Higgs in The Cosmos

TL;DR

This paper investigates how the Higgs boson behaves in the primordial quark-gluon plasma during the early Universe, focusing on chemical and kinetic nonequilibrium and their potential role in baryogenesis. By analyzing production, decay, and scattering rates against the Hubble expansion, it shows that the Higgs remains out of chemical equilibrium with a fugacity $Υ_h \approx 0.69$, and that its momentum distribution becomes non-thermal for $T \lesssim 25$ GeV because scattering cannot keep pace with production and decay. The results reveal a prolonged nonequilibrium Higgs population that could sustain irreversibility and influence the duration and nature of the electroweak phase transition, highlighting the need for a full kinetic treatment that accounts for a temperature-dependent vacuum expectation value $v_0(T)$. Overall, the work suggests Higgs-driven nonequilibrium dynamics as a potentially important facet of baryogenesis in the early Universe and calls for further detailed kinetic studies.

Abstract

We explore the Higgs particle in the cosmic quark-gluon plasma (QGP) below the electroweak phase transition temperature $T_\mathrm{EW}\simeq 125\mathrm{\,GeV}$. We show that Higgs is neither in abundance (chemical) nor in momentum distribution equilibrium in certain stages of the Universe evolution. Nonequilibrium originates in: For chemical nonequilibrium in the always present irreversible decays into virtual heavy gauge bosons, and; For $T<25$\,GeV in relatively rapid $2\leftrightarrow 1$ formation and decay processes yielding momentum distribution as created in these reactions. As heavy particles disappear, the minimal Higgs coupling to abundant low mass particles fails in $2\to2$ (two-particle) scattering processes to assure a kinetic distribution equilibrium. The expansion of the Universe is by more than 10 orders of magnitude slower compared to microscopic processes. All other particles in the Universe are in full thermal equilibrium, with exception of the late in QGP evolution of the bottom flavor near to hadronization condition.

Figures (4)

• Figure 1: Thermal primordial QGP heavy particle abundance, presented as ratios with the baryon asymmetry, as a function of temperature $T$. For particle masses used see Ref. ParticleDataGroup:2024cfk.
• Figure 2: Lowest order Feynman diagrams in primordial QGP for: $(a)$ dominant Higgs production $b\bar{b}\leftrightarrow h$; (b) dominant Higgs decay $h\to WW^\ast,ZZ^\ast$ and $(c)$ Higgs on $b,\bar{b}, t, \bar{t}$ quark scattering.
• Figure 3: The total fusion rate for Higgs production (horizontal blue line) compared to the dominant scattering rates and the Hubble expansion rate (black line), scaled up with factor $10^{10}$.
• Figure 4: Higgs speed $p/E$ distributions at $T=80,60,40,20$ GeV. The dotted line shows the thermal distribution, while the solid line represents the $b\bar{b}\to h$ production distribution.