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Non-Thermal Production of Sexaquark Dark Matter

Marianne Moore, Stefano Profumo

Abstract

Standard thermal freeze-out scenarios with QCD-scale interaction rates predict a $uuddss$ sexaquark relic abundance many orders of magnitude below the observed dark matter density, representing a key challenge for sexaquark dark matter models. Additionally, if the maximum post-inflationary temperature never exceeds the QCD confinement scale, the usual thermal/chemical-equilibrium production of the sexaquark near ${T \sim T_{\rm QCD} \simeq 150 \text{-} 170~\mathrm{MeV}}$ never occurs. In this work we show that non-thermal mechanisms can naturally overcome this obstacle. Using late-decaying reheatons as a representative case (while noting the broader applicability), we demonstrate that the final abundance is determined by two quantities: the branching fraction into strange-quark-rich matter and the coalescence probability into sexaquarks during the matter-dominated or early radiation-dominated epoch. We provide compact expressions and benchmark calculations for reheating temperatures $T_R \in [10, 100]~\mathrm{MeV}$ and reheaton masses above the QCD confinement scale. Unlike the predictive but unsuccessful thermal scenario, non-thermal production is sensitive to injection microphysics, coalescence efficiency, and residual entropy dilution. We delineate the viable parameter space, evaluate collider and precision constraints on representative reheaton models, and derive indirect detection bounds on residual antisexaquark populations. Our results establish non-thermal production as a viable pathway to sexaquark dark matter and highlight broader implications for non-equilibrium mechanisms in the early universe.

Non-Thermal Production of Sexaquark Dark Matter

Abstract

Standard thermal freeze-out scenarios with QCD-scale interaction rates predict a sexaquark relic abundance many orders of magnitude below the observed dark matter density, representing a key challenge for sexaquark dark matter models. Additionally, if the maximum post-inflationary temperature never exceeds the QCD confinement scale, the usual thermal/chemical-equilibrium production of the sexaquark near never occurs. In this work we show that non-thermal mechanisms can naturally overcome this obstacle. Using late-decaying reheatons as a representative case (while noting the broader applicability), we demonstrate that the final abundance is determined by two quantities: the branching fraction into strange-quark-rich matter and the coalescence probability into sexaquarks during the matter-dominated or early radiation-dominated epoch. We provide compact expressions and benchmark calculations for reheating temperatures and reheaton masses above the QCD confinement scale. Unlike the predictive but unsuccessful thermal scenario, non-thermal production is sensitive to injection microphysics, coalescence efficiency, and residual entropy dilution. We delineate the viable parameter space, evaluate collider and precision constraints on representative reheaton models, and derive indirect detection bounds on residual antisexaquark populations. Our results establish non-thermal production as a viable pathway to sexaquark dark matter and highlight broader implications for non-equilibrium mechanisms in the early universe.

Paper Structure

This paper contains 35 sections, 46 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Review of thermal sexaquark production
  3. Factorizing the sexaquark production
  4. Strange quark injection from reheaton decay
  5. Scalar or pseudoscalar reheaton with Yukawa-like couplings
  6. Strange quark injection from reheaton decay
  7. Reheaton production pathways and observable signatures
  8. Gluon-coupled ("gluophilic") scalar reheaton
  9. Strange quark injection from reheaton decay
  10. Reheaton production pathways and observable signatures
  11. Flavor-universal vector or axial reheaton
  12. Strange quark injection from reheaton decay
  13. Reheaton production pathways and observable signatures
  14. Increasing the strangeness budget
  15. Coalescence probability for compact uuddss formation
  16. ...and 20 more sections

Figures (3)

  • Figure 1: Dark matter abundance in sexaquarks, $\Omega_S / \Omega_\text{DM}$, as a function of the reheating temperature $T_R$ and the fraction of reheaton decays into sexaquarks $f_S$. The bold lines indicate the parameter combinations where sexaquarks constitute the entirety of dark matter (${\Omega_S = \Omega_\text{DM}}$) for two representative reheaton masses: ${m_\phi = 50~\text{GeV}}$ (solid) and ${m_\phi = 300~\text{GeV}}$ (dashed).
  • Figure 2: Branching ratios as a function of the reheaton mass $\phi$ from Eqs. \ref{['eq:Bs_Yukawa']} (Yukawa scalar in orange and pseudoscalar in yellow) and \ref{['eq:Bs_universal']} (flavor-universal $Z'$ vector in magenta and axial-vector in violet). The thresholds for the reheaton to decay into two charm quarks ($2m_c$) and two bottom quarks ($2m_b$) are illustrated.
  • Figure 3: Fraction $f_S$ of reheaton decay producing sexaquarks as a function of the reheaton mass. The grey band indicates the region where the full dark matter abundance in sexaquarks can be obtained for the reheating temperature range considered throughout this work, ${10 \leq T_R \leq 100~\text{MeV}}$, with smaller reheating temperatures corresponding to the top of the band. The colored bands indicate the range obtained for two scenarios; a scalar Yukawa and a flavor-universal vector reheaton. The width of each region is determined by the uncertainty on the parameters that control $f_S$.