arXiv subjects

Jean-Yves Ollitrault

Publications and source records attributed to Jean-Yves Ollitrault.

At least 19 recordsLinked to original sources

arXiv2026

Accessing zero-point fluctuations in high-energy nuclear collisions

Ultracentral high-energy collisions between identical nuclei are very sensitive to the size and shape of the frozen configuration of nucleons at the instant of the collision, an information that is encoded in the ground state wave functions of the colliding nuclei. The relative standard deviation of the size of the colliding system soon after the collision has recently been inferred from event-by-event fluctuations of the momentum per particle in collisions of $^{208}$Pb nuclei at the LHC, and since a single collision event produces thousands of particles, this information is very precise. We connect this measurement to the magnitude of the zero-point fluctuations of the radius of the lead nucleus. The value that is extracted from the data is compatible with predictions from nuclear-structure calculations, but is overestimated by classical Glauber-type modeling of high-energy collisions. The role of the Pauli principle in suppressing the fluctuations is underlined. It leads to a distinctive $A$ dependence that could be checked in collisions of smaller systems.

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arXiv2026

Enhanced hydrodynamic predictions for $v_{02}(p_T)$

We present hydrodynamic predictions for the new observable $v_{02}(p_T)$, which measures the correlation of particle spectra with elliptic flow. We implement a data-driven correction so as to match hydrodynamic calculations to elliptic flow ($v_2(p_T)$) data. The corrected results are in fair agreement with $v_0(p_T)$ data up to high $p_T$. We make predictions for $v_{02}(p_T)$ of unidentified charged hadrons up to $p_T=10$~GeV$/c$, and of pions, kaons and protons up to $p_T=5-6$~GeV$/c$, in several centrality windows, for Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$~TeV. We predict a decrease of $v_{02}(p_T)$ of charged hadrons for $p_T>4$~GeV$/c$ and meson-baryon splitting for $p_T>3$~GeV$/c$ in mid-central collisions. We also predict a non-monotonic variation of $v_{02}(p_T)$ for protons at low $p_T$ above $30\%$ centrality. This is a specific feature of this new observable, which is not observed for the usual flow observables $v_2(p_T)$ and $v_0(p_T)$.

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arXiv2026

Extracting the speed of sound of QCD from transverse momentum fluctuations

We extract the speed of sound ($c_s$) in the quark-gluon plasma from ATLAS data on the probability distribution of the transverse momentum per particle, $[p_T]$, in ultra-central Pb+Pb collisions. With an ideal detector, $c_s$ can be inferred from the rise of the mean $[p_T]$ with the collision multiplicity. In practice, however, low-$p_T$ particles escape detection, which biases the analysis. We show how to correct for this bias by using data on the variance of $[p_T]$, as well as information from the recently-measured $v_0(p_T)$. We also introduce a systematic method for deblurring the noise from the hadronization process. Assuming that the size of the quark-gluon plasma is independent of the hadron multiplicity in collisions at zero impact parameter, which is the scenario preferred both by high-energy QCD and heavy-ion data, we obtain $c_s/c=0.496\pm 0.008$ at temperature $T=221\pm 13$~MeV, in perfect agreement with first-principles calculations from lattice QCD.

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arXiv2026

Explaining higher-order correlations between elliptic and triangular flow

The ALICE and CMS Collaborations have analyzed a number of cumulants mixing elliptic flow ($v_2$) and triangular flow ($v_3$), involving up to $8$ particles, in Pb+Pb collisions at the LHC. We unravel an unexpected simplicity in these complex mathematical quantities for collisions at fixed impact parameter. We show that as one increases the order in $v_2$, for a given order in $v_3$, the changes in the cumulants are solely determined by the mean elliptic flow in the reaction plane, which originates from the almond-shaped geometry of the overlap area between the colliding nuclei. We derive simple analytic relations between cumulants of different orders on this basis. These relations are in good agreement with recent data from the CMS Collaboration. We argue that agreement will be further improved if the analysis is repeated with a finer centrality binning. We make quantitative predictions for cumulants of order 10 which have not yet been analyzed.

