Scaled transverse-momentum spectra as a probe of collective dynamics in heavy-ion collisions

TL;DR

This paper introduces a scaling analysis of transverse-momentum spectra in heavy-ion collisions, revealing universal patterns that serve as new probes of the quark-gluon plasma's collective behavior and properties.

Contribution

It presents a novel scaling method for spectra that uncovers universality and constrains medium properties using Bayesian analysis with emulators.

Findings

Spectral shape exhibits approximate universality across centralities and energies.

Hydrodynamic models reproduce the scaled spectra on an event-by-event basis.

Scaling of transverse-mass spectra shows similar universality.

Abstract

We investigate a scaling property of transverse-momentum spectra in ultrarelativistic heavy-ion collisions obtained by removing the global scales of multiplicity and mean transverse momentum. The resulting dimensionless observable isolates the intrinsic shape of the spectrum and reveals an approximate universality across collision centralities, systems, and energies. Hydrodynamic simulations reproduce this scaling on an event-by-event basis, indicating that it may originate from the collective dynamics of the quark-gluon plasma. Using Gaussian-process emulators trained on the JETSCAPE hybrid model, we perform a Bayesian analysis incorporating the scaled spectra as observables. The results demonstrate that the spectral shape provides independent constraints on key properties of the medium, including pre-equilibrium dynamics and initial-state granularity, while exposing tensions with parameter regions preferred by traditional $p_T$-integrated observables. We further explore an analogous scaling of transverse-mass spectra and observe a comparable universality across centralities and hadron species. These results suggest that scaled spectra provide a powerful new probe of collective dynamics and offer complementary constraints for the quantitative characterization of QCD matter created in heavy-ion collisions.