Electroweak Phase transitions impact on dark matter

The relic density of a dark matter candidate is usually computed with today’s particle spectrum, that is, in the electroweak broken phase. At freeze-out the Universe was hotter, and masses, mixings and open annihilation channels depend on temperature through the scalar vacuum expectation values.

This work measures the impact of this approximation in the Standard Model extended by two real scalar singlets with a Z2 × Z ′ 2 symmetry, one of which is the dark matter candidate. A pipeline was built that chains ScannerS, which generates viable points, BSMPT, which follows the vacuum and the scalar spectrum with temperature, and micrOMEGAs, modified to solve the Boltzmann equation along this evolving spectrum.

The differences between the Standard and Thermal relic densities concentrate where the singlet is heavier than the dark matter, at the χχ → h2h2 threshold, and at the scalar-mediator resonance, where the singlet mass is close to twice the dark matter mass. For points that match the observed abundance and respect the LUX-ZEPLIN limits, the corrections reach 4.6%.

The Planck interval is only ±3% wide, so a point close to its edge can be wrongly accepted or excluded by the Standard calculation relative to the Thermal. This change of verdict occurs in both directions, for corrections no larger than 5%. Far from the observed abundance the corrections reach 45%, when an annihilation channel that needs electroweak symmetry breaking is missing at freeze-out. The zero-temperature approximation cannot be taken for granted.