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arXiv2026

Impact parameter dependence of anisotropic flow: Bayesian reconstruction in ultracentral nucleus-nucleus collisions

Peculiar phenomena have been observed in analyses of anisotropic flow ($v_n$) fluctuations in ultracentral nucleus-nucleus collisions: The fourth-order cumulant of the elliptic flow ($v_2$) distribution changes sign. In addition, the ATLAS collaboration has shown that cumulants of $v_n$ fluctuations of all orders depend significantly on the centrality estimator. We show that these peculiarities are due to the fact that the impact parameter $b$ always spans a finite range for a fixed value of the centrality estimator. We provide a quantitative determination of this range through a simple Bayesian analysis. We obtain excellent fits of STAR and ATLAS data, with a few parameters, by assuming that the probability distribution of $v_n$ solely depends on $b$ at a given centrality. This probability distribution is almost Gaussian, and its parameters depend smoothly on $b$, in a way that is constrained by symmetry and scaling laws. We reconstruct, thus, the impact parameter dependence of the mean elliptic flow in the reaction plane in a model-independent manner, and assess the robustness of the extraction using Monte Carlo simulations of the collisions where the impact parameter is known. We argue that the non-Gaussianity of $v_n$ fluctuations gives direct information on the hydrodynamic response to initial anisotropies, ATLAS data being consistent with a smaller response for $n=4$ than for $n=2$ and $n=3$, in agreement with hydrodynamic calculations.

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arXiv2026

The size of the quark-gluon plasma in ultracentral collisions: impact of initial density fluctuations on the average transverse momentum

Recent experiments have shown that the mean transverse momentum $\langle p_T\rangle$ of outgoing particles increases as a function of the particle multiplicity in ultracentral nucleus-nucleus collisions at collider energies. This increase was originally predicted on the basis of simulations where the multiplicity increase occurred at constant volume, so that it implied a larger density and temperature. However, recent state-of-the-art simulations have shown that, for some models of initial condition, the volume may vary with the multiplicity in ultracentral collisions. We elucidate this effect by analytically relating the variation of the volume to the radial distribution of the one- and two-point functions of the fluctuating density field. We show that the volume variation is small if the total entropy of the ultracentral collisions scales with the mass number of the colliding isotopes. We argue that probing detailed transverse distributions of initial-state fluctuations through the ultracentral $\langle p_T\rangle$ has nontrivial implications for models of nuclear structure and of the pre-equilibrium stages.

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arXiv2026

The collectivity of transverse momentum fluctuations

We study the observable $v_0(p_T)$, which quantifies the relative change of $p_T$ spectra induced by event-by-event density fluctuations in the medium created in heavy-ion collisions. This quantity provides a direct measure of radial flow and serves as a probe of collectivity, complementing anisotropic flow coefficients. Using hydrodynamic model calculations, we predict the behavior of $v_0(p_T)$ and show that the scaled quantity $v_0(p_T)/v_0$ exhibits very little dependence on centrality and transport coefficients. We further find that the apparent influence of transport coefficients$-$particularly bulk viscosity$-$ on $v_0(p_T)$ largely originates from modifications of the event-averaged mean transverse momentum, $\langle p_T \rangle$. By expressing $v_0(p_T)/v_0$ as a function of $p_T/\langle p_T \rangle$, the genuine sensitivity of $v_0(p_T)$ to transport coefficients can be isolated. Moreover, since $v_0(p_T)$ is the $p_T$-differential measure of event-by-event $[p_T]$ fluctuations, it naturally explains the observed $p_T$-cut dependence of $σ_{p_T}$ measured by ATLAS collaboration.

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arXiv2025

Understanding the correlation between elliptic and triangular flow

The relative correlation between the magnitudes of elliptic flow ($v_2$) and triangular flow ($v_3$) has been accurately measured in nucleus-nucleus collisions at the LHC collider. As a function of the centrality of the collision, it changes sign and varies non-monotonically. We show that this is naturally explained by two combined effects. The first effect is a skewness in initial-state fluctuations, which is quantified by the correlation between the geometry-driven elliptic deformation in the reaction plane and the fluctuation-driven triangularity $\varepsilon_3$. We introduce an intensive measure of this skewness, which is generically of order unity and depends weakly on the system size and centrality. We evaluate its magnitude using Monte Carlo simulations of the initial state, which show that it is sensitive to the nucleon width. The second effect is the fluctuation of impact parameter relative to centrality classifiers used by experiment. The ATLAS collaboration uses two different centrality classifiers, the multiplicity $N_{ch}$ and the transverse energy $E_T$. We fit both sets of results for Pb+Pb collisions up to $\approx 40\%$ centrality with a single parameter, the intensive mixed skewness. Its value inferred from experiment agrees with theoretical expectations.

