arXiv subjects

Giuliano Giacalone

Publications and source records attributed to Giuliano Giacalone.

At least 55 records · Page 3Linked to original sources

arXiv2021

Turning up and down strong magnetic fields in relativistic nuclear collisions

I show that the average transverse momentum, $\langle p_t \rangle$, of the hadrons emitted in relativistic nuclear collisions can be used as a ``knob'' to control the strength of the magnetic field induced by the spectator and the participant protons over the overlap region. I thus argue that any observable sensitive to this magnetic field is nontrivially correlated with $\langle p_t \rangle$ at a given collision centrality.

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arXiv2021

Constraining the quadrupole deformation of atomic nuclei with relativistic nuclear collisions

Preliminary data by the STAR collaboration at the BNL Relativistic Heavy Ion Collider shows that the elliptic flow, $v_2$, and the average transverse momentum, $\langle p_t \rangle$, of final-state hadrons produced in high-multiplicity $^{238}$U+$^{238}$U collisions are negatively correlated. This observation brings experimental evidence of a significant prolate deformation, $β\approx 0.3$, in the colliding $^{238}$U nuclei. I show that a quantitative description of this new phenomenon can be achieved within the hydrodynamic framework of heavy-ion collisions, and that thus such kind of data in the context of high-energy nuclear experiments can help constrain the quadrupole deformation of the colliding species.

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arXiv2021

Correlation between mean transverse momentum and anisotropic flow in heavy-ion collisions

The correlation between the mean transverse momentum of outgoing particles, $\langle p_t \rangle$, and the magnitude of anisotropic flow, $v_n$, has recently been measured in Pb+Pb collisions at the CERN Large Hadron Collider, as a function of the collision centrality. We confirm the previous observation that event-by-event hydrodynamics predicts a correlation between $v_n$ and $\langle p_t \rangle$ that is similar to that measured in data. We show that the magnitude of this correlation can be directly predicted from the initial condition of the hydrodynamic calculation, for $n=2,3$, if one replaces $v_n$ by the corresponding initial-state anisotropy, $\varepsilon_n$, and $\langle p_t\rangle$ by the total energy per unit rapidity of the fluid at the beginning of the hydrodynamic expansion.

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arXiv2021

Skewness of mean transverse momentum fluctuations in heavy-ion collisions

We propose the skewness of mean transverse momentum, $\langle p_t \rangle$, fluctuations as a fine probe of hydrodynamic behavior in relativistic nuclear collisions. We describe how the skewness of the $\langle p_t \rangle$ distribution can be analyzed experimentally, and we use hydrodynamic simulations to predict its value. We predict in particular that $\langle p_t \rangle$ fluctuations have positive skew, which is significantly larger than if particles were emitted independently. We elucidate the origin of this result by deriving generic formulas relating the fluctuations of $\langle p_t \rangle$ to the fluctuations of the early-time thermodynamic quantities. We postulate that the large positive skewness of $\langle p_t \rangle$ fluctuations is a generic prediction of hydrodynamic models.

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arXiv2021

Accessing the shape of atomic nuclei with relativistic collisions of isobars

Nuclides sharing the same mass number (isobars) are observed ubiquitously along the stability line. While having nearly identical radii, stable isobars can differ in shape, and present in particular different quadrupole deformations. We show that even small differences in these deformations can be probed by relativistic nuclear collisions experiments, where they manifest as deviations from unity in the ratios of elliptic flow coefficients taken between isobaric systems. Collider experiments with isobars represent, thus, a unique means to obtain quantitative information about the geometric shape of atomic nuclei.

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arXiv2021

A matter of shape: seeing the deformation of atomic nuclei at high-energy colliders

This work establishes a deep connection between two seemingly distant branches of nuclear physics: nuclear structure and relativistic heavy-ion collisions. At the heart of this connection is the recent discovery made at particle colliders that the elliptic flow of outgoing hadrons in central nucleus-nucleus collisions is strongly impacted by the quadrupole deformation of the colliding nuclear species. I review the physics of the soft sector of relativistic heavy-ion collisions, and I explain that the interpretation of elliptic flow data in central $^{197}$Au+$^{197}$Au, $^{238}$U+$^{238}$U, and $^{129}$Xe+$^{129}$Xe collisions requires a deep understanding the structure of these ions. Subsequently, I introduce a technique that permits one to isolate collision configurations in which the deformed shapes of the colliding nuclei maximally break rotational (azimuthal) symmetry in the interaction region. This allows me to construct observables that possess an unparalleled sensitivity to the quadrupole deformation of the colliding ions, and thus to conclude that nuclear experiments at high energy can be used to place new quantitative constraints on the deformation of atomic nuclei. I emphasize the great opportunities offered by potential collider experiments aimed at the systematic study of nuclear deformation across the valley of stability.

