arXiv subjects

Andre V. Giannini

Publications and source records attributed to Andre V. Giannini.

15 recordsLinked to original sources

arXiv2026

The shape of transverse momentum spectra in hybrid hydrodynamic models

We study the scaled transverse momentum spectra over a wide parameter space of state-of-the-art hydrodynamic simulation models in order to learn what information can be obtained from the shape of identified-particle spectra -- previously observed to be surprisingly universal across centrality and collision systems in both experimental data and hydrodynamic simulations. We study its sensitivity to each of 17 model parameters in the context of 4 different models for particlization when switching from the hydro description to the kinetic theory afterburner. We find that the strongest sensitivity is to parameters relating to bulk viscosity, free-streaming time, and the $\texttt{T$_\mathrm{R}$ENTo}$ nucleon width parameter $w$. However, we find that the model generally has surprisingly little flexibility in describing the scaled spectrum observable, despite the large number of parameters. Within this small range of parameter dependence, we further find significant tension in a simultaneous description of momentum-integrated observables. In particular, while the mean transverse momentum prefers a large value of the nucleon width parameter $w$, a small value is required to obtain scaled spectra that are consistent with experimental measurements. We speculate on the origin of these model tensions and possible missing physics in the commonly-used $\texttt{T$_\mathrm{R}$ENTo}$+free streaming+hydro+afterburner simulation model.

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arXiv2026

A study of charged-particle multiplicity distribution in high energy p-O collisions

This study investigates the multiplicity distribution of charged particles generated in $p$-O collisions, employing Pythia (Angantyr) and $k_T$-factorization approach. Oxygen nucleus configurations are sampled using a $α$-cluster model to evaluate both formalisms and assess how initial nucleus configuration influences the properties of the produced final states. Results obtained through clustering are systematically compared to those derived from the Woods-Saxon nuclear distribution. The analysis encompasses various pseudorapidity intervals ($|η|<$ 0.5, 1.0, 2.0, 3.0) and center-of-mass energies ($\sqrt{s}=$ 2.36, 5.02, 7.0, 13.0 TeV). Based on the resulting distributions, we examine the KNO scaling effect and fit the distributions with the double NBD model for parameterization, aiming to accurately characterize the observed results and elucidate contributions from both soft and semi-hard processes. Our results indicate that different geometric descriptions of the oxygen nucleus project significantly different multiplicities of charged particles, especially for large multiplicities and higher pseudorapidity. We also observed that multiplicity of charged particles calculated with Pythia reveals significantly different behavior from that calculated with $k_T$-factorization.

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arXiv2026

Universality of scaled particle spectra in ultrarelativistic heavy-ion collisions

We study the transverse momentum spectra of identified particles in ultrarelativistic collisions of large and small collision systems. In order to isolate information contained in the momentum dependence, we propose to scale the spectra by the total particle number and mean transverse momentum -- global quantities which are already well studied. We observe an interesting, nearly universal, centrality-independent shape in the scaled spectra, similar to scalings that have been studied previously. This scaling behavior breaks down at large transverse momentum and for very small systems, such as those produced in p-p collisions. We perform hybrid hydrodynamic simulations and show that, in these simulations, a centrality-independent shape is a consequence of an event-by-event independence. Our results motivate further theoretical and experimental investigations of the regime of validity of this scaling phenomenon and their physical interpretation at different collision energies and systems.

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arXiv2025

Probing the onset of collectivity via scaled particle spectra in ultrarelativistic nuclear collisions

We identify a novel scaling in the transverse momentum spectra of produced particles, obtained by removing the global scales of multiplicity and mean transverse momentum. Hydrodynamic simulations and experimental data reveal an almost universal scaled spectrum across centralities, systems, and even small systems, pointing to its origin in the collective, fluid-like dynamics of the QGP. Comparing this observable with Bayesian a priori distributions shows its independent constraining power on QCD transport properties, while also exposing limitations of current models. A detailed posterior analysis will be pursued in future work, opening a new avenue to refine our understanding of collectivity in heavy-ion collisions.

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arXiv2024

Exploring high-multiplicity events in high-energy proton-proton collisions

It is known that the proton is overpopulated by gluons and is characterized as a highly dense medium at high collision energies. From this, the formation of a new state of matter called Color Glass Condensate (CGC) is expected, and an open question is whether the nonlinear effects predicted by this state are identifiable at the LHC. The multiplicity of particles produced in a hadronic collision presents as a means to adequately investigate this problem. Currently, the description of the available data for different multiplicity regimes remains a challenge. Even though different experimental collaborations have identified that the production of certain final states, in $pp$ collisions, present a modification in the behavior of high multiplicity events in relation to the case of minimum bias we still lack a way to identify the nature of those high multiplicity events: are those driven by initial-state effects, final-state effects or a mix of both? We argue that a analyzing different particle production process that can be described CGC framework, in particular, isolated photon production which is not sensitive to final-state effects, may provide a path forward in answering this question.

