Probing light dark matter with a hadrophilic scalar mediator

We investigate the thermal cosmology and terrestrial and astrophysical phenomenology of a sub-GeV hadrophilic dark sector. The specific construction explored in this work features a Dirac fermion dark matter candidate interacting with a light scalar mediator that dominantly couples to the up-quark. The correct freeze-out relic abundance may be achieved via dark matter annihilation directly to hadrons or through secluded annihilation to scalar mediators. A rich and distinctive phenomenology is present in this scenario, with probes arising from precision meson decays, proton beam dump experiments, colliders, direct detection experiments, supernovae, and nucleosynthesis. In the future, experiments such as NA62, REDTOP, SHiP, SBND, and NEWS-G will be able to explore a significant portion of the cosmologically motivated parameter space.

Figure 1

Left: Branching ratio of the scalar $S$ to SM final states, assuming that there are no dark sector decays of $S$. Right: Contours of constant decay length of $S$, in m, assuming only decays to SM final states.

Figure 2

Top left: Limits on an up-philic scalar mediator. Top right: Limits on an invisibly decaying up-philic scalar. The thermal dark matter and direct detection lines are drawn assuming $m_{\chi }=(1/3)m_S$ and $g_{\chi }=1$. Bottom left: Same as top right panel but for a visibly decaying scalar, with all dark matter lines assuming $m_{\chi }=(3/4)m_S$. Bottom right: Limits on the up-philic scalar with $m_{\chi }=3m_S$. The DM coupling $g_{\chi }$ is chosen at each point such that secluded annihilation sets the correct thermal relic density. Note that some of the bounds are at 90% C.L. (direct detection, E787/E949, MiniBooNE) or 95% CL (BBN, CHARM, SHiP, MAMI, KLOE), while the SN 1987A exclusion region has no defined confidence level.