arXiv subjects

Somadutta Bhatta

Publications and source records attributed to Somadutta Bhatta.

13 recordsLinked to original sources

arXiv2026

A Method to Constrain Preferential Emission and Spectator Dynamics in Heavy-Ion Collisions

Longitudinal particle production in heavy-ion collisions is influenced both by preferential emission from participating nucleons and by the breakup of spectator matter, yet quantifying these effects experimentally remains challenging. We introduce a Pearson correlation between spectator and charged-particle forward-backward asymmetries as an experimental probe of these phenomena. Using Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV simulated with A Multi-Phase Transport (AMPT) model, we validate that this correlator provides a robust, pseudorapidity-differential measure of the influence of preferential emission on the longitudinal structure of particle production. We further demonstrate that the correlation strength is sensitive to fluctuations in spectator number, which in experiments arise from evaporation and fragmentation of the spectator remnants. The proposed observable therefore offers a data-driven handle for constraining models of preferential emission and spectator breakup, thereby improving our understanding of the mechanisms that shape the final-state longitudinal distributions in heavy-ion collisions.

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arXiv2025

Novel Probes of Quark-Gluon Plasma Evolution in Heavy-Ion Collisions With the ATLAS Detector: From Structure of Colliding Nuclei to Collective Expansion of Medium

Understanding the properties of the quark-gluon plasma (QGP) offers insights into the strong interaction and the conditions of the early universe.Since the QGP cannot be observed directly, its properties must be inferred from the particles emitted as it cools. This dissertation introduces a suite of novel probes based on event-by-event multi-particle correlations allowing us to work backward from final-state particles to constrain the collective evolution, transport properties, and initial conditions of the QGP. The studies utilize $^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ and $^{129}\mathrm{Xe}+{}^{129}\mathrm{Xe}$ collisions recorded by ATLAS detector at the LHC. First, evidence for the collective nature of the QGP's radial expansion is provided using a transverse momentum ($p_{\mathrm{T}}$)-differential observable, $v_0(p_{\mathrm{T}})$. This observable exhibits genuine long-range correlations and factorizes into a single-particle property. Its strong sensitivity to bulk viscosity providing more direct constraints on this medium property than traditional measures. Second, the sources of event-wise fluctuations in radial flow within the initial state are disentangled by analyzing higher moments of the event-wise mean-$p_{\mathrm{T}}$ distribution. The average of this distribution is shown to constrain the speed of sound in the QGP. Finally, nuclear deformations, which control overlap area geometry, are constrained by comparing correlations between the anisotropic flow, $v_{n}$ and mean-$p_{\mathrm{T}}$ in spherical Pb+Pb and deformed Xe+Xe collisions. This comparison provides the first experimental evidence for triaxial deformation in $^{129}$Xe. Together, these three complementary analyses significantly improve our understanding of the initial conditions, transport properties, and collective behavior of the QGP, establishing new avenues for precision studies of QGP.

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arXiv2025

The shape of differential radial flow $v_0(p_T)$, not its zero-crossing, carries physical information

Radial flow, a key collective phenomenon in heavy-ion collisions, manifests itself through event-by-event fluctuations of transverse-momentum ($p_{\mathrm{T}}$) spectra. The $p_{\mathrm{T}}$-differential radial flow observable, $v_0(p_{\mathrm{T}})$, was introduced to quantify local spectral-shape fluctuations, but it is unavoidably influenced by global multiplicity fluctuations. Using the HIJING model, we show that different event-activity definitions for centrality classification and different spectral normalization schemes generate a constant vertical offset in $v_0(p_{\mathrm{T}})$ without altering its shape. This offset reflects the impact of residual volume/centrality fluctuations rather than genuine dynamical radial flow fluctuations. Accordingly, only the shape of $v_0(p_{\mathrm{T}})$, or equivalently its derivative $dv_0(p_{\mathrm{T}})/dp_{\mathrm{T}}$, carries physical information about radial-flow dynamics; its zero crossing does not. Practical implications include the need to vertically align measurements from different experiments before comparison, thereby removing normalization ambiguities when constraining QGP properties.

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arXiv2024

Energy dependence of heavy-ion initial condition in isobar collisions

Collisions of isobar nuclei, those with the same mass number but different structure parameters, provide a new way to probe the initial condition of the heavy ion collisions. Using transport model simulation of $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr collisions at two energies $\sqrt{s_{\mathrm{NN}}}=0.2$ TeV and 5.02 TeV, where $^{96}$Ru and $^{96}$Zr nuclei have significantly different deformations and radial profiles, we identify sources of eccentricities contributing independently to the final state harmonic flow $v_n$. The efficacy for flow generation differs among these sources, and explains the modest energy dependence of the isobar ratios of $v_n$. Additionally, a significant component of $v_n$ is found to be uncorrelated with the eccentricity but is instead generated dynamically during system evolution. Experimental measurement of these ratios at the LHC energy and comparison with RHIC energy can provide insight into the collision-energy dependence of the initial condition.

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arXiv2024

Sources of longitudinal flow decorrelations in high-energy nuclear collisions

The longitudinal structure of the quark-gluon plasma (QGP) consists of several components spanning various scales. However, its short-range features are often obscured by final-state non-flow correlations. Here, we introduce a data-driven approach to separate initial state structures from non-flow effects. The longitudinal structure is found having two distinct components: one that reflects the global twisted geometry of the QGP, and another that captures localized fluctuations in rapidity. The characteristics of this second component, contributing to short- and medium-range flow decorrelations, can be quantified by comparing collisions of nuclei with different shapes. This study represents the first successful attempt to disentangle long- and short-range flow decorrelations from non-flow backgrounds, providing new insights into the initial conditions of heavy-ion collisions.

