Results from the K2K Experiment

Results from the K2K experiment

T. Nakadaira KEK

1 K2K experiment lKEK to Kamioka long baseline Neutrino Oscillation Experiment

250km

áEnñ~1.3GeV

Super-Kamiokande Far Detector Fiducial vol. : 22.5kt KEK 12GeV PS n beam line Near Detector(ND)

Started in 1999 2 K2KK2K CollaborationCollaboration

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 U.S.A.: Duke University

3 Principle of K2K

11 12GeV protons ~10 nm/2.2sec ~1 event/2days (/10m´10m) nm + p m+ SK nt Target+Horn 200m 100m ~250km decay pipe p monitor Near n detectors m monitor 2 2 2 1.27Dm L •Confirmation of n àn oscillation prob.=×sin2q sin() m x E reported by Super-K atm-n n nm spectrum@SK Observables: For Dm2=3x10-3, 2 lReduction of events sin 2q=1 lDistortion of spectrum à If sin22q¹0 and Dm2¹0,

Neutrino oscillation. En [GeV]4 Neutrino beam and the directional control l ~1GeV neutrino beam by a dual horn system with 250kA. The beam direction monitored by muons spill by spill X center £ ±1 mrad

<±1mrad Y center

99 Jun ~5 years 04 Feb5 Near Detector l 1KT: water Cerenkov detector [25t fiducial] l SciFi: scintillating fiber and water target [6t fiducial] l LG: Lead glass calorimeter (removed in 2002) à SciBar: fully-active scintillator detector [10t fiducial] (installed in 2003) l MRD: muon range detector

Lead Glass à

Muon Range Detector (MRD)

300m from target 6 Super-Kamiokande (Far detector on K2K)

50kt total volume 22.5kt fiducial volume

20’PMT photocathode coverage SK-I 11,146 40% SK-II 5,182 19%

41.4m

39.3m 1000m underground 7 Delivered Protons On Target ) 18 IIa IIb - - LG removal SciBar Installation Accumulated POT (x10 ) K2K-I SK Rebuilding K2K K2K 12

POT= Protons On Target Protons/Pulse (x10 1999 2000 2001 2002 2003 2004 19 8.9´10 POT are used for oscillation analysis 8 Analysis Flow

Number of n event @ Near Detectors (pm, qm) distributions n interaction Neutrino flux/spectrum at Near Detector Near to Far extrapolation Neutrino flux/spectrum @ Super-K n interaction Prediction Observation exp obs lNSK : Number of SK events üNSK rec rec lEn distribution for 1ring-m ü1Rm En Null oscillation probability, 2 2 (sin 2q23, Dm 23) 9 Near Detector Measurements

10 Event Rate Measurement @1KT l Use total # of events in the fiducial volume (25t).

FKT ´ Far/Near Ratio

F (E)se()EdE exp obs ò SKnnnSK M SK NNSK=KT ·· F (E)se()EdE M ò KTnnnKT KT

measured spectrum

M: Fiducial mass MSK=22,500ton, MKT=25ton e: efficiency eSK-I(II)=77.1(78.2)%, eKT=74.5% expect +11.6 NSK =150.9 -10.0 (w/o oscillation) 11 En Reconstruction

m- l CC Quasi Elastic (CC-QE) nm + n à m + p q (Em, pm) m l 1Rm in W-Cherenkov n p l can reconstruct En ¬(qm,pm) (undetected in 1KT & SK) 2 2 rec (mN -V )Em - mm 2 + mNV -V 2 En = (mN -V ) - Em + pm cosqm V: nuclear potential energy - m l nm + n à m + p + p CC nQE q (Em, pm) m l Background for En meas. n p’s p n nm + N à n + N + p’s l NC

n N p’s 12 Used Data for Spectrum Meas.

p Dq p 1KT n p (1) Fully Contained m 1 ring m-like events m SciFi SciBar (2) 1-track m events (5) 1-track m events (3) 2-track QE-like (6) 2-track QE-like (4) 2-track nonQE-like (7) 2-track nonQE-like 7 sets of (pm,qm) distributions simultaneously fit

• n spectrum FNear(En) (8 bins) • n interaction model (nQE/QE) 13 Measured Spectrum

F(En) at ND

En c2=638.1 for 609 d.o.f nQE/QE = 1.02 ± 0.10 pm/qm after fitting Assigned based on the (SciBar 1track) variation by the fit condition. 14 Super-K Observation and Oscillation Analysis

