Evidence for dynamical chiral condensate in high-energy heavy ion collisions

TL;DR

This paper provides theoretical and experimental evidence for the partial restoration of chiral symmetry through a dynamical chiral condensate in high-energy heavy ion collisions, affecting pion spectra.

Contribution

It introduces a new theoretical model linking chiral condensate deviations to observable pion spectra, supporting chiral symmetry restoration at high temperatures.

Findings

Characteristic soft pion spectrum contribution observed

Experimental data aligns with the theoretical model

Supports partial chiral symmetry restoration at high temperature

Abstract

Quantum chromodynamics with light quarks features an approximate global symmetry, known as chiral symmetry, that is believed to be spontaneously broken by the vacuum expectation value of a scalar and isoscalar composite field, in addition to a small explicit breaking due to finite quark masses. For a high enough temperature, as achieved in the early universe or the fireball created by a high-energy heavy ion collision, this symmetry is expected to be restored. We show theoretically that a coherent deviation of the corresponding quantum field from its usual vacuum expectation value on the freeze-out hypersurface of a heavy-ion collision leads, after resonance decays, to a characteristic contribution to the transverse momentum spectrum of charged pions, in the very soft regime, consistent with experimental data from the Relativistic Heavy Ion Collider and the Large Hadron Collider. Taken together, the experimental data with the new theoretical results provide compelling support for the existence of a chiral condensation mechanism with partial restoration of chiral symmetry at high temperature.