Atmospheric Neutrino Oscillations in Soudan 2

Peter Litchfield

For the Soudan 2 Collaboration

Neutrino and background data

Azimuthal, zenith and L/E distributions

Bin-free likelihood analysis

Feldman-Cousins analysis for confidence limits on m2 and sin22

Results submitted to Phys Rev D and more details in hep-ex/0307069

Soudan 2 Detector

1 kiloton fine grained drifting calorimeter

Individual tracks reconstructed, good energy and pointing resolution

Excellent particle id, ~3% flavour misidentification

Veto shield flags cosmic ray generated events

e quasi-elastic inelastic

Data Sets

5.90 kiloton years of data taken between 1989 and 2001. 488 contained and partially contained and e candidate events with

no activity in the surrounding veto shield. 516 contained events with in time shield activity, produced by

secondary neutrons and gammas from cosmic ray muon interactions. Used to simulate the background in the neutrino events.

3624 MC neutrino events generated using the Bartol 96 flux and the NEUGEN neutrino event generator, inserted in the data stream and processed identically to the data events.

Data divided into; high resolution sample (high energy events with reconstructed

recoil protons and pions) which have good pointing accuracy The rest, low energy, low resolution sample.

Backgrounds

Neutrino events distinguished statistically from neutron and gamma events via the depth distribution in the detector

Background depth distribution represented by the shield tagged sample, neutrino by the MC

Neutron interaction length ~80cm, gamma conversion length ~15cm

Backgrounds dependent on event energy and topology

Data divided into 6 samples and fraction of background fitted independently in each.

~10% overall background

MC+background

MC Background

Zenith and Azimuthal Angles

Oscillation probability

High resolution events L depends only on the zenith

angle, not the azimuthal angle e-flavor events agree with the

MC prediction up to a normalization factor

-flavor events show a zenith angle dependent depletion

Soudan is near the magnetic pole; no geomagnetic azimuthal dependence

E

LmP 222 27.1sin2sin

Unoscillated MC+background

Best fit oscillation parameters +background

Background contribution

L/E distribution

High resolution events e-flavor events follow the MC

prediction up to a ~90% normalization factor

-flavor events show a depletion above log10(L/E) of ~ 1.5

Consistent with neutrino oscillations with m2<~0.025 eV2

Unoscillated MC+background

Best fit oscillation parameters +background

Background contribution

Likelihood Analysis

Data assumed to be represented by the sum of MC neutrino events, weighted by the oscillation probability plus background represented by the shield tagged events

Bin-free likelihood analysis performed. Likelihood given by (d = depth)

Shape factor Q given by

k sums taken over the different data sets, background fraction fitted independently in each

X is a pdf describing the L/E distributions and containing the oscillation probability for the neutrino events

D is a pdf describing the depth distribution

eee1,5k N1,i

kkkii

k MMNMMNdELQLk

lnln),/(ln

)()/()()2sin,,/(1

, 22i

kR

kR

kRi

kkkkii

k dDELXNdDmELXACM

dxQ

Likelihood results

Likelihood calculated on a 15x80 grid of sin22, log10m2

Difference (L) from best likelihood plotted.

No oscillations gives a likelihood rise of 11.3

Best likelihood sin22=0.97 m2=0.0052 Normalization of MC relative

to Bartol 96=0.91 Fraction of background=9.6%

Feldman-Cousins analysis

Proposed by Feldman and Cousins for the calculation of confidence levels to allow for statistics, systematic errors, physical constraints and any biases in the analysis procedure.

At each grid square MC experiments are generated and analyzed just like the data.

Neutrino events and background events are incorporated at each grid square according to the fitted amounts at that square in the data

Systematic effects are incorporated into the MC experiments.

For the uncertainty in energy calibration

For the uncertainty in the neutrino flux as a function of energy

For the uncertainty in the /e flux ratio

For the uncertainty in the neutrino cross-sections

Feldman-Cousins analysis

1000 experiments generated at each grid point and the difference between the best fit likelihood and the likelihood at the generated grid square (LMC)

histogramed.

This is the likelihood distribution expected if the truth was at this grid square

The likelihood containing 90% of the MC experiments (L90) is obtained. If the data likelihood is less than L90 the point is within the 90% confidence region.

The probability of the no oscillation hypothesis is given by the fraction of experiments at the no oscillation grid square with likelihood > data likelihood (5.8x10-4)

“no oscillation” (min m2,sin22)

100000 experiments

L90 surface

Shape is mostly a function of the background and Monte Carlo normalization fitting.

Some contribution from the systematic effects on the neutrino flux, neutrino cross-sections and detector energy calibration

L90 is significantly greater than 2.3 over most of the surface

Confidence regions

68%,90% and 95% confidence level contours are plotted

The 90% sensitivity (defined as the 90% confidence contour calculated from the MC experiments) is slightly bigger than the data 90% contour, more corresponding to the 95% contour. Due to the data electron and muon best normalisation being slightly different.

The contour defined by a 2.3 rise in data likelihood corresponds closely to the 68% Feldman-Cousins confidence level contour

Comparison with other experiments

There is excellent agreement at the 90% level between Soudan 2, Super-K and MACRO.

At the 68% confidence level Soudan 2 prefers the lower part of the Super-K allowed region

The 3-d Battistoni prediction for the neutrino flux gave very similar results for the confidence regions but required a normalization factor of 1.05 to be compared with 0.91 for the Bartol 96 calculation

Summary

Soudan 2 observes a depletion of the atmospheric flux, consistent with oscillations.

A bin-free likelihood analysis has been carried out using the Feldman-Cousins prescription.

The probability of the no oscillation hypothesis is 5.8x10-4.

The 90% confidence level contours in the sin22,m2 plane are consistent with those found by Super-K and MACRO.

This is the first experiment to confirm the Super-K results on fully reconstructed neutrino events covering the full angular and L/E range.

Soudan 2 is a drifting iron calorimeter at a different site to the Super-K water cherenkov. All detector based systematics are quite different.

The excellent agreement is a strong confirmation that neutrino oscillations, and thus neutrino mass, exist