Six Flavor Quark Matter
  Macroscopic nuggets of quark matter were proposed several decades ago as a
candidate for dark matter. The formation of these objects in the early universe
requires the QCD phase transition to be first order - a requirement that is not
satisfied in the Standard Model where lattice simulations reveal a continuous
crossover instead. In this article we point out that new physics may supercool
the electroweak phase transition to below the QCD scale, and the QCD phase
transition with six massless quarks becomes first-order. As a result, the quark
nuggets composed of six-flavor quark matter (6FQM) may survive as a viable dark
matter candidate. The size of a 6FQM nugget is estimated to be around $10^{10}$
grams in mass and $10^{-2}$ cm in radius. The calculated relic abundance of
6FQM nuggets is comparable to the observed dark matter energy density;
therefore, this scenario provides a compelling explanation for the coincident
energy densities of dark and baryonic matter. We have explored various
potential signatures - including a gravitational wave background, gravitational
lensing, and transient photon emission from collisions with compact stars and
other nuggets - and demonstrated that the favored region of parameter space is
still allowed by current constraints while discovery of 6FQM nugget dark matter
may require new experimental probes.