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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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arXiv2023

Causality violations in simulations of large and small heavy-ion collisions

Heavy-ion collisions, such as Pb-Pb or p-Pb, produce extreme conditions in temperature and density that make the hadronic matter transition to a new state, called quark-gluon plasma (QGP). Simulations of heavy-ion collisions provide a way to improve our understanding of the QGP's properties. These simulations are composed of a hybrid description that results in final observables in agreement with accelerators like LHC and RHIC. However, recent works pointed out that these hydrodynamic simulations can display acausal behavior during the evolution in certain regions, indicating a deviation from a faithful representation of the underlying QCD dynamics. To pursue a better understanding of this problem and its consequences, this work simulated two different collision systems, Pb-Pb and p-Pb at $\sqrt{s_{NN}} = 5.02$ TeV. In this context, our results show that causality violation, even though always present, typically occurs on a small part of the system, quantified by the total energy fraction residing in the acausal region. In addition, the acausal behavior can be reduced with changes in the pre-hydrodynamic factors and the definition of the bulk-viscous relaxation time. Since these aspects are fairly arbitrary in current simulation models, without solid guidance from the underlying theory, it is reasonable to use the disturbing presence of acausal behavior in current simulations to guide improvements towards more realistic modeling. While this work does not solve the acausality problem, it sheds more light on this issue and also proposes a way to solve this problem in simulations of heavy-ion collisions.

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arXiv2021

Perturbative structure of two- and four-point functions of color charge in a non-Gaussian small-$x$ action

We compute the perturbative expansion of the two- and four-point functions of color charges in the Color Glass Condensate framework considering the quartic correction to the McLerran-Venugopalan (MV) model of Gaussian color charge fluctuations. Expressions for these correlators in the perturbative expansion for small and large non-Gaussian color charge fluctuations are derived for arbitrary orders in perturbation theory. We explicitly show that the perturbative series does not converge at higher orders as expected. We apply the Borel-Padé resummation method to our problem to construct a convergent series. It is shown that the fully non-perturbative solution can be described by the Borel-Padé approximants constructed from the first few terms of the perturbative series for small non-Gaussian fluctuations.

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arXiv2021

Non-perturbative renormalization of the average color charge and multi-point correlators of color charge from a non-Gaussian small-$x$ action

The McLerran-Venugopalan (MV) model is a Gaussian effective theory of color charge fluctuations at small-$x$ in the limit of large valence charge density, {\it i}.{\it e}., a large nucleus made of uncorrelated color charges. In this work, we explore the effects of the first non-trivial (even C-parity) non-Gaussian correction on the color charge density to the MV model ("quartic" term) in SU(2) and SU(3) color group in the non-perturbative regime. We compare our (numerical) non-perturbative results to (analytical) perturbative ones in the limit of small or large non-Gaussian fluctuations. The couplings in the non-Gaussian action, $\barμ$ for the quadratic and $κ_4$ for the quartic term, need to be renormalized in order to match the two-point function in the Gaussian theory. We investigate three different choices for the renormalization of these couplings: i) $κ_{4}$ is proportional to a power of $\barμ$; ii) $κ_4$ is kept constant and iii) $\barμ$ is kept constant. We find that the first two choices lead to a scenario where the small-$x$ action evolves towards a theory dominated by large non-Gaussian fluctuations, regardless of the system size, while the last one allows for controlling the deviations from the MV model.

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arXiv2019

Limiting fragmentation as an initial state probe in heavy ion collisions

We discuss limiting fragmentation within a few currently popular phenomenological models. We show that popular Glauber-inspired models of particle production in heavy ion collisions, such as the two-component model, generally fail to reproduce limiting fragmentation when all energies and system sizes experimentally available are considered. This is due to the energy-dependence of number of participants and number of collisions. We quantify this violation in terms of the model parameters. We also make the same calculation within a Color Glass Condensate scenario and show that the dependence of the saturation scale on the number of participants generally leads to violation of limiting fragmentation. We further argue that wounded parton models, provided the nucleon size and parton density vary predominantly with Bjorken $x$, could in principle reproduce both multiplicity dependence with energy and limiting fragmentation. We suggest, therefore, that an experimental measurement of deviation from limiting fragmentation in heavy ion collisions, for different system sizes and including the experimentally available range of energies, is a powerful test of initial state models.