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arXiv2025

Correlation between particle spectra and elliptic flow

We introduce a new observable to probe the collective nature of the radial expansion of the quark-gluon plasma. This observable, dubbed $v_{02}(p_T)$, represents the correlation of the spectrum with elliptic flow, in the same way as the recently measured $v_0(p_T)$ represents the correlation of the spectrum with the transverse momentum per particle. The advantage of $v_{02}(p_T)$ over $v_0(p_T)$ is that it is measured using a three-particle cumulant, as opposed to a pair correlation, which significantly reduces the sensitivity to nonflow effects. We predict non-trivial differences between $v_{02}(p_T)$ and $v_0(p_T)$ in semi-central Pb+Pb collisions at the Large Hadron Collider (LHC) on the basis of hydrodynamic simulations. A hint of these differences can be seen in the modification of $p_T$ spectra observed by ALICE in event-shape-engineered events.

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arXiv2025

Nuclear Physics Confronts Relativistic Collisions Of Isobars

High-energy collisions involving the $A=96$ isobars $^{96}$Zr and $^{96}$Ru have been performed in 2018 at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) as a means to search for the chiral magnetic effect in QCD. This would manifest itself as specific deviations from unity in the ratio of observables taken between $^{96}$Zr+$^{96}$Zr and $^{96}$Ru+$^{96}$Ru collisions. Measurements of such ratios (released at the end of 2021) indeed reveal deviations from unity, but these are primarily caused by the two collided isobars having different radial profiles and intrinsic deformations. To make progress in understanding RHIC data, nuclear physicists across the energy spectrum gathered in Heidelberg in 2022 as part of an EMMI Rapid Reaction Task Force (RRTF) to address the following question. Does the combined effort of low-energy nuclear structure physics and high-energy heavy-ion physics enable us to understand the observations made in isobar collisions at RHIC?

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arXiv2025

Bayesian reconstruction of anisotropic flow fluctuations at fixed impact parameter

The cumulants of the distribution of anisotropic flow are measured accurately in Pb+Pb collisions at the LHC as a function of centrality classifiers (charged multiplicity and/or transverse energy). Using Bayesian inference, we reconstruct from these measurements the probability distribution of anisotropic flow in the ``theorists' frame'' where the impact parameter has a fixed magnitude and orientation, up to $\sim 70\%$ centrality. The variation of flow fluctuations with impact parameter displays direct evidence of viscous damping, which is larger for higher Fourier harmonics, in line with expectations from hydrodynamics. We use intensive measures of non-Gaussian flow fluctuations, which have reduced dependence on centrality. We infer from ATLAS data the magnitude of these intensive non-Gaussianities in each Fourier harmonic. They provide data-driven estimates of response coefficients to initial anisotropies, without resorting to any specific microscopic model of initial conditions. These estimates agree with viscous hydrodynamic calculations.

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arXiv2024

Hydrodynamic-like behaviour of glasma

At the earliest stage of ultrarelativistic heavy-ion collisions the produced matter is a highly populated system of gluons called glasma which can be approximately described in terms of classical chromodynamic fields. Although the system's dynamics is governed by Yang-Mills equations, glasma evolution is shown to strongly resemble hydrodynamic behaviour.

