1 M. Yokoyama (Kyoto University) for K2K collaboration June 10 th, U.

2 K2K experiment since 1999 First accelerator-based long baseline (250km) neutrino experiment. 12GeV PS   beamline Beam monitor Near detector 50 kton Water Cherenkov detector Search for  disappearance and e appearance 250km Pure  beam (99%) w/ ~1.3GeV 1. Introduction

3 Flavor mixing in lepton sector  Flavor eigenstates ≠ mass eigenstates Parameters: 3 mixing angle (  12,  23,  13 ) 1 complex phase (  ) S ij :sin  ij, C ij ;cos  ij e   m1m1 m2m2 m3m3

4 Neutrino Oscillation  Time evolution of neutrino Consider mixing b/w two flavors If there is neutrino oscillation →  m 2 ≠0 →m≠0! If m 2 ≠m 3, is in mixed state after time evolution! Consider neutrino generated as pure  cos    sin   

5 Super-Kamiokande atmospheric 1.3x10 -3 <m 2 <3.0x10 -3 eV 2 sin 2 2=1, 90% CL)  →  oscillation?

6  1995 Proposed to study neutrino oscillation for atmospheric neutrinos anomaly.  1999 Started taking data.  2000 Detected the less number of neutrinos than the expectation at a distance of 250 km. Disfavored null oscillation at the 2 level.  2002 Observed indications of neutrino oscillation. The probability of null oscillation is less than 1%.  2004 This result! Brief history of K2K

7 K2K Collaboration JAPAN: High Energy Accelerator Research Organization (KEK) / Institute for Cosmic Ray Research (ICRR), Univ. of Tokyo / Kobe University / Kyoto University / Niigata University / Okayama University / Tokyo University of Science / Tohoku University KOREA: Chonnam National University / Dongshin University / Korea University / Seoul National University U.S.A.: Boston University / University of California, Irvine / University of Hawaii, Manoa / Massachusetts Institute of Technology / State University of New York at Stony Brook / University of Washington at Seattle POLAND: Warsaw University / Solton Institute Since 2002 JAPAN: Hiroshima University / Osaka University CANADA: TRIUMF / University of British Columbia Italy: Rome France: Saclay Spain: Barcelona / Valencia Switzerland: Geneva RUSSIA: INR-Moscow

8 2. K2K experiment overview  monitor  monitor Near detectors ++  Target+Horn 200m decay pipe SK 100m ~250km  12GeV protons ~10 11   /2.2sec (/10m  10m) ~10 6   /2.2sec (/40m  40m) ~1 event/2days  Extrapolation Near detectors at KEK  monitor + simulation （ＭＣ） no oscillation oscillation Super-K Energy 250km

9 Bird ’ s Eye Neutrino Beam Line 200m 100 m Neutrino KEK

10 Neutrino beam and the directional control  ~1GeV neutrino beam by a dual horn system with 250kA. <1mrad The beam direction monitored by muons Y center X center ≤  1 mrad 99 Jun04 Feb ~5 years

11 Accumulated POT (Protons On Target) Jan 99 Jan 00 Jan 01 Jan 02 Jan 03 Jan 04 protons/pulse (×10 12 ) Accumulated POT (×10 18 ) K2K-I K2K-II 8.9×10 19 POT for Analysis (previous results: 4.8 ×10 19 POT)

12 Near detector system at KEK  1KT Water Cherenkov Detector (1KT)  Scintillating-fiber/Water sandwich Detector (SciFi)  Lead Glass calorimeter (LG) before 2002  Scintillator Bar Detector (SciBar) from 2003  Muon Range Detector (MRD) Muon range detector

13 SciBar Detector  Full-active, fine-segment detector made of Scintillator Bars 2.5x1.3x300cm 3 cell ~15000 channels WLS fiber+MAPMT readout Detect short (~10cm) track p/ separation using dE/dx  Precise spectrum measurement  interaction study Extruded scintillator (15t) Multi-anode PMT (64 ch.) Wave-length shifting fiber EM calorimeter 1.7m 3m

14 Detector Photos Scintillators (64layers) EM calorimeter Fibers and front-end elec.

Just Completed! Aug. 22, 2003

Start data taking (SK- I) 1999 K2K start Stop data taking for detector upgrade Accident (~7000 inner PMTs, 1100 outer PMTs were destroyed) partial reconstruction of the detector resume data taking (SK- II) resume K2K beam (K2K-II) SK- II K2K-II SK- I ~5years C Scientific American 42m 39m Water Cherenkov detector 1000 m underground 50,000 ton (22,500 ton fid.) 11, inch PMTs 1,885 anti-counter PMTs Far detector : Super-Kamiokande

17 SK is back ! Sep.-2002, before water filling Jan.-2003, fully contained event Full water on 10-Dec Acrylic + FRP vessel

18 SK Events (BG: 1.6 events within  500  s 2.4×10 -3 events in 1.5  s) T SK T spill GPS SK TOF=0.83msec 108 events Decay electron cut.  20MeV Deposited Energy No Activity in Outer Detector Event Vertex in Fiducial Volume More than 30MeV Deposited Energy Analysis Time Window  500  sec  5  sec T DIFF. (s)  T SK - T spill - TOF  1.3  sec (K2K-1:56)

