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New results from K2K Makoto Yoshida (IPNS, KEK) for the K2K collaboration NuFACT02, July 4, 2002 London, UK

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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 Soltan Institute for Nuclear Study

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Neutrino beam almost pure (98%) ~1.3GeV Near detectors Measure flux/spectrum Far detector Super-Kamiokande (SK) 250 km far from KEK disappearance and e appearance K2K long baseline neutrino oscillation experiment KEK beam line Beam monitor Near detectors Kamioka Super-K 250km

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Neutrino oscillations in K2K Spectrum distortion disappearance K2K aims to observe deficit in number of events distortion of spectrum after 250 km travel Fixed to 250 km probability E =1.3 GeV

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Super-Kamiokande 39 m 41 m 50kton Water Cherenkov detector 1000m underground 22.5kton fiducial mass Inner detector 11146 PMTs(20’’) Outer detector 1885 PMTs(8’’) Atmospheric B.G. against K2K ~10 -5 events/day in beam spill (1.1 s/2.2s)

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Neutrino Beam Line @KEK 200m 12GeV PS 1.1 sec / 2.2sec beam spill 6x10 12 protons/spill Pion monitor p , after Horn p+Al + Near neutrino detectors Flux/spectrum Near to Far flux ratio R FN Double Horn 250kA x20

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Muon Range Detector Near neutrino detectors 1kton water Cherenkov detector (1KT)same type as SK (25ton fiducial) Fine Grained Detectors (FGD) Scintillation fiber tracker (SciFi)w/ water target (6ton fiducial) CCQE identification Lead glass calorimeter (LG)detect electrons from SciFi Muon range detector (MRD)measure muon momentum beam monitor (Fe, 330ton fiducial)

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K2K Results in 2001 T spill T SK GPS w/o pre-activity p.e. >200 1.5 s 500 sec 5 sec T ( sec) Fully Contained events in Fiducial Volume (FCFV) 56 events observed 80 expected in null oscillation case Probability < 3% Number of FCFV events (N SK )

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What’s new Same data set as in last year spectrum at near site Oscillation analysis (N SK + Shape) only N SK in last year Full error treatment to include correlation

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Far site Total number events Spectrum shape Strategy Near site Number of neutrino events muon distributions of neutrino events Fit for neutrino spectrum & interaction model Near to Far extrapolation R FN (E )

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Event categories for spectrum measurement at near site 1KT: single ring -like FCFV events (1R FGD: single track events proton pp pp pp p : muon momentum : muon angle FGD: QE-like 2-track events FGD: nQE-like 2-track events

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QE and nQE in SciFi 2track events nQE-like QE-like Water Target + Al(20%) Fiducial Mass : 6 ton E ~150MeV Event Rate ~ 1/1000pulses

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Fitting Method Fitted (p ) dist. of 1KT and FGD 2=227 for 197 d.o.f. (90 from 1KT, 137 from FGD) 0 - 0.5 GeV 0.5 - 0.75GeV 0.75 - 1.0GeV E QE (MC) nQE(MC) MC templates for neutrino spectrum (E ) (8bin) and interaction model (QE/nQE) Fitted (p ) = weighted mean of MC templetes Measured (p )

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1KT distributions with fitted results Reasonable fitted distribution

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FGD distributions with fitted results 1-track 2-track QE-like 2-track nQE-like CCQE (MC) PP Good agreement for ALL sample

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proton : muon angle p : muon momentum Reconstruct neutrino energy assuming CCQE Use single-ring -like FCFV events (1R ) Neutrino spectrum reconstruction in SK

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Oscillation analysis Data Sets For number of events (N SK ) June ’99-July ’01 FCFV events (56 events) For spectrum shape Nov ’99-July ’01 1R events (29 events) Method-1 Maximum Likelihood method Normalization term N exp = 80.1 +6.2 -5.4 Shape termConstraint term for syst. parameter. (error matrices) L tot = L norm (f) L shape (f) L syst (f) Method-2 different treatment of syst. term. Generate many MC samples with syst. parm. changed within error. L tot = mean value for the MC samples

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Allowed m 2 by N SK and Shape only 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 Merged analysis (N SK and Shape) to get allowed region.

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K2K Allowed region (Shape+Norm) 1.5~3.9x10 -3 eV 2 for sin 2 2 =1 @90%CL Consistent with atm. result cf. Atmospheric results m2=(1.6~3.9)x10-3 eV 2 for sin 2 2 =1.0

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Best fit oscillation parameter (sin 2 2 , m 2 ) method-1method-2 Shape only (1.09, 3.0x10 -3 eV 2 )(1.05, 3.2x10 -3 eV 2 ) N SK +Shape (1.03, 2.8x10 -3 eV 2 )(1.05, 2.7x10 -3 eV 2 ) Shape and N SK @ Best Fit Best fit normalized by area method 1 N SK prediction =54 (obs 56) KS test prob.(shape)= 79% method 2 N SK 82% shape 93% N SK +shape50% Null oscillation N SK and Shape are reasonable at best fit point

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Null Oscillation Probability method-1method-2 N SK only 1.3% 0.7% Shape only15.7% 14.3% N SK +Shape 0.7% 0.4% Prob. for null oscillation case is less than 1%

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Conclusion Neutrino spectrum in K2K has been measured by using near detectors Oscillation analysis with number of events and spectrum shape on June99 ~July01 data Analysis for N SK and for shape indicate consistent m 2 region for sin 2 2 =1 Analysis for N SK +Shape gives m 2 =1.5~3.9x10 -3 eV 2 for sin 2 2 =1 @ 90%CL Null oscillation probability is less than 1% Resume data taking this winter and make statistics 2 times more

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