New constraints on equation of state of hot QCD matter

TL;DR

This paper introduces a new observable based on transverse momentum fluctuations that is highly sensitive to the equation of state of quark-gluon plasma, providing a novel method to constrain its properties in heavy-ion collisions.

Contribution

It proposes a novel fluctuation measure sensitive to the QGP's EoS and demonstrates its effectiveness through advanced hydrodynamic simulations, offering a new way to study QGP properties.

Findings

The new observable is highly sensitive to the QGP EoS.

It shows negligible dependence on QGP transport coefficients.

Simulations confirm the observable's potential to constrain the EoS.

Abstract

The longitudinal structure of the quark-gluon plasma(QGP) remains a key challenge in heavy-ion physics. In this Letter, we propose a novel observable, event-by-event mean transverse momentum fluctuations Var$_{\langle p_{T} \rangle}$, which is sensitive to the local pressure gradients and serves as a probe of longitudinal dynamics in the initial state of QGP. We demonstrate that the covariance of averaged transverse momentum at two rapidities $\mathrm{Cov}_{\langle p_T \rangle}(\eta_1, \eta_2)$ and its associated decorrelation measures, $R_{p_T}(\eta_1, \eta_2)$ and $r_{p_T}(\eta, \eta_{\mathrm{ref}})$, exhibit strong sensitivity to the stiffness of equation of state (EoS) of QGP, while showing negligible dependence on the QGP transport coefficients. This distinctive behavior, revealed through state-of-the-art (3+1)-dimensional hydrodynamic simulations, establishes a powerful approach for constraining the EoS of QCD matter. In the meantime, our results provide new insights into the longitudinal structure of the QGP and its properties under high baryon density.