Detecting scale anomaly in chiral phase transition of QCD: new critical endpoint pinned down

TL;DR

This paper proposes a novel method to detect electromagnetic scale anomalies in QCD's chiral phase transition by observing changes in phase transition order under varying strange quark masses and weak magnetic fields, revealing a new critical endpoint.

Contribution

It introduces a new approach to identify scale anomalies in QCD through electromagnetic effects and predicts a new critical endpoint in the phase diagram.

Findings

Electromagnetic scale anomaly is most detectable in weak magnetic fields.

A new critical endpoint associated with the quantum scale anomaly is identified.

Methodology links phase transition order changes to scale anomaly detection.

Abstract

Violation of scale symmetry, scale anomaly, being a radical concept in quantum field theory, is of importance to comprehend the vacuum structure of QCD, and should potentially contribute to the chiral phase transition in thermal QCD, as well as the chiral and U(1) axial symmetry. Though it should be essential, direct evidence of scale anomalies has never been observed in the chiral phase transition. We propose a methodology to detect a scale anomaly in the chiral phase transition, which is an electromagnetically induced scale anomaly: apply a weak magnetic field background onto two-flavor massless QCD with an extremely heavy strange quark, first observe the chiral crossover; second, adjusting the strange quark mass to be smaller and smaller, observe the second-order chiral phase transition, and then the first-order one in the massless-three flavor limit. Thus, the second-order chiral phase transition, observed as the evidence of the quantum scale anomaly, is a new critical endpoint. It turns out that this electromagnetic scale anomaly gets most operative in the weak magnetic field regime, rather than a strong field region. We also briefly address accessibility of lattice QCD, a prospected application to dense matter system, and implications to astrophysical observations, such as gravitational wave productions provided from thermomagnetic QCD-like theories.