The order of the quantum chromodynamics transition predicted by the standard model of particle physics

TL;DR

This study uses lattice QCD calculations to show that the transition in the early Universe's quark-gluon plasma was an analytic crossover, not a true phase transition, complicating experimental detection.

Contribution

The paper provides the first definitive lattice QCD evidence that the QCD transition in the early Universe was an analytic crossover rather than a phase transition.

Findings

No dramatic increase in susceptibilities observed

Finite-size scaling confirms crossover nature

Transition involves rapid change, not a jump

Abstract

We determine the nature of the QCD transition using lattice calculations for physical quark masses. Susceptibilities are extrapolated to vanishing lattice spacing for three physical volumes, the smallest and largest of which differ by a factor of five. This ensures that a true transition should result in a dramatic increase of the susceptibilities.No such behaviour is observed: our finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied). As such, it will be difficult to find experimental evidence of this transition from astronomical observations.