Dark Matter Particle in QCD
  We report on the possibility that the Dark Matter particle is a stable,
neutral, as-yet-undiscovered hadron in the standard model. The existence of a
compact color-flavor-spin singlet sexaquark (S, uuddss) with mass ~2m_p, is
compatible with current knowledge. The S interacts with baryons primarily via a
Yukawa interaction of coupling strength alpha_SN, mediated by omega and phi
vector mesons having mass ~1 GeV. If it exists, the S is a very attractive DM
candidate. The relic abundance of S Dark Matter (SDM) is established when the
Universe transitions from the quark-gluon plasma to the hadronic phase at ~150
MeV and is in remarkable agreement with the observed Omega_DM/Omega_b =
5.3+-0.1; this is a no-free-parameters result because the relevant parameters
are known from QCD. Survival of this relic abundance to low temperature
requires the breakup amplitude gtilde <~ 2 10^-6, comfortably compatible with
theory expectations and observational bounds because the breakup amplitude is
dynamically suppressed and many orders of magnitude smaller, as we show. The
scattering cross section can differ by orders of magnitude from Born
approximation, depending on alpha_SN, requiring reanalysis of observational
limits. We use direct detection experiments and cosmological constraints to
determine the allowed region of alpha_SN. For a range of allowed values, we
predict exotic nuclear isotopes at a detectable level with mass offset ~2 amu.
The most promising approaches for detecting the sexaquark in accelerator
experiments are to search for a long-interaction-length neutral particle
component in the central region of relativistic heavy ion collisions or using a
beam-dump setup, and to search for evidence of missing particle production
characterized by unbalanced baryon number and strangeness using Belle-II or
possibly GLUEX at J-Lab.
