Cosmic neutrino background detection in the minimally extended Standard Model

TL;DR

This paper explores the detection of relic neutrinos within an extended Standard Model, analyzing various methods and how their sensitivity varies with lepton number violation, providing insights into neutrino nature.

Contribution

It introduces a comprehensive analysis of relic neutrino detection methods considering right-chiral neutrinos and the transition between Dirac and Majorana regimes.

Findings

Sensitivity varies smoothly between Dirac and Majorana regimes.

Neutral current interactions cause transitions between light and heavy neutrinos.

Relic neutrinos can constrain lepton number violation scale to below 10^{-33} eV.

Abstract

We investigate the sensitivity of relic neutrino detection methods within the Standard Model, extended to include right-chiral neutrino singlets with Majorana mass terms. In particular, we study neutrino capture on unstable nuclei, the Stodolsky effect, coherent scattering, and an accelerator experiment. We demonstrate that the sensitivity transitions smoothly between Dirac and Majorana regimes, depending on the scale of lepton number violation. Importantly, neutral current interactions lead to transitions between the light and heavy neutrino states, necessitating the use of a density matrix formalism for accurate sensitivity calculations. As the oldest source of neutrinos in the universe, relic neutrinos would be able to provide an ultimate constraint on the lepton number violating scale, $m_R\gtrsim 10^{-33}~{\rm eV}$, below which neutrinos would behave as Dirac fermions for all practical purposes.