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Recent results from the K2K experiment

Recent results from the K2K experiment. Contents. 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.

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Recent results from the K2K experiment

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  1. Recent results from the K2K experiment Contents 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

  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. IntroductionPrinciple of the long baseline experiment Oscillation Probability= (2 flavor) 1 long baselineexperiment Oscillation maximum Probability Fixed distance 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 Compared to the NULL oscillation case, No oscillation Oscillated 1)Reduced number of events clear dip # of interactions 2)Distorted energy spectrum dip around 0.6 ~ 0.7 GeV 0 0.5 1 1.5 2 2.5 3 3.5

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

  5. Neutrino beamline Front (Near) Detectordirection (n) spectrum , rate m-monitor direction (pm) Al target 200m 12 GeV PS fast extraction every 2.2sec beam spill1.1ms ~6x1012 protons/spill Double Horn(250kA) ~20 x flux Pion monitor （Pp , qp after Horn）　 Near to Far flux ratioRFN

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

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

  8. GPS TOF=0.83ms Tspill TSK Event selection at Super-Kamiokande From June ‘99 to July ’01 (4.8 x 1019 protons on target) High energy trigger w/o pre-activity Fully contained events in the fiducial volume (FCFV) (FC = no activity in the outer detector) requiring Number of FCFV events Expected number of atmospheric n BG Nsk=56 events (1ringm-like=26) <10-3 events

  9. Summary of K2K results in 2001 From June ’99 to July ‘01 accumulated number of protons 4.8x1019POT for the analysis Neutrino beam was very stable • direction of the beam : controlled less than1mrad. confirmed by m profile monitor (pm decay) muon range detector (MRD) (n interaction vertex) • energy spectrum of n confirmed by 1kt water Cherenkov detector (1kt) & MRD

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

  11. m pm nm qm proton pm: muon momentum qm: muon angle Neutrino interactions around 1GeV Charged current quasi-elastic scattering (QE) Encan be reconstructed from Pm and qm. 1kt : single ring m-like FCFV events : QE-like2track events SciFi (when protons are identified) or single track events (when protons are not observed) pion productions 1kt : FCFV events pm m (single-ring or multi-rings) nm qm : non-QE-like events SciFi nucleon p

  12. Observables at SK • number of events (NSK ) • Reconstructed energy of n Flow of the oscillationanalysis Observed quantities at the near detectors (Pm , qm) for each event category Neutrino interaction models Obtainneutrino spectrumatnear detectors Near to Far extrapolation (RFN(En)) (fSK(En)) Predictneutrino spectrum without oscillation (sin22q,Dm2) with Fittheobserved results at SK Use maximum likelihood fit

  13. QE and non-QE events in SciFi 2track events m track 2nd track • QE/non-QE selection • 1. Select 2track events • Select muon track • Calculate expected direction • of protonqexp • (assuming QE interaction) • 4. Compare with the observed • direction of 2nd trackqobs QE -like non-QE-like Dqp<25 deg. : QE-like Dqp>30 deg. : non-QE-like Dqp= |qexp- qobs|

  14. Used data for fnear(En) SciFi (2) 1-track m events (3) 2-track QE-like events (4) 2-track non QE-like events • KT • Fully Contained Fiducial • Volume (FCFV) events • (0) No. of events • (Evis >100MeV) • (1) Single m-like events 4 sets of (pm, qm) distributions Pion monitor & Beam simulation p distribution in (pp , qp )  flux estimationfnear(En)w. error • nfluxfnear(En) (8 bins) • interaction model (parameterized as QE/non-QE ratio)

  15. • • • • • • Fitting method (n flux at KEK) Measured (pm , qm) Prepare MC templates neutrino spectrumf (En) and n interaction model (QE/non-QE ratio). qm En QE(MC)non-QE(MC) qm 0 ~0.5 GeV pm 0.5 ~0.75GeV pm Fit the parameters. f(En) , QE/non-QE ratio … 0.75 ~1.0GeV c2=227for197 d.o.f. (90 from 1kt, 137 from FGD) for fitted (pm , qm) dist. of 1KT and FGD (124 data points) 8bins 7bins

  16. Fitted results (1kt) pm andqm distributions of 1kt 1ring m-like FCFV events Both distributions agree well with the fitted MC. 0 400 800 1200 0 20 40 60 80 100 100 qm (deg.) pm (MeV/c)

  17. Very good agreements Fitted results (SciFi) 1track 2track QE like 2track non-QE like 0 1 2 0 25 50 pm(GeV/c) qm (deg.)

  18. Reconstruction of neutrino energy at SK Usesingle-ring m-like FCFV(1Rm)events Assuming QE interactionandreconstruct En (~50%ofK2K 1Rmevents areCCQE) m pm nm qm proton Em: muon energy pm: muon momentum qm: muon angle

  19. Oscillation analysis 1) Used data sets • 1. Number of events • June ’99 - July ’01 • FCFV events • (56 events) • 2. Spectrum shape • Nov. ’99 - July ’01 • 1Rm events • (29 events) 2) Analysis methods 1.Maximum Likelihood method Constraint term for systematic parameters. (error matrices) Ltot= Lnorm(f)Lshape(f)Lsyst(f) Shape term (for 1Rm) Normalization term (Nexp= 80.1+6.2-5.4 ) 2.different treatment of systematic term. Generate many MC samples. (changing systematic parameters within the error.) Ltot= weighted mean of L for the MC samples

  20. Allowed Dm2 by NSK and shape analysis NSK + Shape Number of Events only Spectrum Shapeonly NSK and shape analysisindicate the same Dm2 region for sin22q=1

  21. Allowed regions Dm2=1.5~3.9x 10-3 eV2 @sin22q=1(90%CL) Best fit parameters 90%CL (sin22q,Dm2) =(1.0 , 2.8x10-3 eV2 ) [Method-1] =(1.0 , 2.7x10-3 eV2 ) [Method-2] dashed: method1 solid : method2

  22. Best fit results Reconstructed energy of neutrino for 1Rm and NSK Best fit parameters (sin22q,Dm2) =(1.0 , 2.8x10-3 eV2 ) Without Oscillation • NSK Expected (W/osc.) = 54 Best fit Observed = 56 • Shape • KS test79% Very good agreements Both shape and NSK

  23. Null oscillation probability Use Dlog(likelihood) from best fit point in the physical region method-1method-2 NSK only 1.3% 0.7% Shape only 15.7% 14.3% NSK + Shape0.7%0.4% Probability of null oscillation is less than 1%.

  24. Summary K2K Oscillation analysis on June’99 ~ July ’01 data Full and Improved error estimations and spectrum shape analysis Use both Number of events + Spectrum shape (June ’99 – July ’01) (Nov. ’99 – July ’01) • Null oscillation probability is less than 1%. • Spectrum shape distortion and observed # of events @SK indicatesconsistent oscillation parameter regions. • Oscillation parameters (sin22q and Dm2) are consistent with the atmospheric n results. Dm2=1.5~3.9x 10-3eV2@sin22q=1(90%CL) (c.f. ATM n : Dm2=1.6~3.9x 10-3 eV2@sin22q=1(90%CL))

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