012/02/272

3. A refresher on neutrino mixing K Mahn, Les Rencontres de Physique de la Vallée d'Aoste The flavor state of the neutrino, ν α, is related to the mass states, ν i, by a mixing matrix, U αi Since there are three observed flavors of neutrinos (  e,  ,   ), U contains three mixing angles (  12,  23,  13 ) and a CP violating phase  “Solar”:  12 ~34° “Atmospheric”:  23 ~37°-53° “CP sector”:  13 <11° c ij = cos  ij, s ij = sin  ij Open questions:  Is  23 exactly 45 degrees, or not?  Is  13 zero, or just small?  Is there CP violation in the neutrino sector? Rather different from quark mixing:  Nearly diagonal unitary matrix  Small angles:  CKM 12 ~13.0°,  CKM 23 ~2.3°,  CKM 13 ~0.2° 2012/02/273

4. Neutrino oscillation: ν μ disappearance K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Because of neutrino mixing, as the neutrinos propagate, the mass states interfere: At the atmospheric Δ m 2 32 ~ 10 -3 eV 2, a beam of   will: Probability to observe   after starting in flavor state   depends on:  L (km): Distance the neutrino has travelled  E (GeV): Energy of the neutrino  Δ m 2 (eV 2 ): Difference of the square of the mass eigenvalues 2012/02/274 ‘’disappear’’ into other neutrino flavors A small fraction of  e will ‘’appear’ ’ Δ m 2 31 ~ Δ m 2 32

5. Produce a beam of   on one side of Japan and detect it on the other The Tokai-to-Kamioka (T2K) experiment K Mahn, Les Rencontres de Physique de la Vallée d'Aoste T2K is designed to measure oscillations at the atmospheric Δ m 2 :  Measure   disappearance (Δm 2 32,  23 )  Aim to discover  e appearance (  13 ) Far detector: Super-Kamiokande located near Kamioka Beam source and near detectors: J-PARC accelerator complex located in Tokai-mura 2012/02/275

6. Creating an (offaxis) neutrino beam K Mahn, Les Rencontres de Physique de la Vallée d'Aoste 30 GeV protons hit a target (carbon) producing secondary mesons (π, K) which decay to a tertiary   beam  Collected 1.43 x 10 20 POT (2% of T2K goal) T2K uses a novel off-axis beam technique:  Off the primary neutrino beam direction, neutrino energy spectrum is narrower, thanks to pion decay kinematics  Peak can be set to ~oscillation maximum  Reduces backgrounds from higher energy neutrino interactions 2012/02/276

7. Neutrino detectors K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Off-axis Super-Kamiokande @295km: Off-axis ND280 detectors @ 280m: Neutrino detectors of T2K On-axis INGRID detector @280m: Measure  beam direction, rate with high statistics  event sample Real-time monitor stability of beam Neutrino detectors measure  ,  e candidates before and after oscillation: proton beam direction 2012/02/277

8. Neutrino interactions at T2K K Mahn, Les Rencontres de Physique de la Vallée d'Aoste W l ν N N’ Δ π CC π + / CC π 0 W CCQE l ν p n Z ν N N’ Δ π ν NC π + /NC π 0 Primary interactions are Charged Current Quasi-Elastic events  Neutrino flavor determined from flavor of outgoing lepton i.e. e for  e,  μ for    Reconstruct neutrino energy from outgoing lepton CCπ (single pion production) and NCπ are backgrounds    disappearance: Same as CCQE if pion is not identified   e appearance: NC backgrounds are flux dependant and can mimic a CC  e interaction  Final state interactions also alter how the underlying event is observed, e.g. absorption or charge exchange of π + NC elastic CC other 2012/02/278

9. Off-axis near detectors: ND280 P0D ECAL Barrel ECAL K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Suite of near detectors sit within UA1 (B=0.2T, 850 tons) magnet  Current analysis use Tracker (TPCs+FGDs) to measure the unoscillated CC ν μ rate  Neutrinos interact on FGDs (water or carbon)  TPC information used to measure outgoing lepton angle, momentum and flavor  Future analyses will use electromagnetic calorimeters, muon range detectors in magnet and P0D (π 0 detector) W νμνμ μ μ νμνμ TPC2 TPC3 FGD1 FGD2 CC   candidate Downstream ECAL 2012/02/279

10. Far detector: Super-Kamiokande (SK) K Mahn, Les Rencontres de Physique de la Vallée d'Aoste  50 kton water Cherenkov detector  Inner tank lined with 11,129 PMTs (40% photocathode coverage)  Outer veto region has 1885 PMTs to reject particles entering the tank  Charged leptons from neutrino interactions emit Cherenkov light recorded as rings on the inner tank wall by PMTs  Angle, momentum of lepton can be reconstructed from PMT pulse height and time information  Muons from CC interactions produce well defined rings  Electrons scatter and produce ``fuzzy`` rings 2012/02/2710

