Fluctuations of v2 versus fluctuations of ε2: Beyond linear and cubic response
Abstract
Elliptic flow (v2) is the most prominent signature of collective flow in heavy-ion collisions. Its event-by-event fluctuations are accurately measured in Pb+Pb collisions at the LHC. They originate from the early stages of the collision, and are usually understood in terms of linear response to the initial anisotropy ε2, with a small cubic correction. We reveal deviations from this scenario on the basis of extensive event-by-event hydrodynamic simulations. We analyze the hydrodynamic response order by order in the cumulant expansion of ε2 and v2 fluctuations. We carry out this expansion in the "intrinsic frame" where the magnitude and direction of the impact parameter are fixed. We find that the asymmetry of v2 fluctuations is opposite to that of ε2 fluctuations. We also find that the response to ε2 fluctuations is larger by ≈ 10−20% than the response to the average geometry. This effect, which is not explained by the cubic response, increases the splitting between v2{2} and v2{4}.
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BibTeX
@misc{kirchner2026fluctuations,
title = {{Fluctuations of $v_2$ versus fluctuations of $\varepsilon_2$: Beyond linear and cubic response}},
author = {Andreas Kirchner and Mubarak Alqahtani and Jean-Yves Ollitrault},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26621},
eprint = {2610.26621},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26621},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Fluctuations of v_2 versus fluctuations of ε_2: Beyond linear and cubic response
AU - Andreas Kirchner
AU - Mubarak Alqahtani
AU - Jean-Yves Ollitrault
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2610.26621
UR - https://arxiv.org/abs/2610.26621
AB - Elliptic flow (v_2) is the most prominent signature of collective flow in heavy-ion collisions. Its event-by-event fluctuations are accurately measured in Pb+Pb collisions at the LHC. They originate from the early stages of the collision, and are usually understood in terms of linear response to the initial anisotropy ε_2, with a small cubic correction. We reveal deviations from this scenario on the basis of extensive event-by-event hydrodynamic simulations. We analyze the hydrodynamic response order by order in the cumulant expansion of ε_2 and v_2 fluctuations. We carry out this expansion in the "intrinsic frame" where the magnitude and direction of the impact parameter are fixed. We find that the asymmetry of v_2 fluctuations is opposite to that of ε_2 fluctuations. We also find that the response to ε_2 fluctuations is larger by ≈ 10−20% than the response to the average geometry. This effect, which is not explained by the cubic response, increases the splitting between v_2{2} and v_2{4}.
ER -