Recent Results from K2K Masashi Yokoyama (Kyoto U.) For K2K collaboration 2004 SLAC Summer Institute
Introduction
What is K2K? KEK to Kamioka first accelerator-based long baseline neutrino experiment! Super-K (far detector) Started in 1999 L=250km <Eν>~1.3GeV Goal: Confirmation of ν µ ν x oscillation reported by Super-K atm-ν, Precise measurement of Δm 2 23 Search for ν µ ν e oscillaton KEK PRL 93, 051801 ν beam line Beam monitor Near detectors 3
Accelerator-produced ν beam Controlled beam properties ν µ purity >98% Known distance (L=250km) Observations: Reduction of events (Eν<τ threshold) Distortion of spectrum Principle of K2K Crucial items: understanding of ν beam flux/spectrum/stability ν interaction P(ν µ ν µ ) =1 sin 2 2θ sin 2 ( 1.27Δm2 L E ν ) Δm 2 sin 2 2θ Survival prob. Spectrum@SK For Δm 2 =3x10-3, sin 2 2θ=1 4
Neutrino Beamline To Kamioka Near Detector µ monitor 12GeV PS Target/Horn Decay volume (200m, π+ µ++νµ) 1.1µs spill/2.2s ~5.5x1012 p/spill 5
Near Detectors 300m from target (MRD) 6
Event Displays ν µ p µ Area of circle proportional to energy 1KT water Cherenkov SciBar detector (new from 2003) 7
Accumulated Data POT=Protons On Target 120 8.9 10 19 POT for Analysis Accumulated POT ( 10 18 ) protons/pulse ( 10 12 ) 60 5 K2K-I SK accident Confirmed to be stable for whole period K2K-II Jan 99 Jan 00 Jan 01 Jan 02 Jan 03 Jan 04 8
Far Detector: Super-K 50,000ton water-cherenkov detector 41.4m Inner detector 11146 20 PMTs Outer detector 1885 8 PMTs 39m We just heard about it.. 9
Previous Result (K2K-I) Use both Number of events + Spectrum shape (June 99 July 01) (Nov. 99 July 01) 56obs 80.1 +6.2-5.4 exp Normalized by area Expectation w/o oscillation Best Fit PRL 90(2003)041801 Null oscillation probability: less than 1%. Δm 2 =1.5~3.9 x 10-3 ev 2 @sin 2 2θ=1(90%CL) Best fit: Δm 2 =2.8 x10 3 ev 2 Reconstructed Eν Allowed region 10
Analysis Flow Number of ν interaction @ Near Detectors (p µ, θ µ ) distributions ν interaction Neutrino flux/spectrum at Near Detector Near to Far extrapolation Neutrino flux/spectrum @ Super-K ν interaction Prediction N SK exp : Number of SK events E ν rec distribution for 1ring-µ Observation N SK obs 1Rµ E ν rec Null oscillation porbability, (sin 2 2θ 23, Δm 2 23) 11
Near Detector Measurements
Event Rate Measurement @1KT Use all type of interaction in the fiducial volume (25t). N exp SK = N obs KT Φ SK (E ν )σ(e ν )de ν Φ KT (E ν )σ(e ν )de ν M SK M KT ε SK ε KT Far/Near Ratio (by MC)~1 10-6 (verified w/ pion monitor) M: Fiducial mass M SK =22,500ton, M KT =25ton ε: efficiency ε SK-I(II) =77.0(78.2)%, ε KT =74.5% N SK exp =150.9 +11.6-10.0 (w/o oscillation) 13
Eν Reconstruction ν µ + n µ + p ν θ µ µ (E µ, p µ ) p (undetected in W-Ch) CC Quasi Elastic (CC-QE) 1Rµ in W-Cherenkov can reconstruct Eν (θ µ,p µ ) E rec ν = m N m N E E µ µ m + p µ 2 µ 2 cosθ µ ν µ + n µ + p + π θ µ µ - (E µ, p µ ) ν π s p ν µ + N ν + N + π s ν CC nqe Background for Eν meas. ν π s N NC 14
Used Data for Spectrum Meas. 1KT (1) Fully Contained 1 ring µ-like events SciFi (2) 1-track µ events (3) 2-track QE-like (4) 2-track nonqe-like Δθ p p µ SciBar (5) 1-track µ events (6) 2-track QE-like (7) 2-track nonqe-like 7 sets of (pµ,θµ) distributions ν spectrum Φ Near (E ν ) (8 bins) ν interaction model (nqe/qe) 15
θµ (degree) Actual procedure KT data Eν QE (MC) nqe(mc) 0-0.5 GeV MC templates 0.5-0.75GeV 0.75-1.0GeV Pµ (MeV/c) 1.0-1.5GeV ν spectrum Φ KEK (E ν ) (8 bins) ν interaction (nqe/qe)
Measured Spectrum Φ(E ν ) at ND preliminary pµ/θµ after fitting (SciBar 1track) E ν χ 2 =638.1 for 609 d.o.f nqe/qe = 1.02 ± 0.10 Assigned based on the variation by the fit condition. 17