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arXiv2020

Hydrodynamic attractors, initial state energy and particle production in relativistic nuclear collisions

We exploit the concept of hydrodynamic attractors to establish a general relation between the initial state energy and the produced particle multiplicities in high-energy nuclear collisions. When combined with an ab initio model of energy deposition, the entropy production during the pre-equilibrium phase naturally explains the universal centrality dependence of the measured charged particle yields in nucleus-nucleus collisions. We further estimate the energy density of the far-from-equilibrium initial state and discuss how our results can be used to constrain non-equilibrium properties of the quark-gluon plasma.

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arXiv2020

Thermodynamics of hot strong-interaction matter from ultrarelativistic nuclear collisions

Collisions between heavy atomic nuclei at ultra-relativistic energies are carried out at particle colliders to produce the quark-gluon plasma, a state of matter where quarks and gluons are not confined into hadrons, and colour degrees of freedom are liberated. This state is thought to be produced as a transient phenomenon before it fragments into thousands of particles that reach the particle detectors. Despite two decades of investigations, one of the big open questions is to obtain an experimental determination of the temperature reached in a heavy-ion collision, and a simultaneous determination of another thermodynamic quantity, such as the entropy density, that would give access to the number of degrees of freedom. Here we obtain the first such determination, utilizing state-of-the-art hydrodynamic simulations. We define an effective temperature, averaged over the space-time evolution of the medium. Then, using experimental data, we determine this temperature, the corresponding entropy density and speed of sound in the matter created in lead-lead collisions at the Large Hadron Collider. Our results agree with first-principles calculations from lattice quantum chromodynamics and confirm that a deconfined phase of matter is indeed produced.

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arXiv2020

The mean transverse momentum of ultracentral heavy-ion collisions: A new probe of hydrodynamics

We predict that the mean transverse momentum of charged hadrons $\langle p_t\rangle$ rises as a function of the charged-particle multiplicity in ultracentral nucleus-nucleus collisions. We explain that this phenomenon has a simple physical origin and represents an unambiguous prediction of the hydrodynamic framework of heavy-ion collisions. We argue that the relative increase of $\langle p_t \rangle$ is proportional to the speed of sound squared $c_s^2$ of the quark-gluon plasma. Based on the value of $c_s^2$ from lattice QCD, we expect $\langle p_t\rangle$ to increase by approximately $18$ MeV between 1\% and 0.001\% centrality in Pb+Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV.

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arXiv2020

Observable signatures of initial state momentum anisotropies in nuclear collisions

We show that the correlation between the elliptic momentum anisotropy, $v_2$, and the average transverse momentum, $[p_T]$, at fixed multiplicity in small system nuclear collisions carries information on the origin of the observed momentum anisotropy. A calculation using a hybrid IP-Glasma+\textsc{Music}+UrQMD model that includes contributions from final state response to the initial geometry as well as initial state momentum anisotropies of the Color Glass Condensate, predicts a characteristic sign change of the correlator $\hatρ(v_2^2,[p_T])$ as a function of charged particle multiplicity in p+Au and d+Au collisions at $\sqrt{s}=200\,{\rm GeV}$, and p+Pb collisions at $\sqrt{s}=5.02\,{\rm TeV}$. This sign change is absent in calculations without initial state momentum anisotropies. The model further predicts a qualitative difference between the centrality dependence of $\hatρ(v_2^2,[p_T])$ in Au+Au collisions at $\sqrt{s}=200\,{\rm GeV}$ and Pb+Pb collisions at $\sqrt{s}=5.02\,{\rm TeV}$, with only the latter showing a sign change in peripheral events. Predictions for O+O collisions at different collision energy show a similar behavior. Experimental observation of these distinct qualitative features of $\hatρ(v_2^2,[p_T])$ in small and large systems would constitute strong evidence for the presence and importance of initial state momentum anisotropies predicted by the Color Glass Condensate effective theory.

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arXiv2020

Observing the deformation of nuclei with relativistic nuclear collisions

I show that particle collider experiments on relativistic nuclear collisions can serve as direct probes of the deformation of the colliding nuclear species. I argue that collision events presenting very large multiplicities of particles and very small values of the average transverse momentum of the emitted hadrons probe collision geometries in which the nuclear ellipsoids fully overlap along their longer side. By looking at these events one selects interaction regions whose elliptic anisotropy is determined by the deformed nuclear shape, which becomes accessible experimentally through the measurement of the elliptic flow of outgoing hadrons.

