Atmospheric and Astrophysics Neutrinos with MACRO

TL;DR

The MACRO experiment observed neutrino oscillations through deficits and angular distribution distortions in atmospheric neutrino events, providing evidence for muon-tau neutrino oscillations with specific mass differences, and set limits on dark matter particle signals.

Contribution

This study provides experimental evidence for neutrino oscillations and constrains neutrino mass differences, while also searching for dark matter signals with no positive detection.

Findings

Observed fewer upward muons than expected without oscillations.

Best fit for neutrino oscillation parameters is maximal mixing with Δm^2 around 10^-3 eV^2.

No evidence of dark matter signals was found, setting limits on neutralino mass.

Abstract

MACRO can detect three topologies of neutrino induced events, corresponding to different parent neutrino energies. The most numerous sample is made of upward throughgoing muons induced by atmospheric neutrinos of average energy 100 GeV. We have observed less events than expected events and the observed zenith distribution does not fit the expected one in the no-oscillation hypothesis, giving a maximum probability of 0.1 percent. Considering the neutrino (numu-nutau) oscillation hypothesis the best probability (17 percent) is obtained for maximum mixing and difference of square masses of a few times 10-3 eV2. The other detected samples are due to internally produced events and upward-going stopping muons, corresponding to an average neutrino energy of around 4 GeV. The results concerning these lower energy sample show a deficit and a shape of the angular distributions in agreement with that predicted by the oscillation model suggested by the higher energy sample. No evidence of a neutrino signal due to dark matter particles in the direction of the core of the Earth and of the Sun has been found among the background due to atmospheric neutrinos and limits have been set. The neutralino hypothesis is investigated and limits on its mass are given.