1. Recent results from the K2K experiment Yoshinari Hayato (KEK/IPNS) for the K2K collaboration Introduction Summary of the results in 2001 Overview of the new analysis (Number of events + Spectrum) Flux measurements at KEK Oscillation analysis and results Summary Contents

2. K2K Collaboration High Energy Accelerator Research Organization(KEK) Institute for Cosmic Ray Research(ICRR), University of Tokyo Kobe University Kyoto University Niigata University Okayama University Tokyo University of Science Tohoku University Chonnam National University Dongshin University Korea University Seoul National University 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 Warsaw University Solton Institute

3. Introduction Principle of the long baseline experiment Oscillation Probability= 1)Reduced number of events 2)Distorted energy spectrum Compared to the NULL oscillation case, (2 flavor) Fixed distance Probability 1 0.5 0 122.531.503.5 0.5122.531.503.5 # of interactions No oscillation Oscillated dip around 0.6 ~ 0.7 GeV clear dip Oscillation maximum long baseline experiment

4. The K2K experiment 250km Super Kamiokande (far detector) 50kt water Cherenkov detector 12GeV PS@KEK  beamline beam monitors near detectors Neutrino beam Beam monitors & near detectors beam direction Far detector  disappearance spectrum distortion  monitor  flux & spectrum

5. 200m Front (Near) Detector direction ( ) spectrum, rate 12 GeV PS fast extraction every 2.2sec beam spill 1.1  s ~6x10 12 protons/spill Double Horn(250kA) ~20 x flux Neutrino beamline Pion monitor （ P ,   after Horn ） Near to Far flux ratio R FN  -monitor direction (p  ) Al target

6. 1kt water Cherenkov detector Fine grained detectors Scintillating fiber tracker Muon range detector same type as SK (25t fid. vol.) water target (330t fid. vol.)Iron target (6t fid. vol.) beam monitor (mom. & dir.) Muon range detector CCQE identification water target (SciFi) (MRD) (1kt) Near neutrino detectors

7. Super-Kamiokande (Far detector) 39m 41.4m 50kt water Cherenkov detector Outer detector Inner detector 1885 8” PMTs 11146 20” PMTs Atmospheric ~8 events/day (FCFV) 1000m under the ground Fiducial volume 22.5kt accidental coincidence events/day ~10 -5

8. Event selection at Super-Kamiokande T spill T SK GPS TOF=0.83ms w/o pre-activity High energy trigger (FC = no activity events in the fiducial volume (FCFV) in the outer detector) requiring Number of FCFV events N sk =56 events Expected number of atmospheric BG <10 -3 events (1ring  -like=26) From June ‘99 to July ’01 (4.8 x 10 19 protons on target) Fully contained

9. Summary of K2K results in 2001 From June ’99 to July ‘01 accumulated number of protons 4.8x10 19 POT Neutrino beam was very stable direction of the beam : controlled less than 1mrad. confirmed by  profile monitor muon range detector (MRD) ( interaction vertex) energy spectrum of 1kt water Cherenkov detector (1kt) & MRDconfirmed by for the analysis (   decay)

10. # of FCFV events in Super-K Observed : 56Expected : Probability of null oscillation < 3% Full and Improved error estimations and spectrum shape analysis Neutrino flux @ SK estimated by Monte-Carlo  monitor confirmed by normalization was done by 1kt water Cherenkov detector [number of protons was measured by Current Transformer (CT)] Next step Summary of K2K results in 2001

11. Neutrino interactions around 1GeV Charged current quasi-elastic scattering (QE) pion productions p  : muon momentum   : muon angle   proton  pp   nucleon  pp  1kt SciFi : single ring  -like FCFV events or single track events : QE-like 2track events (when protons are not observed) (when protons are identified) SciFi : non-QE-like events 1kt: FCFV events (single-ring or multi-rings) E can be reconstructed from P  and  .