11. T2K ν μ disappearance method K Mahn, Les Rencontres de Physique de la Vallée d'Aoste For a fixed baseline (L=295km) oscillation probabilities depend on the neutrino energy E  Disappearance observable as distortion to neutrino energy spectrum and reduction of overall rate at far detector Note different scale! 2012/02/2711

12. T2K ν μ disappearance method K Mahn, Les Rencontres de Physique de la Vallée d'Aoste  Calculate unoscillated event rate using flux (Φ), cross section (σ) predictions and detector (ε) efficiencies  Correct far detector   spectrum (N’ FD ) by observed ND280 rate R(CC   ) to reduces flux, cross section uncertainties and set normalization  Extract Δ m 2, sin 2 2  23 oscillation parameters from a fit to N’ FD * Formula is a simplification, ignores summations over neutrino flavor types and interaction modes 2012/02/2712

13. Unoscillated flux at Super-K is ν μ  Also   (~6%) and    components (~1%) Neutrinos at peak from π + decay, high energy flux from K decay Neutrino flux prediction K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Geant3/FLUKA based flux prediction Total   signal/ND uncertainty: 4.8%  Δm 2 32 =2.4 x 10 -3 eV 2 sin 2 2θ 23 =1.0 Flux uncertainties (%) NDν e bkrd ν e / ND Proton beam 2.2% 0.0% 2.2% Pion production 5.7% 6.2% 2.5% Kaon production 10.0% 11.1% 7.6% Other hadronic interactions 9.7% 9.5% 1.5% Meson focusing, beam direction 2.8% 2.2% 0.8% Total15.4% 16.1% 8.5% Prediction and uncertainties are determined by external or in-situ measurements Largest uncertainty is π, K production from the target, constrained by NA61 exp’t Phys.Rev.C 84, 034604 (2011) 2012/02/2713

14. Neutrino interaction uncertainties K Mahn, Les Rencontres de Physique de la Vallée d'Aoste  FSI alters the energy dependence of how backgrounds are reconstructed  Modify π re-interaction probabilities within cross section model according to external data to determine FSI uncertainty   signal @ Δm 2 32 =2.4 x 10 -3 eV 2 sin 2 2θ 23 =1.0 External data on π + interaction cross sections Cross section model, uncertainties set from external data at E  ~ 1 GeV e.g. MiniBooNE, SciBooNE, K2K and constrained by ND280 rate σ uncert. (%) (after ND) ν μ signalν e bkrd CCQE nuclear model (@lowE) 2.5% 3.1% CC1π +0.4% -0.5% 2.2% CC coherent π - 3.1% CC other +4.1% -3.6% 4.4% NC all 0.9% - NC1π 0 - 5.3% NC coherent π - 2.3% NC other - 2.3% σ(ν e ) N/A 3.4% Final State Interactions (FSI) 6.7% 10.1% Total +8.3% -8.1% 14.0% 2012/02/2714

15. ND280 CC ν μ sample K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Reconstructed momentum and angle of the CC   candidates: 1.Require no tracks in TPC1 (veto  interactions upstream of FGDs) 2.Select events which originate in FGD1 or FGD2 fiducial volume 3.Use the highest momentum, negative TPC2 or TPC3 track 4.Select μ from TPC dE/dx information to determine flavor of neutrino Rate used to normalize expected number of events at far detector: Dataset shown here: 2.88 x 10 19 POT R(data/MC) = 1.036 ± 0.028 (stat) +0.044 -0.037 (detector sys) ± 0.038 (xsec model) R(data/MC) = 1.036 ± 0.028 (stat) +0.044 -0.037 (detector sys) ± 0.038 (xsec model) No tuning to flux or cross section applied 2012/02/2715

16. T2K far detector ν μ event selection K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Select CCQE   candidates: 1. Apply basic neutrino selection a)Event time within beam time window b)No veto activity c)Within fiducial volume d)Single reconstructed ring 2. Muon-like ring a)Sufficient momentum for e-  separation 3. 0 or 1 decay electrons, to reject CC1π a) Search for low energy electron events after primary event Data event in Super-K: single muon 2012/02/2716 CCQE Signal CCQE   Efficiency = 72% Background CCnonQE Rejection = 79% CC1π  μ- e-  μ- e- π

17. T2K far detector ν e event selection Similar selection for CC  e candidates: a)Basic neutrino selection b)Single electron-like ring c)0 decay electrons (reject CC   ) d)Remove E  >1.25 GeV events as beam CC   have higher average energy than signal K Mahn, Les Rencontres de Physique de la Vallée d'Aoste MC event: electron MC event: π 0 Additional background from NCπ 0 events, where π 0 decay to two photons (electron- like rings) e) Calculate invariant mass assuming 2 nd ring, and reject invariant mass consistent with π 0 2012/02/2717 Rejection efficiency for all NC bkrds: 99% Signal efficiency: 66%