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arXiv2019

Bulk quantities in nuclear collisions from running coupling $k_{T}$-factorization and hybrid simulations

Starting from a Color Glass Condensate (CGC) framework, based on a running-coupling improved $k_T$-factorized formula, we calculate bulk observables in several heavy-ion collision systems. This is done in two ways: first we calculate the particle distribution directly implied from the CGC model, and we compare this to the case where it is instead used as initial conditions for a hybrid hydrodynamic simulation. In this way, we can assess the effects of hydrodynamic and hadronic evolution by quantifying how much they change the results from a pure initial state approach and, therefore, to what extent initial condition models can be directly compared to experimental data. We find that entropy production in subsequent hydrodynamic evolution can increase multiplicity by as much as 50\%. However, disregarding a single overall normalization factor, the centrality, energy, and system size dependence of charged hadron multiplicity is only affected at the $\sim$5\% level. Because of this, the parameter-free prediction for these dependencies gives reasonable agreement with experimental data whether or not hydrodynamic evolution is included. On the other hand, our model results are not compatible with the hypothesis that hydrodynamic evolution is present in large systems, but not small systems like p-Pb, in which case the dependence of multiplicity on system size would be stronger than seen experimentally. Moreover, we find that hydrodynamic evolution significantly changes the distribution of momentum, so that observables such as mean transverse momentum are very different from the initial particle production, and much closer to measured data. Finally, we find that a good agreement to anisotropic flow data cannot be achieved due to the large eccentricity generated by this model.

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arXiv2018

Centrality and energy dependence of charged particles in p+A and A+A collisions from running coupling $k_{T}$-factorization

We extend the numerical analysis of the energy and centrality dependence of particle multiplicities at midrapidity in high-energy p+A and A+A collisions from a running coupling $k_T$-factorization formula made in~\cite{Dumitru:2018gjm} by considering two unintegrated gluon distributions that were left out. While a good agreement with the experimental data in A+A collisions is achieved, improving the description of those observables in p+A collisions calls for a better understanding of the proton unintegrated gluon distribution at larger values of $x$ and also the use of a realistic impact parameter dependence.

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arXiv2018

Particle multiplicities in the central region of high-energy collisions from $k_T$-factorization with running coupling corrections

Horowitz and Kovchegov have derived a $k_T$-factorization formula for particle production at small $x$ which includes running coupling corrections. We perform a first numerical analysis to confront the theory with data on the energy and centrality dependence of particle multiplicities at midrapidity in high-energy p+A (and A+A) collisions. Moreover, we point out a strikingly different dependence of the multiplicity per participant on $N_\text{part}$ in p+Pb vs.\ Pb+Pb collisions at LHC energies, and argue that the observed behavior follows rather naturally from the convolution of the gluon distributions of an asymmetric vs. symmetric projectile and target.

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arXiv2015

Anisotropic particle production and azimuthal correlations in high-energy pA collisions

We summarize some recent ideas relating to anisotropic particle production in high-energy collisions. Anisotropic gluon distributions lead to anisotropies of the single-particle azimuthal distribution and hence to disconnected contributions to multi-particle cumulants. When these dominate, the four-particle elliptic anisotropy $c_2\{4\}$ changes sign. On the other hand, connected diagrams for $m$-particle cumulants are found to quickly saturate with increasing $m$, a ``coherence'' quite unlike conventional ``non-flow'' contributions such as decays. Finally, we perform a first exploratory phenomenological analysis in order to estimate the amplitude ${\cal A}$ of the $\cos(2φ)$ anisotropy of the gluon distribution at small $x$, and we provide a qualitative prediction for the elliptic asymmetry from three-particle correlations, $c_2\{3\}$.

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arXiv2014

Initial state angular asymmetries in high energy p+A collisions: spontaneous breaking of rotational symmetry by a color electric field and C-odd fluctuations

We present a simple model for generating initial-state azimuthal asymmetries in pA collisions from dipole scattering on an anisotropic dense target. Parity even angular harmonics arise from the C-even real part of the dipole S-matrix which spontaneously breaks rotational symmetry in two dimensions due to a condensate for the color electric field. This leads to an angular correlation with the direction of $\vec E$. Parity odd harmonics are generated by the C-odd imaginary part (odderon) due to coupling to coherent target fluctuations which again break rotational invariance. We perform a first qualitative extraction of the amplitude and cutoff of C-odd fluctuations in the dense target.

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