15 Super-K Event Selection GPS

SK ±500msec Tspill TOF=0.83msec TSK Fully contained in 22.5kt fiducial volume 107 events

±5msec Analysis Time Window in 1.5 msec window w/ negligible (2x10-3) background All data are (re-)processed with consistent software. 16 TDIFF. (ms) Super-K Event Summary K2K-all DATA MC (K2K-I, K2K-II) (K2K-I, K2K-II) (K2K-I, K2K-II) FC 22.5kt 107 150.9 (55, 52) (79.1, 71.8) 1ring 67 94.0 (33, 34) (48.9, 45.1 ) m-like 57 85.4 rec for En (30, 27) (44.9, 40.5) e-like 10 8.6 (3, 7) (4.0, 4.5) Multi Ring 40 56.9 (22, 18) (30.2, 26.7) Ref; K2K-I(47.9´1018POT), K2K-II(41.2´1018POT)

17 Reconstructed En for 1Rm events

K2K-I + K2K-II ... 57 events

Expected shape for Null Oscillation case

18 Maximum likelihood fit l Total Number of events rec l En spectrum shape of FC-1ring-m events l Systematic error term L(Dmf2 ,sin2q,)x 22xxx =Lnorm(Dm,sin2qq,f)×Lshape(D×m,sin2,fLf)syst () f x : Systematic error parameters Normalization, Flux, and nQE/QE ratio are in fx

Near Detector measurements, Pion Monitor constraint, beam MC estimation, and Super-

K systematic uncertainties. 19 Fit results of n oscillation parameters lBest fit in all parameter region nsin22q = 1.51 Null oscillation nDm2 [eV2] = 2.19 ´10-3 l# of FC events Best fit nNexp = 101.5 ßà Nobs=107 lGoodness of fit (KS test) nKS prob.(Data & fit): 63% nKS prob.(Data & Null osci.): 0.08%

DlnL=0.75 (within 1s) lBest fit in physical region nsin22q = 1.00 Null oscillation nDm2 [eV2] = 2.79´10-3 l# of FC events Best fit nNexp = 103.8 ßà Nobs=107 lGoodness of fit nKS prob. (data & fit) : 36% 20 Allowed region (Contour of DlnL)

Log scale linear scale

Dm2 @ sin22q=1 : 68% … 2.14´10-3 £ Dm2 £ 3.37´10-3 [eV2] 90% … 1.87´10-3 £ Dm2 £ 3.58´10-3 [eV2] 21 Event Rate vs. Spectrum

NSK (#nm) only En shape only ] ] 2 2 [eV [eV 2 2 m m D D

2 sin 2q sin22q

Consistent answer. 22 Null Oscillation Probability

The null oscillation probabilities calculated based on DlnL. K2K-I K2K-II K2K-I+II number of events 1.4% 3.7% 0.26%(3.0s)

En spectrum distortion 12.0% 5.8% 0.74% (2.6s) Combined 0.58% 0.56% 0.005% (2.7s) (2.7s) (4.0s)

Disappearance of nm and distortion of the energy spectrum as expected in neutrino oscillation. K2KK2K confirmedconfirmed neutrinoneutrino oscillationoscillation discovereddiscovered inin SuperSuper--KK atmosphericatmospheric neutrinos.neutrinos. 23 K2K & SK Atmospheric results

24 Other Physics in K2K (based on K2K-I data)

0 nm +H2O®NC1p nm®ne search 90%CL limit

] 90%CL sensitivity 2 [eV 2

0 m

not NC1p D

M (MeV) 2 0 gg sin 2qme s (n m ® NC1p ) =0.064±0.001±0.007 s (n ® CCall ) PRL 93, 051801 (2004) m =0.065 (prediction) hep-ex/0408134 Submitted to PLB

25 Summary and Prospects l K2K confirmed neutrino oscillation at 4.0s with 8.9x1019POT. n Both number of events and En spectrum distortion are consistent with neutrino oscillation

n Allowed region for nm dis-appearance is 1.9´10-3 £ Dm2 £ 3.6´10-3 (90%C.L.) @ sin22q=1.0 ß Consistent with SK atm. results. n This result will be published soon. hep-ex/0411038 (Accepted by PRL) l K2K physics run resumed in Oct. 2004, but … n The horn was broken in Nov. 2004. It is hard to recover because we can not access the horn area due to high radiation. à K2K physics run is finished. l Next long-baseline n experiment in Japan (T2K) will starts in 2009. n ´100 intense n beam using 0.75MW 50GeV PS (JPARC).

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