Ratios of collective flow observables in high-energy isobar collisions are insensitive to final state interactions

The ratios of bulk observables, such as harmonic flow $v_2$ and $v_3$, between high-energy $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr collisions were recently argued to be a clean probe of the nuclear structure differences between $^{96}$Ru and $^{96}$Zr. Using a transport model simulation of isobar collisions, we quantify this claim from the dependence of the ratios $v_{2,\mathrm{Ru}}/v_{2,\mathrm{Zr}}$ and $v_{3,\mathrm{Ru}}/v_{3,\mathrm{Zr}}$ on various final state effects, such as the shear viscosity, hadronization and hadronic cascade. Although the $v_2$ and $v_3$ change by more than 50% when varying the final state effects, the ratios are unchanged. In addition, these ratios are independent of the transverse momentum $p_{\mathrm{T}}$ and hadron species, despite of up to a factor of two change in $v_n$. The ratio of mean transverse momentum $\left\langle p_{\mathrm{T}}\right\rangle$ is found to be controlled by the nuclear skin and nuclear radius, but is only slightly impacted by the final state effects. Therefore, these isobar ratios serve as a clean probe of the initial condition of the quark-gluon plasma, which in turn is controlled by the collective structure of the colliding nuclei.Comment: 6 pages and 6 figures, replaced with version accepted by PR

Nuclear shape fluctuations in high-energy heavy ion collisions

Atomic nuclei often exhibit a quadrupole shape that fluctuates around some average profile. We investigate the impact of nuclear shape fluctuation on the initial state geometry in heavy ion collisions, particularly its eccentricity $\varepsilon_2$ and inverse size $d_{\perp}$, which can be related to the elliptic flow and radial flow in the final state. The fluctuation in overall quadrupole deformation enhances the variances and modifies the skewness and kurtosis of the $\varepsilon_2$ and $d_{\perp}$ in a controllable manner. The fluctuation in triaxiality reduces the difference between prolate and oblate shape for any observable, whose values, in the large fluctuation limit, approach those obtained in collisions of rigid triaxial nuclei. The method to disentangle the mean and variance of the quadrupole deformation is discussed.Comment: 12 pages 9 figure

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

A collision between two atomic nuclei accelerated close to the speed 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 direct evidence that this fluid reaches thermalization, at least to some extent, using recent data from the Large Hadron Collider. The ATLAS Collaboration has measured the variance of the momentum per particle across Pb+Pb collision events with the same particle multiplicity. It decreases steeply over a narrow multiplicity range corresponding to central collisions, which hints at an emergent phenomenon. We show that the observed pattern is explained naturally if one assumes that, for a given multiplicity, the momentum per particle increases as a function of the impact parameter of the collision. 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. This is a generic property of relativistic fluids, thus observed for the first time in a laboratory experiment.Comment: 9 pages, 3 figure

Measurement of collective dynamics in small and large systems with the ATLAS detector

The investigation of bulk observables in heavy-ion collisions has been a long-standing pursuit to understand the properties of Quark-Gluon Plasma (QGP). A key question regarding the property of the QGP is the existence of collective phenomena in smaller collision systems, specifically proton-nucleus ($p$+A) and proton-proton ($pp$) collisions. Various theoretical models propose that the sources of collectivity in these smaller systems differ from those in nucleus-nucleus (A+A) collisions. To address these questions, the ATLAS collaboration has investigated two-particle correlations in $pp$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 13 TeV to investigate it's sensitivity to the presence of jets. The exclusion of particles associated with low-transverse momentum ($p_{\mathrm{T}}$) jets have revealed no discernible impact on the measured flow coefficients, establishing that collectivity in small systems does not originate from hard processes. Moreover, analyses of flow decorrelation in both Xe+Xe and $pp$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.44 TeV and 5 TeV, respectively, have revealed a more pronounced decorrelation in $pp$ collisions compared to Xe+Xe collisions at the same multiplicity. This observation underscores the role played by sub-nucleonic structure and fluctuations in the longitudinal energy deposition when characterizing the longitudinal structure of the colliding system. Moreover, ATLAS has also measured the correlation, $\rho_{n}$, between the event-wise average transverse momentum and harmonic flow for Xe+Xe and Pb+Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.44 TeV and 5 TeV, respectively, showing a large correlation between flow and transverse momentum fluctuations in heavy-ion collisions. This measurement also provided the first experimental evidence from a high-energy experiment for a significant triaxiality in $^{129}$Xe using $\rho_{2}$ and constraints $\gamma_{\mathrm{Xe}}\sim 30^{\circ}$. Thus, this measurement highlights the importance of nuclear structure for a more accurate description of bulk observables measured in heavy-ion collisions

