L/E analysis of the atmospheric neutrino data from Super-Kamiokande Itaru Higuchi ICRR for Super-Kamiokande collaboration ICRC2005 Aug.5, 2005

Motivation Other models can explain zenith angle dependent muon deficit. How can we distinguish oscillation from other hypotheses ? Muon neutrino disappearance probability as a function of neutrino flight length L over neutrino energy E was studied to distinguish neutrino oscillation from other hypotheses. A dip in the L/E distribution was observed in the data from Super-Kamiokande-I during 1489 live-days exposure. We report preliminary result from SK-II during 627 days live-days exposure.

Use events with high resolution in L/E Survive probability Dm2L Neutrino oscillation : Pmm = 1 – sin22qsin2(1.27 ) E m L Neutrino decay : Pmm = (cos2q + sin2q x exp(– ))2 2t E 1 L Neutrino decoherence : Pmm = 1 – sin22q x (1 – exp(–g0 )) 2 E Use events with high resolution in L/E The first dip can be observed Direct evidence for oscillations Strong constraint to oscillation parameters, especially Dm2 value

Event samples in L/E analysis FC (tracks are contained inside the ID) Single-ring , μ-like Multi-ring , μ-like observed charge / expectation from through-going PC (deposits visible energy in the OD) OD stopping OD through-going Classify PC events using OD charge OD through-going MC OD stopping OD through going OD stopping MC

Reconstruction of E and L Neutrino energy Neutrino direction En Eobserved Eobserved  En Zenith angle  Flight length Neutrino energy is reconstructed from observed energy using relations based on MC simulation Neutrino flight length is estimated from zenith angle of particle direction

L/E resolution cut Select events with high resolution in L/E Full oscillation 1/2 oscillation Bad L/E resolution for horizontally going events  due to large dL/dcosq low energy events  due to large scattering angle D(L/E)=70%

Event summary of L/E analysis SK-I 1489 days SK-II 627 days FC Data MC CC nm Data MC CC nm single-ring multi-ring stopping through-going 1619 2105.6 (98.3%) 502 813.0 (94.2%) 114 137.0 (95.4%) 491 670.1 (99.2%) 686 876.6 (98.6%) 222 329.4 (93.6%) 51 48.5 (94.4%) 162 269.7 (99.2%) PC Total 2726 3725.7 1121 1524.2

L/E in atmospheric neutrino data Mostly upward Mostly upward SK-I FC+PC SK-II FC+PC Preliminary Null oscillation MC Null oscillation MC Mostly downward Mostly downward Best-fit expectation Best-fit expectation

L/E in atmospheric neutrino data SK-I FC+PC SK-II FC+PC Best fit expectation w/ systematic errors Best fit expectation w/ systematic errors Preliminary First dip is observed as expected from neutrino oscillation

Constraint to neutrino oscillation parameters SK-I SK-II Preliminary Best fit result Dm2=2.4x10-3,sin22q=1.00 c2min=37.8/40 d.o.f (sin22q=1.02, c2min=37.7/40 d.o.f) Dm2=2.6x10-3,sin22q=1.00 c2min=54.8/40 d.o.f (sin22q=1.02, c2min=54.7/40 d.o.f) 1.9x10-3 < Dm2 < 3.0x10-3 eV2 0.90 < sin22q at 90% C.L. 1.8x10-3 < Dm2 < 4.0x10-3 eV2 0.83 < sin22q at 90% C.L.

