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The detection of cosmic neutrino background with helicity-changing decays

Jihong Huang, Shun Zhou

TL;DR

This work addresses the detection of the cosmic neutrino background (CνB) in the presence of invisible decays mediated by a Majoron, incorporating helicity-preserving and helicity-changing channels for both Dirac and Majorana neutrinos. It provides explicit expressions for decay rates and a suppression factor that governs the cosmological evolution of relic neutrino number densities $n_i(h_i)$ during the expansion of the Universe, then translates these dynamics into predicted capture rates for PTOLEMY-like experiments. The key results show that helicity-changing decays can substantially modify the CνB capture rates, with Majorana-NO scenarios reaching around $16~{ m yr}^{-1}$ and Dirac-NO around $11~{ m yr}^{-1}$, while inverted ordering is strongly suppressed (<$1~{ m yr}^{-1}$). These predictions offer a concrete avenue to probe Majoron couplings and the absolute neutrino mass scale via future CνB measurements, enabling tests of physics beyond the Standard Model.

Abstract

In this talk, we present the investigation of the invisible decays of a heavy massive neutrino into a lighter neutrino and a massless Nambu-Goldstone boson, i.e., $ν_i^{} \to ν_j^{} + φ$. The total decay rates are calculated in the most general case, where the individual helicities of both parent and daughter neutrinos are specified. We then examine the evolution of the number densities of cosmological relic neutrinos throughout the expansion of the Universe, and explore the consequent impacts on the capture rates in PTOLEMY-like experiments. The total event rates can be significantly modified compared to those in the scenario of stable neutrinos, with helicity-changing decays playing an especially important role in the Dirac neutrino case.

The detection of cosmic neutrino background with helicity-changing decays

TL;DR

This work addresses the detection of the cosmic neutrino background (CνB) in the presence of invisible decays mediated by a Majoron, incorporating helicity-preserving and helicity-changing channels for both Dirac and Majorana neutrinos. It provides explicit expressions for decay rates and a suppression factor that governs the cosmological evolution of relic neutrino number densities during the expansion of the Universe, then translates these dynamics into predicted capture rates for PTOLEMY-like experiments. The key results show that helicity-changing decays can substantially modify the CνB capture rates, with Majorana-NO scenarios reaching around and Dirac-NO around , while inverted ordering is strongly suppressed (<). These predictions offer a concrete avenue to probe Majoron couplings and the absolute neutrino mass scale via future CνB measurements, enabling tests of physics beyond the Standard Model.

Abstract

In this talk, we present the investigation of the invisible decays of a heavy massive neutrino into a lighter neutrino and a massless Nambu-Goldstone boson, i.e., . The total decay rates are calculated in the most general case, where the individual helicities of both parent and daughter neutrinos are specified. We then examine the evolution of the number densities of cosmological relic neutrinos throughout the expansion of the Universe, and explore the consequent impacts on the capture rates in PTOLEMY-like experiments. The total event rates can be significantly modified compared to those in the scenario of stable neutrinos, with helicity-changing decays playing an especially important role in the Dirac neutrino case.
Paper Structure (4 sections, 8 equations, 1 figure)

This paper contains 4 sections, 8 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Neutrino Invisible Decays
  3. Cosmic Neutrino Background
  4. Summary

Figures (1)

  • Figure 1: The capture rates for Majorana (left) and Dirac (right) neutrinos in the case of NO Huang:2024tbo. The green dot-dashed curves represent the total capture rates, while the contributions from each mass eigenstate with a specific helicity are shown separately. For Dirac neutrinos, the capture rates with all decays being helicity-preserving are plotted in the green dotted curve.