Testing thermal equilibrium in heavy-ion collisions with electric-charge fluctuations
Abstract
Relativistic heavy-ion collisions exhibit several phenomena commonly associated with the formation of a thermalized quark--gluon plasma. However, the observation of similar phenomena in small systems, and their non-thermal microscopic descriptions there, raises questions about this assumption. Recent continuum-extrapolated lattice-QCD calculations of the second- and fourth-order electric-charge susceptibilities, χ2ΔQ and χ4ΔQ, reveal a striking tension with hadron-resonance-gas expectations, particularly for χ4ΔQ. As the ratio χ4ΔQ/χ2ΔQ is experimentally measurable with net-charge cumulants, this opens up completely new ways to test whether thermal QCD applies in central heavy-ion collisions. However, achieving a measurement that can be interpreted with high precision requires further work on both the phenomenological and experimental sides. Here, the focus is on an experimental counterpart to recent phenomenological work on the D-measure of net-charge fluctuations: how restricted detector acceptance in rapidity and transverse momentum can be controlled experimentally. We demonstrate this for the second-order net-charge cumulant, κ2(ΔQ), by rewriting it in terms of the charge balance function and the charged-particle rapidity density. Both are event-averaged observables for which experimental efficiency corrections and extrapolations into unmeasured kinematic regions can be studied systematically. This provides a complementary experimental route for connecting measurements of D = 4κ2(ΔQ)/⟨ Nch ⟩ to theory and a possible starting point for extending the same strategy to the fourth-order cumulant.
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BibTeX
@misc{christiansen2026testing,
title = {{Testing thermal equilibrium in heavy-ion collisions with electric-charge fluctuations}},
author = {Peter Christiansen},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37829},
eprint = {2610.37829},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37829},
}
RIS
TY - UNPB
TI - Testing thermal equilibrium in heavy-ion collisions with electric-charge fluctuations
AU - Peter Christiansen
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37829
UR - https://arxiv.org/abs/2610.37829
AB - Relativistic heavy-ion collisions exhibit several phenomena commonly associated with the formation of a thermalized quark--gluon plasma. However, the observation of similar phenomena in small systems, and their non-thermal microscopic descriptions there, raises questions about this assumption. Recent continuum-extrapolated lattice-QCD calculations of the second- and fourth-order electric-charge susceptibilities, χ_2^ΔQ and χ_4^ΔQ, reveal a striking tension with hadron-resonance-gas expectations, particularly for χ_4^ΔQ. As the ratio χ_4^ΔQ/χ_2^ΔQ is experimentally measurable with net-charge cumulants, this opens up completely new ways to test whether thermal QCD applies in central heavy-ion collisions. However, achieving a measurement that can be interpreted with high precision requires further work on both the phenomenological and experimental sides. Here, the focus is on an experimental counterpart to recent phenomenological work on the D-measure of net-charge fluctuations: how restricted detector acceptance in rapidity and transverse momentum can be controlled experimentally. We demonstrate this for the second-order net-charge cumulant, κ_2(ΔQ), by rewriting it in terms of the charge balance function and the charged-particle rapidity density. Both are event-averaged observables for which experimental efficiency corrections and extrapolations into unmeasured kinematic regions can be studied systematically. This provides a complementary experimental route for connecting measurements of D = 4κ_2(ΔQ)/⟨ N_ch ⟩ to theory and a possible starting point for extending the same strategy to the fourth-order cumulant.
ER -