On the strength of the $U_A(1)$ anomaly at the chiral phase transition in $N_f=2$ QCD

TL;DR

This study uses lattice QCD simulations to analyze the chiral phase transition in two-flavor QCD, finding evidence for a crossover and insights into the $U_A(1)$ anomaly's role, challenging the $O(4)$ universality class.

Contribution

It provides new lattice simulation results on the chiral transition, focusing on the $U_A(1)$ anomaly's strength and its impact on the transition's nature.

Findings

Transition is a broad crossover, not a sharp phase change.

Degeneracy in vector and axial-vector channels indicates chiral symmetry restoration.

Scalar-pseudoscalar mass splitting reduces significantly, questioning $O(4)$ universality.

Abstract

We study the thermal transition of QCD with two degenerate light flavours by lattice simulations using $O(a)$-improved Wilson quarks. Temperature scans are performed at a fixed value of $N_t = (aT)^{-1}=16$, where $a$ is the lattice spacing and $T$ the temperature, at three fixed zero-temperature pion masses between 200 MeV and 540 MeV. In this range we find that the transition is consistent with a broad crossover. As a probe of the restoration of chiral symmetry, we study the static screening spectrum. We observe a degeneracy between the transverse isovector vector and axial-vector channels starting from the transition temperature. Particularly striking is the strong reduction of the splitting between isovector scalar and pseudoscalar screening masses around the chiral phase transition by at least a factor of three compared to its value at zero temperature. In fact, the splitting is consistent with zero within our uncertainties. This disfavours a chiral phase transition in the $O(4)$ universality class.