L/E analysis of the atmospheric neutrino data from Super-

Kamiokande

Itaru Higuchi

ICRR

for Super-Kamiokande collaboration

ICRC2005 Aug, 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.

P = (cos2sin2x exp(– ))2m2

Survive probability

Neutrino oscillation :

P = 1 – sin22sin2(1.27 )

m2LE

Neutrino decoherence :

P = 1 – sin22 x (1 – exp(–))

21

Use events with high resolution in L/E

Neutrino decay :

The first dip can be observed

Direct evidence for oscillations Strong constraint to oscillation parameters, especially m2 value

LELE

Event samples in L/E analysis

Classify PC events using OD charge

I. OD stoppingII. OD through going

FC (tracks are contained inside the ID)

PC (deposits visible energy in the OD)

observed charge / expectation from through-going

OD stopping

OD through-going

OD through-going MC

OD stopping MC

I. Single-ring , μ-likeII. Multi-ring , μ-like

Reconstruction of E and L

E

Eobserve

d

Eobserved E

Neutrino energy is reconstructed from observed energy using relations based on MC simulation

Zenith angle Flight length

Neutrino flight length is estimated from zenith angle of particle direction

Neutrino energy Neutrino flight length

L/E resolution cut

horizontally going events due to large dL/dcos

low energy events due to large scattering angle

Bad L/E resolution for

Select events with high resolution in L/E

Full oscillation

1/2 oscillation

(L/E)=70%

Event summary of L/E analysis

1619 2105.6 (98.3%)

502 813.0 (94.2%)

114 137.0 (95.4%)

491 670.1 (99.2%)

single-ring

multi-ring

stopping

through-going

FC

PC

Data MC CC

2726 3725.7Total

686 876.6 (98.6%)

222 329.4 (93.6%)

51 48.5 (94.4%)

162 269.7 (99.2%)

Data MC CC

1121 1524.2

SK-I 1489 days SK-II 627 days

L/E in atmospheric neutrino data

Preliminary

Null oscillation MC

Best-fit expectation

Mostly downward

Mostly upward

Null oscillation MC

Best-fit expectation

Mostly downward

Mostly upward

SK-I FC+PC SK-II FC+PC

L/E in atmospheric neutrino dataSK-I FC+PC

First dip is observed as expected from neutrino oscillation

Best fit expectation w/ systematic errors

Preliminary

SK-II FC+PC

Best fit expectation w/ systematic errors

Constraint to neutrino oscillation parameters

m2=2.4x10-3,sin22=1.00

2min=37.8/40 d.o.f

(sin22=1.02, 2min=37.7/40 d.o.f)

1.9x10-3 < m2 < 3.0x10-3 eV2

0.90 < sin22 at 90% C.L.

Preliminary

m2=2.6x10-3,sin22=1.00

2min=54.8/40 d.o.f

(sin22=1.02, 2min=54.7/40 d.o.f)

1.8x10-3 < m2 < 4.0x10-3 eV2

0.83 < sin22 at 90% C.L.

SK-I SK-II

Best fit results

Constraint to neutrino oscillation parameters

m2=2.4x10-3,sin22=1.00

2min=37.8/40 d.o.f

(sin22=1.02, 2min=37.7/40 d.o.f)

1.9x10-3 < m2 < 3.0x10-3 eV2

0.90 < sin22 at 90% C.L.

Preliminary

m2=2.6x10-3,sin22=1.00

2min=54.8/40 d.o.f

(sin22=1.02, 2min=54.7/40 d.o.f)

1.8x10-3 < m2 < 4.0x10-3 eV2

0.83 < sin22 at 90% C.L.

SK-I SK-II

Best fit results

Constraint to neutrino oscillation parameters

m2=2.4x10-3,sin22=1.00

2min=37.8/40 d.o.f

(sin22=1.02, 2min=37.7/40 d.o.f)

1.9x10-3 < m2 < 3.0x10-3 eV2

0.90 < sin22 at 90% C.L.

Preliminary

m2=2.6x10-3,sin22=1.00

2min=54.8/40 d.o.f

(sin22=1.02, 2min=54.7/40 d.o.f)

1.8x10-3 < m2 < 4.0x10-3 eV2

0.83 < sin22 at 90% C.L.

SK-I SK-II

Best fit results

Tests for neutrino decay & decoherence

Oscillation

Decay

Decoherence

2 = 2decay - 2

osc

　　 =11.4(3.4σ)

2 = 2decoherence - 2

osc

=14.6(3.8 σ)

SK-I SK-II

Preliminary

cannot be explained by alternative hypotheses

2 = 2decay - 2

osc

　　 =7.9(2.8σ)

2 = 2decoherence - 2

osc

=8.7(2.9 σ)

Conclusions

Measurement of L/E dependence of flavor transition probability

First dip was observed as expected from neutrino oscillation

The results from SK-I and SK-II agree well.

cannot be explained by alternative hypotheses

gives strong constraint to neutrino oscillation parameters

Next step : combine SK-I and SK-II