Super-K Observation and Oscillation Analysis
Super-K Event Selection GPS ±500µsec T spill TOF=0.83msec SK T SK ±5µsec Analysis Time Window T DIFF. (µs) Fully contained in 22.5kt fiducial volume 108 events in 1.5 µsec window w/ negligible (O(10-3 )) background 19
K2K-all (K2K-I, K2K-II) FC 22.5kt 1ring µ-like e-like Multi Ring Super-K Event Summary DATA (K2K-I, K2K-II) 108 (56, 52) 66 (32, 34) 57 (30, 27) 9 (2, 7) 42 (24, 18) (56) for E rec ν preliminary MC (K2K-I, K2K-II) 150.9 (79.1, 71.8) 93.7 (48.6, 45.1 ) 84.8 (44.3, 40.5) 8.8 (4.3, 4.5) 57.2 (30.5, 26.7) Ref; K2K-I(47.9 10 18 POT), K2K-II(41.2 10 18 POT) 20
Maximum Likelihood Fit Total Number of events E ν rec spectrum shape of FC-1ring-µ events Systematic error term 2 x L( Δm,sin 2θ, f ) = L norm ( Δm 2,sin 2θ, f x ) L shape ( Δm 2,sin 2θ, f x ) L syst ( f x ) f x : Systematic error parameters Normalization, Spectrum, nqe/qe ratio, Super-K systematic uncertainty. 21
Best fit values in all region sin 2 2θ = 1.53 Δm 2 [ev 2 ] = 2.12 10-3 Fit Results Best fit values in the physical region sin 2 2θ = 1.00 Δm 2 [ev 2 ] = 2.73 10-3 Δm 2 A toy MC preliminary 14.4% ΔlogL=0.64 2.73 sin 2 2θ=1.53 can occur by statistical fluctuation with 14.4% probability. 1.00 1.53 sin 2 2θ 22
Fit Results N SK obs =108 N SK exp (best fit)=104.8 N SK exp (null osc.)=150.9 Best Fit KS prob.=52% Null osc. KS prob=0.11% Δm 2 [ev 2 ] preliminary Based on ΔlnL Eν rec [GeV] 1.7x10-3 <Δm 2 <3.5x10-3 @ sin 2 2θ=1.0 (90% C.L.) sin 2 2θ 23
Event Rate vs. Spectrum Δm 2 [ev 2 ] N SK (#ν µ ) only Δm 2 [ev 2 ] E ν shape only sin 2 2θ Consistent answer. sin 2 2θ 24
Comparison with Super-K K2K SK-I 2flavor SK-I L/E K2K result is consistent with SK-I results. 25
Null Oscillation Probability preliminary The null oscillation probabilities calculated based on ΔlnL. ν µ disappearance E ν spectrum distortion Combined K2K-I 2.0% 19.5% 1.3% (2.5σ) K2K-II 3.7% 5.4% 0.56% (2.8σ) K2K-all 0.33%(2.9σ) 1.1% (2.5σ) 0.011% (3.9σ) Disappearance of ν µ and distortion of the energy spectrum as expected in neutrino oscillation. K2K confirmed neutrino oscillation discovered in Super-K atmospheric neutrinos. 26
K2K Prospects Neutrino interaction study. Single π 0 production w/ 1kt. M A measurement w/ SciFi. Coherent pion production w/ SciBar. More will come soon! Oscillation analysis prospects. Improved analysis for ν µ disappearance. Update on ν e appearance search. } Finalizing results with ~half year more data, Then,.. x1.2 x2.5 27
Next Generation Exp.: T2K Approved in 2003! Super-K: 50 kton Water Cherenkov Kamioka ν µ νx disappearance ν µ νe appearance NC measurement second phase w/ upgrade: CP violation in lepton sector Proton decay LOI: http://neutrino.kek.jp/jhfnu ν µ beam of ~1GeV ( off axis beam) Φ~x100 of K2K J-PARC (Tokai-mura) 0.75 MW 50 (40) GeV PS (under construction) Construction of beamline has started this year. Plan to start exp. in 2009 28
T2K Physics Sensitivity δ(sin 2 2θ 23 ) δ(δm 23 2 ) νµ disappearance OAB-2 o OAB-3 o 5 years Sin 2 2θ 13 >0.006 CHOOZ excluded x20 improvement True Δm 23 2 (ev 2 ) ~1% precision!! Sensitivity for νµ νe oscillation 29
Summary K2K, the first accelerator-based long baseline neutrino experiment, confirmed neutrino oscillation observed by Super-K atm-ν. Number of events 2.9σ Eν spectrum distortion 2.5σ Combined 3.9σ Proved that accelerator-based LBL (>100km) experiment can do excellect job! Next generation experiment in Japan, T2K, was approved and is being designed/ constructed for startup in 2009. 30