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arXiv2024

Probing collectivity in heavy-ion collisions with fluctuations of the $p_T$ spectrum

Event-by-event fluctuations in the initial stages of ultrarelativistic nucleus-nucleus collisions depend little on rapidity. The hydrodynamic expansion which occurs in later stages then gives rise to correlations among outgoing particles which depend weakly on their relative rapidity. Azimuthal correlations, through which anisotropic flow ($v_n(p_T)$) is defined, have been the most studied. Here we study a new observable introduced in 2020 by Schenke, Shen and Teaney and dubbed $v_0(p_T)$, which quantifies the relative change in the $p_T$ spectrum induced by a fluctuation. We describe how it can be measured. Using hydrodynamic simulations, we make quantitative predictions for $v_0(p_T)$ of charged and identified hadrons. We then discuss how $v_0(p_T)$ relates to two phenomena which have been measured: The increase of the mean transverse momentum in ultracentral collisions, and the event-by-event fluctuations of the transverse momentum per particle $[ p_T]$. We show that $v_0(p_T)$ determines the dependence of these quantities on the $p_T$ cuts implemented in the analysis. We quantitatively explain the rise of $σ_{p_T}$ observed by ATLAS as the upper $p_T$-cut is increased from $2$ to $5$~GeV/$c$.

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arXiv2024

Accessing the speed of sound in relativistic ultracentral nucleus-nucleus collisions using the mean transverse momentum

It has been argued that the speed of sound of the strong interaction at high temperature can be measured using the variation of the mean transverse momentum with the particle multiplicity in ultracentral heavy-ion collisions. We test this correspondence by running hydrodynamic simulations at zero impact parameter with several equations of state, at several colliding energies from 0.2 TeV to 15 TeV per nucleon pair. The correspondence is found to be precise and robust for a smooth, boost-invariant fluid and an ideal detector. We discuss the differences between this ideal setup and an actual experiment. We conclude that the extraction of the speed of sound from data is reliable, and that the main uncertainty comes from our poor knowledge of the distribution of density fluctuations at the early stages of the collision.

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arXiv2024

Multiplicity fluctuations and rapidity correlations in ultracentral proton-nucleus collisions

A collision between a proton and a heavy nucleus at ultrarelativistic energy creates particles whose rapidity distribution is asymmetric, with more particles emitted in the direction of the nucleus than in the direction of the proton. This asymmetry becomes more pronounced as the centrality estimator, defined from the energy deposited in a calorimeter, increases. We argue that for high-multiplicity collisions, the variation of the impact parameter plays a negligible role, and that the fluctuations of the multiplicity and of the centrality estimator are dominated by quantum fluctuations, whose probability distribution can be well approximated by a correlated gamma distribution. We show that this simple model reproduces existing data, and we make quantitative predictions for collisions in the $0-0.1\%$ and $0-0.01\%$ centrality windows. We argue that by repeating the same analysis with a different centrality estimator, one can obtain direct information about the rapidity decorrelation in particle production.

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arXiv2024

Dilepton polarization as a signature of plasma anisotropy

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio $η/s$ in the early stages of the collision.

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arXiv2024

Thermalization at the femtoscale seen in high-energy Pb+Pb collisions

A collision between two atomic nuclei accelerated at a speed close to that of light creates a dense system of quarks and gluons. Interactions among them are so strong that they behave collectively like a droplet of fluid of ten-femtometer size, which expands into the vacuum and eventually fragments into thousands of particles. We report a new manifestation of thermalization in recent data from the Large Hadron Collider. Our analysis is based on results from the ATLAS Collaboration, which has measured the variance of the momentum per particle across Pb+Pb collision events with the same particle multiplicity. This variance decreases steeply over a narrow multiplicity range corresponding to central collisions. We provide a simple explanation of this newly-observed phenomenon: For a given multiplicity, the momentum per particle increases with increasing impact parameter. Since a larger impact parameter goes along with a smaller collision volume, this in turn implies that the momentum per particle increases as a function of density, which is a generic consequence of thermalization. Our analysis provides the first direct evidence of this phenomenon at the femtoscale.

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arXiv2023

Studying short-range nuclear correlations using relativistic heavy-ion collisions

Recently, a method was developed for implementing arbitrary short-range nucleon-nucleon correlations in Monte Carlo sampled nuclei (as well as deformations of the 1-body nuclear density). We use this method to implement realistic 2-body correlations in a sample of nuclei for use in simulations of relativistic heavy-ion collisions and we quantify the statistical benefits. These results demonstrate that the method can be used to easily implement an arbitrary correlation function, and systematically study the effects of correlations using significantly less resources than is necessary with traditional methods.

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