Flow and transverse momentum correlations in Pb+Pb and Xe+Xe collisions with ATLAS

Fluctuations of event-wise average transverse momentum ([$p_T$]) and the harmonic flow ($v_n$) carry important information about initial-state geometry. Collisions of nuclei with large quadrapole deformation are predicted to produce an initial state with enhanced shape and size fluctuations, and result in non-trivial correlation between $v_n$ and [$p_T$] in the final state. In particular, the $v_2$-[$p_T$] correlations are predicted to be different between collisions of spherical $^{208}$Pb and collisions of deformed $^{129}$Xe. This poster presents new measurement of $v_n$-[$p_T$] correlation in $\sqrt{s_{NN}} = 5.44$ TeV Xe+Xe collisions and compared with Pb+Pb at $\sqrt{s_{NN}} = 5.02$ TeV for harmonics $n = 2,$ 3 and 4. The correlation strength is found to depend strongly on centrality and also on the choice of transverse momentum range of the particles for all harmonics. Comparison with theoretical model calculations are used to shed light on the system-size dependence of this correlation and the influence of deformations. This measurement provides inputs for better understanding of the initial-state nuclear geometry and dynamics of heavy ion collisions

Probing initial state using higher order correlations: vn−[pT]v_{n}-[p_{T}] and [pT][p_{T}] correlations in in ATLAS

Constraining the initial condition of the QGP using experimental observables is one of the most important challenges in our field. Recent studies show that both $\rho_n(v_n, [p_\mathrm{T}])$ (correlation between $v_n$ and event-wise mean $p_\mathrm{T}$) and $[p_\mathrm{T}]$ fluctuations can probe several ingredients of the initial state, such as number and size of sources, volume fluctuation, nuclear deformation, and initial momentum anisotropy. This talk presents new measurements of high-order $[p_\mathrm{T}]$ fluctuations and $v_n-[p_\mathrm{T}]$ correlation for $n$=2-4 in $^{129}$Xe+$^{129}$Xe and $^{208}$Pb+$^{208}$Pb collisions. The results are obtained via the standard and subevents cumulant methods to assess the role of non-flow and flow decorrelations in these observables. The variance and skewness of $[p_\mathrm{T}]$ fluctuations as well as $\rho_n$ show non-monotonic dependence on centrality, $p_\mathrm{T}$ and $\eta$. We also found that the result depends on the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. The $\rho_2$ is found to constrain the triaxiality of the quadrupole deformation, while the $[p_\mathrm{T}]$ fluctuations show significant deviations from the independent source picture. The implication of these results on the nature of heavy ion initial condition, both in terms of deformation and energy deposition mechanism, is discussed

Measurements of collective behavior in pp, Xe+Xe, and Pb+Pb collisions with the ATLAS detector

This talk presents ATLAS measurements of collective, flow phenomena in a variety of collision systems, including pp collisions at 13 TeV, Xe+Xe collisions at 5.44 TeV, and Pb+Pb collisions at 5.02 TeV. A new measurement of -[T] correlations in Xe+Xe and Pb+Pb collisions is presented for harmonics =2, 3, and 4. The correlation between the event-wise average transverse momentum ([T]) and the harmonic flow () carries important information about the initial-state geometry of the quark-gluon plasma. Additionally, the potential quadrupole deformation in Xe+Xe is predicted to produce an initial state with enhanced shape and size fluctuations, and result in non-trivial change in the correlation. A measurement of the sensitivity of two-particle correlations in pp collisions to the presence of jets is presented. By rejecting particles associated with low-T jets, this data can distinguish the role that semi-hard processes play in the collective phenomena observed in pp collisions

