Detecting non-relativistic cosmic neutrinos by capture on tritium: phenomenology and physics potential

TL;DR

This paper explores the potential of the PTOLEMY experiment to detect the Cosmic Neutrino Background via neutrino capture on tritium, offering insights into neutrino mass, nature, and cosmological implications.

Contribution

It provides a detailed phenomenological analysis of neutrino capture on tritium, highlighting the experiment's sensitivity to neutrino mass, nature, and potential new physics.

Findings

Detection is feasible for neutrino masses above 0.1 eV with high energy resolution.

Capture rates differ for Dirac and Majorana neutrinos, enabling potential discrimination.

The experiment could probe neutrino clustering, lepton asymmetry, and sterile neutrinos.

Abstract

We study the physics potential of the detection of the Cosmic Neutrino Background via neutrino capture on tritium, taking the proposed PTOLEMY experiment as a case study. With the projected energy resolution of $\Delta \sim$ 0.15 eV, the experiment will be sensitive to neutrino masses with degenerate spectrum, $m_1 \simeq m_2 \simeq m_3 = m_\nu \gtrsim 0.1$ eV. These neutrinos are non-relativistic today; detecting them would be a unique opportunity to probe this unexplored kinematical regime. The signature of neutrino capture is a peak in the electron spectrum that is displaced by $2 m_{\nu}$ above the beta decay endpoint. The signal would exceed the background from beta decay if the energy resolution is $\Delta \lesssim 0.7 m_\nu $. Interestingly, the total capture rate depends on the origin of the neutrino mass, being $\Gamma^{\rm D} \simeq 4$ and $\Gamma^{\rm M} \simeq 8$ events per year (for a 100 g tritium target) for unclustered Dirac and Majorana neutrinos, respectively. An enhancement of the rate of up to $\mathcal{O}(1)$ is expected due to gravitational clustering, with the unique potential to probe the local overdensity of neutrinos. Turning to more exotic neutrino physics, PTOLEMY could be sensitive to a lepton asymmetry, and reveal the eV-scale sterile neutrino that is favored by short baseline oscillation searches. The experiment would also be sensitive to a neutrino lifetime on the order of the age of the universe and break the degeneracy between neutrino mass and lifetime which affects existing bounds.