19 3. Analysis Overview Observation #, p  and   interaction MC Measurement  (E ), int. KEK Far/Near Ratio (beam MC with  mon.) Observation # and E  rec. Expectation # and E  rec. (sin 2 2 ,  m 2 ) SK

20 4. Near Detector measurements  Event rate measurement (#of int.) Measurement w/ 1KT Cross-checked by other detectors  Spectrum shape measurement 1KT, SciFi, SciBar (p, ) Measure spectrum and nQE/QE ( interaction model) Predict number of event and spectrum shape at SK

21 Neutrino GeV & E reconstruction  CC QE  ~100% efficiency for N SK  can reconstruct E  p    CC nQE (1pi, multi-pi, coherent,DIS)  ~100% efficiency for N SK  Bkg. for E measurement  NC  ~40% efficiency for N SK  + n →  + p -- p ( E , p  )   + n → + p +  ’s p  ’s --  + n →  + p +  p ( E , p  )   ’s  /E ( cm 2 /GeV)

Event rate  The same detector technology as Super-K.  Sensitive to low energy neutrinos. Far/Near Ratio (by MC)~1×10 -6 M: Fiducial mass M SK =22,500ton, M KT =25ton : efficiency  SK-I(II) =77.0(78.2)%,  KT =74.5% N SK exp =150.9 N SK obs =

Measurement with SciBar  Full Active Fine-Grained detector (target: CH). Sensitive to a low momentum track. Identify CCQE events and other interactions (non-QE) separately.  p CCQE Candidate CCQE non-QE 25  p (degree)  p  DATA CC QE CC 1  CC coherent-  CC multi-  2 track events p

Near Detector Spectrum Measurements  1KT Fully Contained 1 ring  (FC1R) sample.  SciBar 2 track nQE ( p >25  ) 1 track, 2 track QE ( p ≤25 ), 2 track nQE ( p >25  ) where one track is   SciFi 1 track, 2 track QE ( p ≤25 ), 2 track nQE ( p >30  ) where one track is 

25 A hint of K2K forward  deficit. SciBar non-QE Events K2K observed forward  deficit. A source is non-QE events. For CC-1,  Suppression of ~q 2 /0.1[GeV 2 ] at q 2 <0.1[GeV 2 ] may exist. For CC-coherent ,  The coherent may not exist. We do not identify which process causes the effect. The MC CC-1 (coherent ) model is corrected phenomenologically. Oscillation analysis is insensitive to the choice. q 2 rec (Data-MC)/MC DATA CC 1  CC coherent-  Preliminary q 2 rec (GeV/c) 2

Near Detectors combined measurements (p    ) for 1track, 2trackQE and 2track nQE samples  (E), nQE/QE  Fitting parameters (E ), nQE/QE ratio Detector uncertainties on the energy scale and the track counting efficiency. The change of track counting efficiency by nuclear effect uncertainties; proton re-scattering and  interactions in a nucleus …  Strategy  Measure (E ) in the more relevant region of   20 for 1KT and   10 for SciFi and SciBar.  Apply a low q 2 correction factor to the CC-1 model (or coherent ).  Measure nQE/QE ratio for the entire   range.

GeV GeV GeV GeV E QE (MC) nQE(MC) MC templates KT data P  (MeV/c)  (MeV/c) flux  KEK (E ) (8 bins) interaction (nQE/QE)

28 Flux measurements  2 =638.1 for 609 d.o.f 1 ( E< 500) = 0.78  0.36 2 ( 500 E < 750) = 1.01  0.09 3 ( 750 E <1000) = 1.12  0.07 4 (1500 E <2000) = 0.90  0.04 5 (2000 E <2500) = 1.07  0.06 5 (2500 E <3000) = 1.33  0.17 6 (3000 E ) = 1.04  0.18 nQE/QE = 1.02  0.10 The nQE/QE error of 10% is assigned based on the variation by the fit condition.  >10(20 ) cut: nQE/QE=0.95 0.04  standard(CC-1 low q 2 corr.): nQE/QE=1.02 0.03  No coherent: =nQE/QE=1.06 0.03 (E ) at KEK E preliminary

29 1KT:  momentum and angular distributions. with measured spectrum   (deg.) 0 90 p  (MeV/c)

30 SciFi (K2K-IIa with measured spectrum) P  1trk P  2trk QE P  2trk non-QE   1trk   2trk QE   2trk non-QE 02(GeV/c)040(degree)

31 SciBar (with measured spectrum) P  1trk P  2trk QE P  2trk nQE   1trk   2trk QE   2trk nQE

32 5. Super-K oscillation analysis  Total Number of events  E rec spectrum shape of FC-1ring- events  Systematic error term f x : Systematic error parameters Normalization, Flux, and nQE/QE ratio are in f x Near Detector measurements, Pion Monitor constraint, beam MC estimation, and Super- K systematic uncertainties.