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arXiv2024

An improved method to access initial states in relativistic heavy-ion collisions

Observables in heavy-ion collisions are generally categorized into centralities, which reflect an average over events within a range of impact parameter including a wide variety of initial state configurations. A multiple binning method using spectator neutrons within each centrality has been previously shown to provide access to events with rare initial state conditions. This work suggests an improvement in quantifying the difference between standard centrality and spectator neutron binning towards accessing the initial state properties. A selection of events with higher initial state density at fixed participating nucleon number was observed to result in larger final state particle production and smaller elliptic flow. The relative difference between observables in centrality and spectator binning shows reduced sensitivity for the observables dominated by impact parameter fluctuations in initial state such as triangular flow. This property makes the spectator binning method a good candidate to separate geometric contributions from random fluctuations in initial state towards final state observables.

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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

Transverse momentum fluctuation in ultra-central Pb+Pb collision at the LHC

The ATLAS collaboration has recently observed that the variance of the transverse momentum per particle ($[ p_t ]$), when measured as a function of the collision multiplicity ($N_{ch}$) in Pb+Pb collisions, decreases by a factor $2$ for the largest values of $N_{ch}$, corresponding to ultra-central collisions. We show that this phenomenon is naturally explained by invoking impact parameter ($b$) fluctuations, which contribute to the variance, and gradually disappear in ultra-central collisions. It implies that $N_{ch}$ and $[ p_t ]$ are strongly correlated at fixed $b$, which is explained by the local thermalization of the QGP medium.

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arXiv2023

Impact of nuclear shape fluctuations in high-energy heavy ion collisions

The shape of atomic nuclei is often interpreted to possess a quadrupole deformation that fluctuates around some average profile. We investigate the impact of nuclear shape fluctuations 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 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.

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arXiv2022

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 between ${\bm V}_n$ and event-wise mean transverse momentum $[p_{\mathrm{T}}]$, $ρ_{n}({\bm V}_n,[p_{\mathrm{T}}])$ can probe number and size of sources, nuclear deformation, volume fluctuation, and initial momentum anisotropy in initial state of heavy-ion collisions. This proceeding presents new, precision measurements of ${\bm V}_n-[p_{\mathrm{T}}]$ correlation in $^{\mathrm{129}}\mathrm{Xe}+^{\mathrm{129}}\mathrm{Xe}$ and $^{\mathrm{208}}\mathrm{Pb}+^{\mathrm{208}}\mathrm{Pb}$ collisions for harmonics $n=2$, 3, and 4 using the ATLAS detector at the LHC. The values of $ρ_{n}$ shows rich and non-monotonic dependence on centrality, $p_{T}$ and $η$, reflecting that different ingredients of the initial state impact different regions of the phase space. The ratio of $ρ_{2}$ between the two systems in the ultra-central region suggests that ${^\mathrm{129}}\mathrm{Xe}$ has large quadrupole deformation and with a significant triaxiality. All current models fail to describe many of the observed trends in the data.

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arXiv2022

Higher-order transverse momentum fluctuations in heavy-ion collisions

In relativistic heavy-ion collisions, the event-by-event mean transverse momentum fluctuations are sensitive to overlap area and energy density fluctuations in initial state. We present a framework to calculate $p_{\mathrm{T}}$ fluctuations up to $4^{\mathrm{th}}$-order using standard and subevent methods, which is validated using the HIJING model. We observe a power-law dependence for cumulants of all orders as a function of charged particle multiplicity $N_{\mathrm{ch}}$, consistent with a simple independent source picture. The fluctuation in $pp$ collisions is observed to be larger than for $p+$Pb, Pb+Pb and Xe+Xe collisions at the same $N_{\mathrm{ch}}$ due to bias in number of contributing sources. The short-range correlations are greatly suppressed in subevent method in comparison to calculations based on the standard method. This study provides a baseline for transverse momentum fluctuations without the presence of final state effects.

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arXiv2022

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.

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arXiv2022

Non-flow effects in correlation between harmonic flow and transverse momentum in nuclear collisions

A large anti-correlation signal between elliptic flow $v_2$ and average transverse momentum $[p_{\mathrm{T}}]$ was recently measured in small collision systems, consistent with a final-state hydrodynamic response to the initial geometry. This negative $v_2$-$[p_{\mathrm{T}}]$ correlation was predicted to change to positive correlation for events with very small charged particle multiplicity $N_{\mathrm{ch}}$ due to initial-state momentum anisotropies of the gluon saturation effects. However, the role of non-flow correlations is expected to be important in these systems, which is not yet studied. We estimate the non-flow effects in $pp$, $p$Pb and peripheral PbPb collisions using {\tt Pythia} and {\tt Hijing} models, and compare them with the experimental data. We show that the non-flow effects are largely suppressed using the rapidity-separated subevent cumulant method (details of the cumulant framework are also provided). The magnitude of the residual non-flow is much less than the experimental observation in the higher $N_{\mathrm{ch}}$ region, supporting the final-state response interpretation. In the very low $N_{\mathrm{ch}}$ region, however, the sign and magnitude of the residual non-flow depend on the model details. Therefore, it is unclear at this moment whether the sign change of $v_2$-$[p_{\mathrm{T}}]$ can serve as evidence for initial state momentum anisotropies predicted by the gluon saturation.

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