Abstract
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 1010 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.
Highlights
Provide a formation mechanism in the early universe
In this article we explore the idea that new physics can affect the order of the quantum chromodynamics (QCD) phase transition and resuscitate quark nugget dark matter
In this article we have studied an exotic form of Standard Model matter, called six-flavor quark matter
Summary
We discuss how nuggets of 6FQM could be formed in the early universe during the quark-hadron phase transition. Since the expectation value of the Higgs field is zero in the 6FQM phase, the cosmological creation of 6FQM requires the electroweak phase transition to be supercooled below the temperature of the quark-hadron transition, which will be assumed to be true
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