Measuring Anisotropies in the Cosmic Neutrino Background

TL;DR

This paper discusses how polarized tritium targets can detect anisotropies in the cosmic neutrino background, providing insights into neutrino properties and cosmological effects.

Contribution

It introduces a method to measure CvB anisotropies using polarized targets, highlighting potential for new physics constraints and tests of neutrino nature.

Findings

Relic neutrinos are captured most efficiently when anti-aligned with target polarization.

Peculiar velocities and gravitational effects induce small dipole anisotropies.

Upcoming PTOLEMY experiment can test isotropy and neutrino properties.

Abstract

Neutrino capture on tritium has emerged as a promising method for detecting the cosmic neutrino background (CvB). We show that relic neutrinos are captured most readily when their spin vectors are anti-aligned with the polarization axis of the tritium nuclei and when they approach along the direction of polarization. As a result, CvB observatories may measure anisotropies in the cosmic neutrino velocity and spin distributions by polarizing the tritium targets. A small dipole anisotropy in the CvB is expected due to the peculiar velocity of the lab frame with respect to the cosmic frame and due to late-time gravitational effects. The PTOLEMY experiment, a tritium observatory currently under construction, should observe a nearly isotropic background. This would serve as a strong test of the cosmological origin of a potential signal. The polarized-target measurements may also constrain non-standard neutrino interactions that would induce larger anisotropies and help discriminate between Majorana versus Dirac neutrinos.