Measurement of the atmospheric neutrino-induced upgoing muon flux using MACRO

TL;DR

This paper reports on the MACRO detector's measurement of atmospheric neutrino-induced upgoing muons, providing evidence for neutrino oscillations through analysis of flux and zenith distribution discrepancies.

Contribution

First measurement of upgoing muon flux with MACRO, showing evidence for neutrino oscillations through flux suppression and zenith angle distribution analysis.

Findings

Observed flux ratio of 0.74 compared to expectations.

Zenith distribution inconsistent with no oscillation hypothesis.

Best-fit oscillation parameters: sin^2(2θ)=1.0, Δm^2≈few×10^-3 eV^2.

Abstract

We present a measurement of the flux of neutrino-induced upgoing muons (<E_nu>~100 GeV) using the MACRO detector. The ratio of the number of observed to expected events integrated over all zenith angles is 0.74 +/- 0.036 (stat) +/- 0.046(systematic) +/- 0.13 (theoretical). The observed zenith distribution for -1.0 < cos(theta) < -0.1 does not fit well with the no oscillation expectation, giving a maximum probability for chi^2 of 0.1%. The acceptance of the detector has been extensively studied using downgoing muons, independent analyses and Monte-Carlo simulations. The other systematic uncertainties cannot be the source of the discrepancies between the data and expectations. We have investigated whether the observed number of events and the shape of the zenith distribution can be explained by a neutrino oscillation hypothesis. Fitting either the flux or zenith distribution independently yields mixing parameters of sin^2 (2theta)=1.0 and delta m^2 of a few times 10^-3 eV^2. However, the observed zenith distribution does not fit well with any expectations giving a maximum probability for chi^2 of 5% for the best oscillation hypothesis, and the combined probability for the shape and number of events is 17%. We conclude that these data favor a neutrino oscillation hypothesis, but with unexplained structure in the zenith distribution not easily explained by either the statistics or systematics of the experiment.