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arXiv2020

Effects of initial state fluctuations on the mean transverse momentum

We show that in ideal hydrodynamic simulations of heavy-ion collisions, initial state fluctuations result in an increase of the mean transverse momentum of outgoing hadrons, $\langle p_t\rangle$. Specifically, $\langle p_t\rangle$ is larger by a few percent if realistic fluctuations are implemented than with smooth initial conditions, the multiplicity and impact parameter being kept fixed. We show that result can be traced back to the fact that for a given total entropy, the initial energy contained in the fluid is larger if the density profile is bumpy. We discuss the implication of these results for the extraction of the equation of state of QCD from experimental data.

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arXiv2019

Evolution of fluctuations in the initial state of heavy-ion collisions from RHIC to LHC

Fluctuations in the initial state of heavy-ion collisions are larger at RHIC energy than at LHC energy. This fact can be inferred from recent measurements of the fluctuations of the particle multiplicities and of elliptic flow performed at the two different energies. We show that an analytical description of the initial energy-density field and its fluctuations motivated by the color glass condensate (CGC) effective theory predicts and quantitatively captures the measured energy evolution of these observables. The crucial feature is that fluctuations in the CGC scale like the inverse of the saturation scale of the nuclei.

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arXiv2019

Primordial non-Gaussianity in heavy-ion collisions

We show that experimental data on cumulants of anisotropic flow in heavy-ion collisions probe the non-Gaussian statistics of the energy density field created right after the collision. We carry out a perturbative expansion of the initial anisotropies of the system in terms of its density fluctuations. We argue that the correlation between the magnitudes of elliptic flow and triangular flow, dubbed $sc(3,2)$, is generically of the same sign and order of magnitude as the kurtosis of triangular flow in a hydrodynamic picture. The experimental observation that these quantities are negative implies that the distribution of energy around a given point has positive skew.

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arXiv2019

Primordial fluctuations in heavy-ion collisions

We present a simple description of the energy density profile created in a nucleus-nucleus collision, motivated by high-energy QCD. The energy density is modeled as the sum of contributions coming from elementary collisions between localized charges and a smooth nucleus. Each of these interactions creates a sharply-peaked source of energy density falling off at large distances like $1/r^2$, corresponding to the two-dimensional Coulomb field of a point charge. Our model reproduces the one-point and two-point functions of the energy density field calculated in the framework of the color glass condensate effective theory, to leading logarithmic accuracy. We apply it to the description of eccentricity fluctuations. Unlike other existing models of initial conditions for heavy-ion collisions, it allows us to reproduce simultaneously the centrality dependence of elliptic and triangular flow.

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arXiv2019

New paradigm for fluctuations in heavy-ion collisions

Since their discovery, fluctuations in the initial state of heavy-ion collisions have been understood as originating mostly from the random positions of nucleons within the colliding nuclei. We consider an alternative approach where all the focus is on fluctuations generated by QCD interactions, that we evaluate at leading logarithmic accuracy in the color glass condensate effective theory. We validate our approach using BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) data on anisotropic flow. In particular, we show that, compared to standard Glauber-inspired calculations, our formalism provides a better description of the centrality dependence of the ratio of elliptic flow and triangular flow. It also naturally explains the evolution of elliptic flow fluctuations between RHIC and LHC energies.

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arXiv2019

Relating eccentricity fluctuations to density fluctuations in heavy-ion collisions

The magnitude of anisotropic flow in a nucleus-nucleus collision is determined by the energy density field, $ρ(x,y,z)$, created right after the collision occurs. Specifically, elliptic flow, $v_2$, and triangular flow, $v_3$, are proportional to the anisotropy coefficients $\varepsilon_2$ and $\varepsilon_3$, which are functionals of $ρ$. We express the mean and the variance of $\varepsilon_2$ and $\varepsilon_3$ as a function of the 1- and 2-point functions of $ρ$. These results generalize results obtained previously, that were valid only for central collisions, or only for identical point-like sources. We apply them to the color glass condensate effective theory, using the recently derived expression of the 2-point function.

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arXiv2019

Kurtosis of elliptic flow fluctuations

Elliptic flow ($v_2$) in ultrarelativistic nucleus-nucleus collisions fluctuates event to event, both in magnitude and in orientation with respect to the reaction plane. Even though the reaction plane is not known event to event in experiment, we show that the statistical properties of $v_2$ fluctuations in the reaction plane can be precisely extracted from experimental data. Previous studies have shown how to measure the mean, variance and skewness using the first three cumulants $v_2\{2\}$, $v_2\{4\}$ and $v_2\{6\}$. We complement these studies by providing a formula for the kurtosis, which requires an accurate determination of the next cumulant $v_2\{8\}$. Using existing data, we show that the kurtosis is positive for most centralities, in contrast with the kurtosis of triangular flow fluctuations, which is negative. We argue that these features are robust predictions of fluid-dynamical models.

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