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Constraints on millicharged particles from nuclear gamma-decays

Ting Gao, Maxim Pospelov

TL;DR

This work investigates millicharged particles (MCPs) as a dark-sector portal by analyzing MCP production in nuclear γ decays, with a focus on reactors as intense MCP sources. It introduces a reactor-based MCP production mechanism via off-shell photons accompanying γ de-excitations, deriving energy-differential production rates and fluxes, and uses near-reactor electron-recoil data to set constraints on the millicharge $\varepsilon$ in the MeV mass range, achieving leading limits around $0.7-2$ MeV. Beyond reactors, the paper estimates MCP fluxes from natural radioactivity (geo-neutrinos) and the Sun, outlining their potential detectability in large, low-threshold detectors and highlighting the distinct mass and flux features of solar MCPs. Additionally, the authors extend the analysis to MeV-scale dark photons produced in nuclear de-excitation, providing near-reactor constraints as an independent cross-check. Overall, the study broadens the experimental reach for MCPs to higher masses and motivates future low-threshold detectors to exploit these MeV-scale pathways.

Abstract

We consider nuclear gamma decays and $γ$-emitting reactions that can be an efficient source of hypothetical millicharged particles ($χ$). In particular, we revisit the production of millicharged particles in nuclear reactor environment, pointing out that $γ$ cascades from $^{239}$U is an overlooked yet a powerful source of $χ\barχ$ pairs. This leads to an increased flux compared to previous studies. We then apply new estimates of the flux to derive novel limits on the value of millicharge, $\varepsilon = Q_χ/e$, from the electron recoil searched for in a variety of experiments placed in proximity to the reactor cores. The derived limits on $\varepsilon$ are the strongest in the interval of masses $\sim 0.7-2 $\,MeV. We also derive the MCP flux from the Sun and point out potential sensitivity of the low-threshold dark matter search experiments.

Constraints on millicharged particles from nuclear gamma-decays

TL;DR

This work investigates millicharged particles (MCPs) as a dark-sector portal by analyzing MCP production in nuclear γ decays, with a focus on reactors as intense MCP sources. It introduces a reactor-based MCP production mechanism via off-shell photons accompanying γ de-excitations, deriving energy-differential production rates and fluxes, and uses near-reactor electron-recoil data to set constraints on the millicharge in the MeV mass range, achieving leading limits around MeV. Beyond reactors, the paper estimates MCP fluxes from natural radioactivity (geo-neutrinos) and the Sun, outlining their potential detectability in large, low-threshold detectors and highlighting the distinct mass and flux features of solar MCPs. Additionally, the authors extend the analysis to MeV-scale dark photons produced in nuclear de-excitation, providing near-reactor constraints as an independent cross-check. Overall, the study broadens the experimental reach for MCPs to higher masses and motivates future low-threshold detectors to exploit these MeV-scale pathways.

Abstract

We consider nuclear gamma decays and -emitting reactions that can be an efficient source of hypothetical millicharged particles (). In particular, we revisit the production of millicharged particles in nuclear reactor environment, pointing out that cascades from U is an overlooked yet a powerful source of pairs. This leads to an increased flux compared to previous studies. We then apply new estimates of the flux to derive novel limits on the value of millicharge, , from the electron recoil searched for in a variety of experiments placed in proximity to the reactor cores. The derived limits on are the strongest in the interval of masses \,MeV. We also derive the MCP flux from the Sun and point out potential sensitivity of the low-threshold dark matter search experiments.

Paper Structure

This paper contains 8 sections, 26 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Pair-production from nuclear de-excitation
  3. Millicharged particle detection via atomic ionization
  4. MCPs from naturally occurring decays and reactions
  5. Dark photon from nuclear de-excitation
  6. Discussion and Conslusions
  7. Acknowledgments
  8. Pair production from E2 and E3 transitions

Figures (5)

  • Figure 1: Differential ratios for E1 (\ref{['fig:rE']}) and M1(\ref{['fig:rB']}) transitions with ${\varepsilon}=1$ for different values of $m_{\chi}/\omega$. Quantities with a bar on top are normalized by $\omega$ for universality: $\bar{m}_{\chi}=m_{\chi}/\omega$, $\bar{E}_{\chi}=E_{\chi}/\omega$. Notice that when $m_\chi \ll \omega/2$, the E1 and M1 yields of MCP are similar, while for heavier MCP (relative to $\omega/2$), the M1 yields are more suppressed, as expected.
  • Figure 2: Atomic ionization differential cross sections for germanium crystal under Photo Absorption Ionization model (PAI), free electron approximation (FEA), and equivalent photon approximation (EPA) for $m_{\chi}=1\mathrm{keV}$, $E_{\chi}=1\mathrm{MeV}$ (\ref{['fig:sigma1']}) and $m_{\chi}=1\mathrm{MeV}$, $E_{\chi}=1.5\mathrm{MeV}$ (\ref{['fig:sigma2']}). The photo-absorption cross section is obtained from Henke:1993eda. When the millicharged particle is ultra-relativistic, PAI agrees with the maximum between FEA and EPA as expected, when $E_{\chi}$ and $m_{\chi}$ are relatively close, EPA ceases to be a good approximation, while PAI continues to capture the enhancement at low energy transfer and the free electron behavior at high energy transfer.
  • Figure 3: (\ref{['fig:fit']}): An example of the comparison between the analysis from TEXONO and this work with $m_{\chi}=1\mathrm{MeV}$, ${\varepsilon}=2.7\times10^{-5}$. (\ref{['fig:constraint']}): The constraint we get on MCPs compared to TEXONO TEXONO:2018nir, CONNIE and Atucha-II CONNIE:2024off, and SLAC Prinz:1998ua. Our result, based on theoretical re-evaluation of the MCP flux in conjunction with past experimental searches, provides the strongest constraint in the mass range $0.7-2\mathrm{MeV}$.
  • Figure 4: Relative production of solar MCPs due to different reactions. For $m_\chi < m_e$ the flux is dominated by the positron annihilation.
  • Figure 5: Excluded region for dark photon from this work and the SLAC E137 experiment Bjorken:1988asAndreas:2012mtMarsicano:2018krp. The entire plot region is also excluded by the constraint based on supernova Chang:2016ntp.