18. T2K far detector selection uncertainties K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Evaluated with atmospheric  control samples detector uncertainties (%) ν e signal ν e bkgd Ring counting3.9%8.3% Electron PID3.8%8.0% Invariant mass5.1%8.7% π 0 rejection-3.6% Fiducial volume1.4% Energy scale0.4%1.1% Decay electron eff0.1%0.3% Muon PID-1.0% Total7.6%15% Total uncertainty for   analysis: 10.3% (predominantly ring counting) Evaluated with a special control sample  Select an atmospheric   candidate or decay electron event  Add a simulated photon to the event to create a ``hybrid π 0 ’’  Difference in π 0 rejection efficiency between hybrid sample and pure simulated π 0 sample is set as uncertainty Hybrid π 0 sample MC π 0 sample Invariant mass (MeV) 2012/02/2718

19. ν μ disappearance results K Mahn, Les Rencontres de Physique de la Vallée d'Aoste  31 events pass   selection criterion  103.6 +13.8 -13.4 expected for no osc, excluded at 4.5σ  Fit E  distribution for 2 flavor osc. parameters (binned χ 2 fit)  Best fit: | Δm 2 32 |=2.65 x 10 -3 eV 2 sin 2 2  23 = 0.98 Reconstructed energy E  (GeV) 2012/02/2719 MINOS: Phys. Rev. Lett. 101, 131802 (2008) Super-K: Phys.Rev.D71:112005 (2005) Summary of uncertainties ν μ signal Δm 2 23 =2.4 x 10 -3 eV 2 sin 2 2θ 23 =1.0 ν flux±4.8% ν interactions+8.3 -8.1% Near detector+6.2 -5.9% Far detector±10.3% Total+15.4 -15.1% Results accepted by PRD Rapid Comm.

20. ν e appearance at T2K K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Extract sin 2 2  13 by comparing observed number of  e candidates to expectation (counting experiment) Background# events beam  e 0.76   CC background 0.03 NC background0.61 osc through  12 0.09 total:1.49±0.34(sys) Signal (ν μ to ν e osc)# events @sin 2 2θ 13 =0.1,δcp=04.11 Uncertaintiesν e bkrdν e sig+bkrd ν flux±8.5% ν interactions±14.0%±10.5% Near detector+5.6 -5.2% Far detector±14.7%±9.4% Total+22.8 -22.7%+17.6 -17.5%   signal@ Δ m 2 32 =2.4 x 10 -3 eV 2, sin 2 2  23 =1.0 2012/02/2720 Event predictions normalized by ND280

21. ν e appearance results  6 candidate events observed for background of 1.49 ± 0.34  Probability to see 6 events or more for sin 2 2  13 =0 is 0.007 (2.5σ equivalent)  For |Δm 2 32 |=2.4 x 10 -3 eV 2 ; sin 2 2  23 = 1 Best fit: sin 2 2  13 = 0.11 0.03 < sin 2 2  13 <0.28 at 90% C.L. Δm 2 >0  Recent results are consistent with T2K: MINOS: sin 2 2  13 <0.12 at 90% C.L. Double Chooz: 0.017 < sin 2 2  13 <0.16  Combined fit to all three experiments: sin 2 2  13 >0 at ~3σ 2012/02/2721 Phys. Rev. Lett. 107,041801(2011 ) References: MINOS: Phys. Rev. Lett. 107, 181802 (2011) Double Chooz: hep-ex/1112.6353 Global fit: hep-ph/1111.3330

22. Earthquake and T2K On March 11 th, 2011, Japan experienced a severe earthquake followed by a tsunami K Mahn, Les Rencontres de Physique de la Vallée d'Aoste  Magnitude 9 earthquake on Richter scale, magnitude 6+ at J-PARC  The tsunami did not reach J-PARC  Accelerator was not operating (maintenance day)  No reported injuries to T2K collaborators or J-PARC employees T2K experiment status:  Repairs made to infrastructure and accelerator  Minor repairs and recommissioning of J-PARC near detectors successful  First beam in late Dec 2011  T2K run scheduled for ~4 months prior to summer 2012 shutdown 2012/02/2722

23. First neutrinos post-earthquake! K Mahn, Les Rencontres de Physique de la Vallée d'Aoste Enormous amount of work by collaborators, labs (KEK, J-PARC) and funding agencies) to make this happen, to which we are very grateful 2012/02/2723

24. Summary K Mahn, Les Rencontres de Physique de la Vallée d'Aoste The T2K experiment is designed to make precision measurements of:    disappearance ( Δm 2 32,  23 )   e appearance (  13 ) With dataset prior to the earthquake (2% of design POT)  Preliminary   disappearance results are inconsistent with no-oscillation at 4.5σ and consistent with previous experiments (MINOS, Super-K,K2K)  6 candidate  e events were observed with expected background is 1.49 ± 0.34 (2.5σ equivalent indication of  13 ) and consistent with recent results from MINOS, Double Chooz T2K will resume running to establish conclusively if  13 is nonzero or not  Run period of ~4months prior to 2012 summer shutdown of accelerator  Improvements to all aspects of the analysis 2012/02/2724