Tests for neutrino decay & decoherence SK-I SK-II Oscillation Decay Decoherence Dc2 = c2decay - c2osc 　　 =11.4(3.4σ) Dc2 = c2decoherence - c2osc =14.6(3.8 σ) Dc2 = c2decay - c2osc 　　 =7.9(2.8σ) Dc2 = c2decoherence - c2osc =8.7(2.9 σ) Preliminary cannot be explained by alternative hypotheses

Next step : combine SK-I and SK-II Conclusions Measurement of L/E dependence of flavor transition probability First dip was observed as expected from neutrino oscillation  cannot be explained by alternative hypotheses  gives strong constraint to neutrino oscillation parameters The results from SK-I and SK-II agree well. Next step : combine SK-I and SK-II

Zenith angle distributions Motivation Zenith angle distributions SK-I m-like sub-GeV < 400 MeV m-like sub-GeV > 400 MeV m-like multi-GeV + PC m-like sub-GeV multi-ring m-like multi-GeV multi-ring Oscillation Decay Decoherence Other models can explain zenith angle dependent muon deficit. How can we distinguish oscillation from other hypotheses ?

Angular resolutions

Lepton scattering angle as a function of momentum ne CC elastic Low energy leptons have weak angular correlation to the parent n direction. 0o 180o nm CC elastic 0o 3000 (MeV/c)

Energy and path length reconstruction Momentum weighted vector sum  Reconstructed neutrino direction Momentum sum  Eobserved Resolution of neutrino direction FC single-ring En Eobserved  En Eobserved Resolution of neutrino energy  flight length Due to large dL/dcosq near horizon, events near horizon have bad L/E resolution

Definition of c2 Poisson with systematic errors Nobs : observed number of events Nexp : expectation from MC ei : systematic error term si: sigma of systematic error Various systematic effects in detector, flux calculation and neutrino interaction are taken into account

Constraint to neutrino oscillation parameters Preliminary 1.9x10-3 < Dm2 < 3.0x10-3 eV2 0.90 < sin22q at 90% C.L. Dm2=2.4x10-3,sin22q=1.00 c2min=37.8/40 d.o.f (sin22q=1.02, c2min=37.7/40 d.o.f) Consistent with standard zenith angle analysis L/E analysis Zenith angle analysis 90% allowed regions

Tests for neutrino decay & decoherence for SK-I Oscillation Decay Decoherence c2min=37.8/40 d.o.f c2min=49.2/40 d.o.f  Dc2 =11.4 c2min=52.4/40 d.o.f  Dc2 =14.6 3.4 s to n decay 3.8 s to n decoherence First dip observed in data cannot be explained by alternative hypotheses

Tests for neutrino decay & decoherence for SK-II Preliminary Oscillation Decay Decoherence c2min=54.8/40 d.o.f c2min=62.8/40 d.o.f  Dc2 =7.9 c2min=63.5/40 d.o.f  Dc2 =8.7 2.8 s to n decay 2.9 s to n decoherence First dip observed in data cannot be explained by alternative hypotheses

Sensitivities to alternative models n decoherence obtained Dc2 Consistent with the expectation n decay Assumption nm nt 2 flavor oscillation (Dm2=2.0x10-3eV2,sin22q=1.0) L/E resolution cut at 70%

SK-I chi2 dist

SK-II chi2 dist

SK-I SK-II Neutrino decay χ2min,Δχ2 χ2min = 49.1/40 d.o.f Neutrino decoherenceχ2min,Δχ2 SK-I χ2min = 49.1/40 d.o.f Δχ2 = 11.3/40 d.o.f (3.4 standard deviations) χ2min = 52.4/40 d.o.f Δχ2 = 14.5/40 d.o.f (3.8 standard deviations) SK-II χ2min = 62.8/40 d.o.f Δχ2 = 7.9/40 d.o.f (2.8 standard deviations) χ2min = 63.5/40 d.o.f Δχ2 = 8.7/40 d.o.f (2.9 standard deviations)

Results from the oscillation analysis on the SK-I and SK-II χ2 at (sin22θ,Δm2) include unphysical region 90% C.L. allowed region SK-I 37.9/40 d.o.f (1.00,2.4×10-3eV2) (1.00,2.4×10-3eV) sin22θ> 0.9 1.9<Δm2<3.0×10-3eV2 SK-II 54.8/40 d.o.f (1.00,2.6×10-3eV) sin22θ> 0.83 1.8<Δm2<4.0×10-3eV2