Exploring new physics with astrophysical neutrinos at icecube

Abstract

In 1914, James Chadwick made a bizarre observation while conducting the beta decay experiment. It was expected that the neutrons decay to protons and electrons, and since it was a two body decay, the electron would manifest having fixed energy. But on observation, he found that electrons, produced during beta decay, had a range of energy as can be seen in fig. 1.1. In his desperation to uphold the law of conservation of energy in 1929, Pauli proposed a neutral fermion, which at that time, he called ‘neutron’ (as neutrons were not discovered yet). Later, after the discovery of neutrons by James Chadwick in 1932, this accompanying particle in beta decay got its name ‘neutrino’ or the little neutral one. This new particle was believed to be undetectable since it interacts very weakly, i.e., the cross-section is much less as compared to electromagnetic interaction. However, in 1956 Reines and Cowan detected electron antineutrinos by directing them to a water detector with CdCl2, via the process ¯νep → ne+ [1]. The first pulse is that the positron annihilates the electron in the detector to produce two 0.5 MeV photons, which were detected. Also, there is a second pulse by the capture of the neutron in the cadmium leading to a delayed coincidence. Later on, muon neutrino was discovered from pion decay at the Brookhaven National Laboratory in 1962 [2] and tau neutrino at the DONUT experiment at CERN in 2000 [3]. These neutrinos are associated with a family of charged leptons whose mass and interactions are successfully explained by the Standard Model (SM) of particle physics. The SM is a theory based on Lorentz and gauge invariance.