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Illuminating sequential freeze-in dark matter with dark photon signal at the CERN SHiP experiment

Xinyue Yin, Sibo Zheng

TL;DR

The paper studies a two-field freeze-in dark matter scenario with a light massive dark photon portal and analyzes its testability at the CERN SHiP experiment. By solving sequential freeze-in Boltzmann equations, it finds a near-constant dark charge $e'\approx 1.3\times 10^{-12}$ and a kinetic-mixing window $\epsilon_{\rm DM}\le \epsilon < \epsilon_{\rm th}$ with $\epsilon_{\rm DM}\sim (1-7)\times 10^{-11}$ and $\epsilon_{\rm th}\sim 10^{-8}-10^{-7.5}$ for $m_{A'}$ in the SHiP range. SHiP projections show 90% CL exclusions of $\epsilon \gtrsim 10^{-8.5}$ (vector meson dominance) or $\epsilon \gtrsim 10^{-7.9}$ (dipole) for $N=2\times 10^{20}$ (5 years) and $N=6\times 10^{20}$ (15 years), leaving only a narrow region near $\epsilon\sim 10^{-11}$. Consequently, SHiP can decisively test the sequential freeze-in DM scenario with a light dark photon, significantly narrowing the viable parameter space for this DM realization.

Abstract

Single-field freeze-in dark matter barely leaves observable footprints in dark matter direct detection, collider or fixed-target experiments, which can be altered in the two-field context. In this work, we consider sequential freeze-in dark matter through signals of dark photon mediator with a mass range of $m_{A'}\sim 10^{-2}-10$ GeV covered by the proposed SHiP experiment. We show that the dark charge is fixed to be $e'\sim 1.3\times 10^{-12}$ and the mixing parameter is restricted to $10^{-11}\leq ε< 10^{-8}-10^{-7.5}$, as a result of the out-of-equilibrium condition of dark photon and the observed relic abundance of dark matter. Within this $ε$ region, the 5(15)-year data of proton bremsstrahlung process for the dark photon, assuming vector meson (dipole) dominance, excludes $ε\geq 10^{-8.5} (10^{-7.9})$ at 90\% confidence level, implying only a narrow region of $ε$ close to $\sim 10^{-11}$ left for alternative tests.

Illuminating sequential freeze-in dark matter with dark photon signal at the CERN SHiP experiment

TL;DR

The paper studies a two-field freeze-in dark matter scenario with a light massive dark photon portal and analyzes its testability at the CERN SHiP experiment. By solving sequential freeze-in Boltzmann equations, it finds a near-constant dark charge and a kinetic-mixing window with and for in the SHiP range. SHiP projections show 90% CL exclusions of (vector meson dominance) or (dipole) for (5 years) and (15 years), leaving only a narrow region near . Consequently, SHiP can decisively test the sequential freeze-in DM scenario with a light dark photon, significantly narrowing the viable parameter space for this DM realization.

Abstract

Single-field freeze-in dark matter barely leaves observable footprints in dark matter direct detection, collider or fixed-target experiments, which can be altered in the two-field context. In this work, we consider sequential freeze-in dark matter through signals of dark photon mediator with a mass range of GeV covered by the proposed SHiP experiment. We show that the dark charge is fixed to be and the mixing parameter is restricted to , as a result of the out-of-equilibrium condition of dark photon and the observed relic abundance of dark matter. Within this region, the 5(15)-year data of proton bremsstrahlung process for the dark photon, assuming vector meson (dipole) dominance, excludes at 90\% confidence level, implying only a narrow region of close to left for alternative tests.

Paper Structure

This paper contains 9 sections, 18 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Freeze-in dark matter model via massive dark photon portal
  3. The Model
  4. Sequential freeze-in
  5. Dark photon signal at the SHiP Experiment
  6. Production
  7. Decay
  8. Signal sensitivities
  9. Discussion

Figures (3)

  • Figure 1: The observed DM relic abundance arising from the sequential freeze-in production projected to the place of $m_{A'}-\epsilon$ for various ratios of $e'/\epsilon=\{10^{-4.6},10^{-4},10^{-3}\}$ and two different mass ratios of $m_{A'}/m_{\chi}=\{3, 10\}$.
  • Figure 2: The values of $\sigma_{pp\to \chi \chi}/(e'\epsilon)^{2}$ in Eq.(\ref{['cs']}) for two different mass ratios of $m_{A'}/m_{\chi}=\{3,10\}$.
  • Figure 3: The sequential freeze-in DM parameter space corresponding to $\epsilon_{\rm{DM}}\leq\epsilon<\epsilon_{\rm{th}} (\rm{in~black})$ compared to the 90% CL limits of the meson, Pbrem (dipole $\&$ VMD) and QCD production processes for the dark photon with $N=\{2, 6\}\times 10^{20}$, and the existing bounds (in shaded gray region). See text for details.