33 K2K-SK events K2K-alll (K2K-I, K2K-II) DATA (K2K-I, K2K-II) MC (K2K-I, K2K-II) FC 22.5kt 108 (56, 52) (79.1, 71.8) 1ring 66 (32, 34) 93.7 (48.6, 45.1 ) -like 57 (30, 27) 84.8 (44.3, 40.5) e-like 9 (2, 7) 8.8 (4.3, 4.5) Multi Ring 42 (24, 18) 57.2 (30.5, 26.7) for E rec Ref; K2K-I(47.9×10 18 POT), K2K-II(41.2×10 18 POT) (56) preliminary

#SK Events Expected shape (No Oscillation) E rec [GeV] KS probability=0.11% V: Nuclear potential Toy MC CC-QE assumption

35  Best fit values. sin 2 21.53 m 2 [eV 2 ] = 2.1210 -3  Best fit values in the physical region. sin 2 21.00 m 2 [eV 2 ] = 2.7310 -3 logL= Results sin 2 2  =1.53 can occur by statistical fluctuation with 14.4% probability. A toy MC sin 2 2 m2m % preliminary 1.53

36  N SK obs =108  N SK exp (best fit)=104.8 E rec [GeV] Best Fit KS prob.=52% m 2 [eV 2 ] sin 2 2 Data are consistent with the oscillation. Based on lnL preliminary

37  disappearance versus E shape distortion  sin 2 2 m 2 [eV 2 ] N SK (#  ) E shape Both disappearance of  and the distortion of E spectrum have the consistent result.

38 Null oscillation probability K2K-IK2K-IIK2K-all  disappearance 2.0%3.7%0.33% (2.9) E spectrum distortion 19.5%5.4%1.1% (2.5) Combined 1.3% (2.5) 0.56% (2.8) 0.011% (3.9) K2K confirmed neutrino oscillation discovered in Super-K atmospheric neutrinos. Disappearance of  and distortion of the energy spectrum as expected in neutrino oscillation. The null oscillation probabilities are calculated based on lnL. preliminary

39 m 2 [eV 2 ] sin 2 2 preliminary 8. Summary has confirmed 3.9  With 8.9×10 19 POT, K2K has confirmed neutrino oscillations at 3.9. 2.9 Disappearance of  2.9 2.5 Distortion of E spectrum 2.5 x10 -3 <m 2 <3.5x10 -3 eV 2 2=1 (90%C.L.)

40 Backup

41 7. Other Physics in K2K (based on K2K-I data) M  (MeV)  +H 2 ONC1 0 sin 2 2 e m 2 [eV 2 ] 90%CL limit 90%CL sensitivity   e search PRL accepted =0.064 (prediction) =0.065 0.0010.007 not NC1 0 preliminary

42 1KT:  momentum and angular distributions. with measured spectrum   (deg.) flux measurement low q 2 corr. p  (MeV/c)

43 SciFi (K2K-IIa with measured spectrum) P  1trk P  2trk QE P  2trk non-QE   1trk   2trk QE   2trk non-QE 02(GeV/c)040(degree) flux measurement 10

44 SciBar (with measured flux) P  1trk P  2trk QE P  2trk nQE   1trk   2trk QE   2trk nQE flux measurement 10

45 Log Likelihood difference from the minimum.  m 2 <(1.7~3.5)×10 -3 eV 2 at sin 2 2=1.0 (90% C.L.) sin 2 2 m 2 [eV 2 ] lnL - 68% - 90% - 99% - 68% - 90% - 99%

46 The change of N SK exp in K2K-I (Bugs)  The detector position 295m  294m -1%  MC difference between KT and SK KT; M A (QE)=1.1 (NC el ) KT =1.1×(NC el ) SK SK; M A (QE)=1.0  Efficiency change! -1% N SK exp Change ~2% 295m 294m

47

48 CC-1  suppression versus coherent 

49 Systematic Bias without the MC correction. Oscillation Results ND (SciBar) measurement DATA; MC w/ CC-1  suppression MC template; Default MC systematic bias flux nQE/QE Toy MC Default MC for ND and SK sin 2 2  =1.00  m 2 =2.73 ×10 -3 eV 2 Prob.(null oscillation)=0.0049% Corrected MC sin 2 2  =1.00  m 2 =2.65×10 -3 eV 2 Prob.(null oscillation)=0.011% There is a small bias in nQE/QE and the low energy flux.

50 Oscillation result with a default MC Without low q 2 MC correction The result w/o low q 2 MC correction gives the better (biased) measurement due to the more low energy flux and the smaller nQE/QE.

51 K2K-I vs K2K-II

52 K2K-I vs K2K-II

53 µ Disappearance Result (K2K-I) Normalized by area Expectation w/o oscillation Best Fit Reconstructed E Best fit:  m 2 =2.8 x10 –3 eV 2 Null oscillation probability: less than 1%.  m 2 =1.5~3.9 x eV 2 2  (90%CL) Allowed region PRL 90(2003)041801

54 Extrapolation from Near to Far sites beam 250km FD (<2m) ×10 10 ×10 4 Energy RadiusEnergy Radius PION moniter Gas Cherenkov detector: (insensitive to primary protons) Measure, N(p ,   ) just after the horns. 1.0×10 -6

55