12. Flow of the oscillation analysis Observed quantities at the near detectors Neutrino interaction models (P ,   ) for each event category Obtain neutrino spectrum at near detectors Near to Far extrapolation(R FN (E )) Predict neutrino spectrum without oscillation (  SK ( E )) Fit the observed results at SK Use maximum likelihood fit (sin 2 2 ,  m 2 ) Observables at SK number of events (N SK ) Reconstructed energy of with

13. non-QE-like QE/non-QE selection 1. Select 2track events 2.Select muon track 3.Calculate expected direction of proton  exp (assuming QE interaction) 4. Compare with the observed direction of 2 nd track  obs   track 2 nd track  p <25 deg. : QE-like  p >30 deg. : non-QE-like  p = |  exp -  obs | QE -like QE and non-QE events in SciFi 2track events

14. Used data for  near (E ) KT Fully Contained Fiducial Volume (FCFV) events (0) No. of events (Evis >100MeV) (1) Single  -like events SciFi (2) 1-track  events (3) 2-track QE-like events (4) 2-track non QE-like events flux  near (E ) (8 bins ) interaction model (parameterized as QE/non-QE ratio) Pion monitor & Beam simulation  distribution in (p    )  flux estimation  near (E ) w. error 4 sets of (p ,   ) distributions

15. Fitting method ( flux at KEK)  2 =227 for 197 d.o.f. (90 from 1kt, 137 from FGD) for fitted (p    ) dist. of 1KT and FGD (124 data points) 0 ~0.5 GeV 0.5 ~0.75GeV 0.75 ~1.0GeV E QE(MC) non-QE(MC) neutrino spectrum  (E ) and interaction model (QE/non-QE ratio). Fit the parameters. Measured (p    ) 7bins 8bins pp  pp  Prepare MC templates  (E ), QE/non-QE ratio …

16. Fitted results (1kt) 0204060100   (deg.) 10080 0 400800 p  (MeV/c) 1200 p  and   distributions of 1kt 1ring  -like FCFV events Both distributions agree well with the fitted MC.

17. Fitted results (SciFi) p  (GeV/c)   (deg.) 1track 2track non-QE 2track QE like like 012 02550 Very good agreements

18. Reconstruction of neutrino energy at SK p  : muon momentum   : muon angle   proton  pp Use single-ring  -like FCFV (1R  ) events Assuming QE interaction and reconstruct E ( ~50% of K2K 1R  events are CCQE) E  : muon energy

19. Oscillation analysis 1.Maximum Likelihood method Normalization term (N exp = 80.1 +6.2 -5.4 ) Shape term (for 1R  ) Constraint term for systematic parameters. (error matrices) L tot = L norm (f) L shape (f) L syst (f) 2.different treatment of systematic term. Generate many MC samples. (changing systematic parameters within the error.) L tot = weighted mean of L for the MC samples 1) Used data sets 1. Number of events June ’99 - July ’01 FCFV events (56 events) 2. Spectrum shape Nov. ’99 - July ’01 1R  events (29 events) 2) Analysis methods

20. Allowed  m 2 by N SK and shape analysis N SK and shape analysis indicate the same  m 2 region for sin 2 2  =1 Number of Events only Spectrum Shape only N SK + Shape

21. Allowed regions  m 2 =1.5~3.9 x 10 -3 eV 2 @sin 2 2  (90%CL) Best fit parameters  sin 2 2 ,  m 2  =(1.0, 2.8x10 -3 eV 2 ) =(1.0, 2.7x10 -3 eV 2 ) [Method-1] [Method-2] 90%CL dashed: method1 solid : method2

22. Best fit results Reconstructed energy of neutrino for 1R  and N SK Without Oscillation Best fit Best fit parameters  sin 2 2 ,  m 2  =(1.0, 2.8x10 -3 eV 2 ) N SK Observed= 56 = 54 Shape KS test 79% Both shape and N SK Very good agreements Expected (W/osc.)

23. Null oscillation probability method-1 method-2 N SK only 1.3% 0.7% Shape only 15.7%14.3% N SK + Shape 0.7% 0.4% Probability of null oscillation is less than 1%. Use  log(likelihood) from best fit point in the physical region

24. Summary K2K Oscillation analysis on June’99 ~ July ’01 data Use both Number of events + Spectrum shape Full and Improved error estimations and spectrum shape analysis (June ’99 – July ’01)(Nov. ’99 – July ’01) Null oscillation probability is less than 1%. Spectrum shape distortion  m 2 =1.5~3.9 x 10 -3 eV 2 @sin 2 2  (90%CL) Oscillation parameters (sin 2 2  and  m 2 ) are  c.f.  m 2 =1.6~3.9 x 10 -3 eV 2 @sin 2 2  (90%CL)  and observed # of events @SK indicates consistent oscillation parameter regions. consistent with the atmospheric results.