Measurements of collective behavior in pp, Xe+Xe, and Pb+Pb collisions with the ATLAS detector

ATLAS measurements of collective, flow phenomena in a variety of collision systems, including $pp$ collisions at 13 TeV, Xe+Xe collisions at 5.44 TeV, and Pb+Pb collisions at 5.02 TeV are presented. A measurement of the sensitivity of two-particle correlations in $pp$ collisions to the presence of jets is presented. By rejecting particles associated with low-\pT jets, this data can distinguish the role that semi-hard processes play in the collective phenomena observed in $pp$ collisions (details in ATLAS-CONF-2020-018). The correlation between the event-wise average transverse momentum ([\pT]) and the harmonic flow ($v_{n}$) carries important information about the initial-state geometry of the Quark-Gluon Plasma. Additionally, the potential quadrupole deformation in Xe+Xe is predicted to produce an initial state with enhanced shape and size fluctuations, and result in non-trivial change in the correlation. A new measurement of v_{n}-[\pT] correlations in Xe+Xe and Pb+Pb collisions is discussed for harmonics $n=2$, 3, and 4 (ATLAS-CONF-2021-001)

Flow and transverse momentum correlation in Pb+Pb and Xe+Xe collisions with ATLAS: assessing the initial condition of the QGP

One important challenge in our field is to understand the initial condition of the QGP and constrain it using sensitive experimental observables. Recent studies show that the Pearson Correlation Coefficient (PCC) between vn and event-wise mean transverse momentum [pT], rho(vn, [pT]), and its centrality dependence can probe several ingredients of the initial state, such as number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy. In particular, a recent calculation shows that the 129Xe nucleus is triaxially deformed, which is expected to enhance rho in 129Xe+129Xe relative to 208Pb+208Pb collisions. This talk presents comprehensive and precision measurements of vn-pT correlation in 129Xe+129Xe and 208Pb+208Pb collisions for harmonics n = 2, 3, and 4. The results are obtained via the standard and subevents cumulant methods to assess the role of non-flow and flow decorrelations in these observables, and they are found to be small in the mid-central and central collisions in these systems. All PCC coefficients, rho_2, rho_3 and rho_4 show rich and non- monotonic dependence on centrality, pT and eta, reflecting the fact that different ingredients of the initial state impact different regions of the phase space. For example, we found the result depends on the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. On the other hand, the ratio of rho_n between the two systems is less sensitive to the centrality estimator, and in the ultra-central region, the value of the ratio suggests that 129Xe has large quadrupole deformation but with a significant triaxiality. All current models fail to describe many of the observed trends in the data, pointing to the unprecedented constraining power enabled by this precision measurement

Flow and transverse momentum correlation in Pb+Pb and Xe+Xe collisions with ATLAS: assessing the initial condition of the QGP

One important challenge in our field is to understand the initial condition of the QGP and constrain it using sensitive experimental observables. Recent studies show that the Pearson Correlation Coefficient (PCC) between and event-wise mean transverse momentum [T], (,[T]), and its centrality dependence can probe several ingredients of the initial state, such as number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy. In particular, a recent calculation shows that the 129Xe nucleus is triaxially deformed, which is expected to enhance 2 in 129Xe+129Xe relative to 208Pb+208Pb collisions. This talk presents new, comprehensive and precision measurements of −[T] correlation in 129Xe+129Xe and 208Pb+208Pb collisions for harmonics =2, 3, and 4. The results are obtained via the standard and subevents cumulant methods to assess the role of non-flow and flow decorrelations in these observables, and they are found to be small in the mid-central and central collisions in these systems. All PCC coefficients, 2, 3 and 4 show rich and non-monotonic dependence on centrality, T and , reflecting the fact that different ingredients of the initial state impact different regions of the phase space. For example, it was found the result depends on the centrality estimator used in the analysis, indicating a strong influence of volume fluctuations. On the other hand, the ratio of 2 between the two systems is less sensitive to the centrality estimator, and in the ultra-central region, the value of the ratio suggests that 129Xe has large quadrupole deformation but with a significant triaxial. All current models fail to describe many of the observed trends in the data, pointing to the unprecedented constraining power enabled by this precision measurement