[Paper Review] Neutron Dark Matter Decays

A. N. Ivanov, Roman Höllwieser+3 more|arXiv (Cornell University)|Jun 26, 2018

Atomic and Subatomic Physics Research63 references4 citations

TL;DR

This paper proposes a new explanation for the neutron lifetime puzzle by introducing two dark matter decay modes: $n \to \chi + \nu_e + \bar{\nu}_e$ and $n \to \chi + e^- + e^+$, where $\chi$ is a dark matter Dirac fermion. Unlike prior models, it shows that the $n \to \chi + e^- + e^+$ mode can be undetectable yet still resolve the discrepancy between bottle and beam experiment results through Fierz interference, with a UV-complete $SU_L(2)\times U_R(1)\times U_R'(1)\times U_L''(1)$ gauge theory model that satisfies LHC and astrophysical constraints.

ABSTRACT

We analyse the discrepancy between the neutron lifetimes measured in the bottle and beam experiments. Following Fornal and Grinstein (Phys. Rev. Lett. 120, 191801 (2018)) we propose an explanation of such a puzzle by the dark matter channels of the neutron decay. However, unlike Fornal and Grinstein in addition to the dark matter decay channel n -> χ+ e^- + e^+, where χis a dark matter Dirac fermion and (e^-e^+) is an electron--positron pair, we assume the existence of the dark matter channel n -> chi + ν_e + \barν_e, where ν_e \barν_e is the electron neutrino-antineutrino pair. This allows to describe the discrepancy between the measurements of the neutron lifetime even in case of an unobservability of the dark matter decay channel n -> χ+ e^- + e^+, which may be below the reaction threshold. The existence of the coupling n -> χ+ e^- + e^+ can be observed experimentally by measuring electron-neutron scattering e^- + n -> χ+ e^- at very low electron energies, induced with the strength as of the decay n -> χ+ ν_e + \barν_e$. We propose a gauge invariant quantum field theory model with SU_L(2) imes U_R(1) imes U_R'(1) imes U''_L(1) symmetry for the UV completion of the effective (nχ\ell \bar{\ell}) interaction, where \ell(\bar{\ell}) is electron (positron) or neutrino(antineutrino).

Motivation & Objective

• To resolve the persistent 1% discrepancy between neutron lifetime measurements in bottle and beam experiments.
• To extend Fornal and Grinstein's dark matter decay hypothesis by including the $n \to \chi + \nu_e + \bar{\nu}_e$ mode as a key contributor.
• To construct a gauge-invariant quantum field theory model that UV-completes the effective $(n\chi\ell\bar{\ell})$ interaction with $SU_L(2)\times U_R(1)\times U_R'(1)\times U_L''(1)$ symmetry.
• To demonstrate that the model avoids conflict with LHC constraints, neutron star cooling, and dark matter production limits.
• To show that the $n \to \chi + e^- + e^+$ mode can be undetectable yet still affect the neutron lifetime via Fierz interference.

Proposed method

• Introduces an effective low-energy interaction Lagrangian for $n \to \chi + \ell + \bar{\ell}$, including both electron-positron and neutrino-antineutrino final states.
• Constructs a UV-complete quantum field theory model with $SU_L(2)\times U_R(1)\times U_R'(1)\times U_L''(1)$ gauge symmetry to realize the effective interaction.
• Derives the amplitude for $e^- + n \to \chi + e^-$ at low energies, induced by the $n \to \chi + \nu_e + \bar{\nu}_e$ mode, to probe the coupling.
• Estimates the suppression scale $\Lambda_{\rm DM} \geq 790\,\text{GeV}$ for $q\bar{q} \to \chi\bar{\chi}$ using quark condensate $\langle \bar{d}d \rangle = -(0.240\,\text{GeV})^3$.
• Uses the $t$-channel $u_j\bar{u}^j \to \chi\bar{\chi}$ process to estimate $|\xi_\chi| \sim 1/\Lambda_\chi^4$ with $\Lambda_\chi \sim 2.4\,\text{GeV}$, consistent with ATLAS constraints.
• Analyzes neutron star cooling via URCA-like processes: $n+n \to \chi+\chi$, $n+n \to \chi+\chi+\nu_e+\bar{\nu}_e$, and $\chi+\chi \to n+n$.

Experimental results

Research questions

• RQ1 Can the $n \to \chi + \nu_e + \bar{\nu}_e$ decay mode alone resolve the neutron lifetime discrepancy even if $n \to \chi + e^- + e^+$ is below threshold?
• RQ2 How can the $n \to \chi + e^- + e^+$ mode be consistent with experimental non-observation while still affecting the neutron lifetime?
• RQ3 What is the UV completion of the effective $(n\chi\ell\bar{\ell})$ interaction that preserves gauge invariance and avoids anomalies?
• RQ4 Do the proposed dark matter decay modes conflict with constraints from the LHC, neutron star cooling, or dark matter production?
• RQ5 Can the model support URCA-like processes in neutron stars that enhance cooling rates?

Key findings

• The model explains the neutron lifetime discrepancy without requiring the $n \to \chi + e^- + e^+$ mode to be observable, as the $n \to \chi + \nu_e + \bar{\nu}_e$ mode alone can account for the 1% difference.
• The Fierz interference term in the neutron $\beta^-$ decay is predicted to be $b \sim -10^{-2}$, indicating a significant beyond-Standard Model contribution.
• The UV-complete model with $SU_L(2)\times U_R(1)\times U_R'(1)\times U_L''(1)$ gauge symmetry realizes the effective interaction and avoids Adler-Bell-Jackiw anomalies.
• The suppression scale for $q\bar{q} \to \chi\bar{\chi}$ is estimated at $\Lambda_{\rm DM} \geq 790\,\text{GeV}$, consistent with ATLAS constraints on dark matter production.
• The model allows for URCA-like processes in neutron stars: $n+n \to \chi+\chi$, $n+n \to \chi+\chi+\nu_e+\bar{\nu}_e$, and $\chi+\chi \to n+n$, which can enhance cooling rates.
• The coupling strength is estimated as $|\xi_\chi| \sim 1/\Lambda_\chi^4$ with $\Lambda_\chi \sim 2.4\,\text{GeV}$, consistent with low-energy electron-neutron scattering signals.