arXiv subjects

S. Plumari

Publications and source records attributed to S. Plumari.

At least 19 recordsLinked to original sources

arXiv2024

Spontaneous breaking of diffeomorphism invariance in conformally reduced quantum gravity

We study the spontaneous breaking of diffeomorphism invariance using the proper-time non-perturbative flow equation in quantum gravity. In particular, we analyze the structure of the UV critical manifold of conformally reduced Einstein-Hilbert theory and observe the occurrence of a non-trivial minimum for the conformal factor at Planckian energies. We argue that our result can be interpreted as the occurrence of a dynamically generated minimal length in quantum gravity.

gr-qc↗

arXiv2022

Letter of Intent: the NA60+ experiment

We propose a new fixed-target experiment for the study of electromagnetic and hard probes of the Quark-Gluon Plasma (QGP) in heavy-ion collisions at the CERN SPS. The experiment aims at performing measurements of the dimuon spectrum from threshold up to the charmonium region, and of hadronic decays of charm and strange hadrons. It is based on a muon spectrometer, which includes a toroidal magnet and six planes of tracking detectors, coupled to a vertex spectrometer, equipped with Si MAPS immersed in a dipole field. High luminosity is an essential requirement for the experiment, with the goal of taking data with 10$^6$ incident ions/s, at collision energies ranging from $\sqrt{s_{\rm NN}} = 6.3$ GeV ($E_{\rm lab}= 20$ A GeV) to top SPS energy ($\sqrt{s_{\rm NN}} = 17.3$ GeV, $E_{\rm lab}= 158$ A GeV). This document presents the physics motivation, the foreseen experimental set-up including integration and radioprotection studies, the current detector choices together with the status of the corresponding R&D, and the outcome of physics performance studies. A preliminary cost evaluation is also carried out.

nucl-ex↗

arXiv2020

Diffusion of heavy quarks in the early stage of high-energy nuclear collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

We study the diffusion of charm and beauty in the early stage of high energy nuclear collisions at RHIC and LHC energies, considering the interaction of these heavy quarks with the evolving Glasma by means of the Wong equations. In comparison with previous works, we add the longitudinal expansion as well as we estimate the effect of energy loss due to gluon radiation. We find that heavy quarks diffuse in the strong transverse color fields in the very early stage (0.2-0.3 fm/c) and this leads to a suppression at low $p_T$ and enhancement at intermediate low $p_T$. The shape of the observed nuclear suppression factor obtained within our calculations is in qualitative agreement with the experimental results of the same quantity for $D-$mesons in proton-nucleus collisions. We compute the nuclear suppression factor in nucleus-nucleus collisions as well, for both charm and beauty, finding a substantial impact of the evolving Glasma phase on these, suggesting that initialization of heavy quarks spectra in the quark-gluon plasma phase should not neglect the early evolution in the strong gluon fields.

nucl-th↗

arXiv2020

Dissipative hydrodynamics of relativistic shock waves in a Quark Gluon Plasma: comparing and benchmarking alternate numerical methods

This paper presents numerical cross-comparisons and benchmark results for two different kinetic numerical methods, capable of describing relativistic dissipative fluid dynamics in a wide range of kinematic regimes, typical of relevant physics applications, such as transport phenomena in quark-gluon plasmas. We refer to relativistic lattice Boltzmann versus Montecarlo Test-Particle methods. Lacking any realistic option for accurate validation vis-a-vis experimental data, we check the consistency of our results against established simulation packages available in the literature. We successfully cross-compare the results of the two aforementioned numerical approaches for momentum integrated quantities like the hydrostatic and dynamical pressure profiles, the collective flow and the heat flux. These results corroborate the confidence on the robustness and correctness of these computational methods and on the accurate calibration of their numerical parameters with respect to the physical transport coefficients. Our numerical results are made available as supplemental material, with the aim of establishing a reference benchmark for other numerical approaches.

hep-ph↗

arXiv2019

Transport properties from Charm to Bottom: $p_T$ suppression, anisotropic flow $v_n$ and their correlations to the bulk dynamics

We study the propagation of heavy quarks (HQs) in the quark-gluon plasma (QGP) by means of a relativistic Boltzmann transport (RBT) approach. The non-perturbative interaction between heavy quarks and light quarks is described by means of a quasi-particle approach able to describe simultaneously the experimental data for the nuclear suppression factor $R_{\rm AA}$ and the elliptic flow $v_2(p_T)$ of D mesons from RHIC to LHC energies. In the same framework we predict the B meson nuclear modification factor at LHC. Finally, we discuss the relevance of initial state fluctuations that allows to extend the analysis to high order anisotropic flows $v_n(p_T)$ as well as to investigate the role of QCD interaction in developing correlations between the light and the heavy flavour anisotropic flows.

hep-ph↗

arXiv2018

Study of hard and electromagnetic processes at CERN-SPS energies: an investigation of the high-$μ_{\mathbf{B}}$ region of the QCD phase diagram with NA60+

The exploration of the phase diagram of Quantum ChromoDynamics (QCD) is carried out by studying ultrarelativistic heavy-ion collisions. The energy range covered by the CERN SPS ($\sqrt{s_{\rm \scriptscriptstyle{NN}}} \sim$ 6-17 GeV) is ideal for the investigation of the region of the phase diagram corresponding to finite baryochemical potential ($μ_{\rm B}$), and has been little explored up to now. We propose in this document a new experiment, NA60+, that would address several observables which are fundamental for the understanding of the phase transition from hadronic matter towards a Quark-Gluon Plasma (QGP) at SPS energies. In particular, we propose to study, as a function of the collision energy, the production of thermal dimuons from the created system, from which one would obtain a caloric curve of the QCD phase diagram that is sensitive to the order of the phase transition. In addition, the measurement of a $ρ$-a$_1$ mixing contribution would provide conclusive insights into the restoration of the chiral symmetry of QCD. In parallel, studies of heavy quark and quarkonium production would also be carried out, addressing the measurement of transport properties of the QGP and the investigation of the onset of the deconfinement transition. The document also defines an experimental set-up which couples a vertex telescope based on monolithic active pixel sensors (MAPS) to a muon spectrometer with tracking (GEM) and triggering (RPC) detectors within a large acceptance toroidal magnet. Results of physics performance studies for most observables accessible to NA60+ are discussed, showing that the results of the experiment would lead to a significant advance of our understanding of strong interaction physics. The document has been submitted as an input to the European Particle Physics Strategy Update 2018-2020 (http://europeanstrategyupdate.web.cern.ch/).

nucl-ex↗

arXiv2018

Extraction of Heavy-Flavor Transport Coefficients in QCD Matter

We report on broadly based systematic investigations of the modeling components for open heavy-flavor diffusion and energy loss in strongly interacting matter in their application to heavy-flavor observables in high-energy heavy-ion collisions, conducted within an EMMI Rapid Reaction Task Force framework. Initial spectra including cold-nuclear-matter effects, a wide variety of space-time evolution models, heavy-flavor transport coefficients, and hadronization mechanisms are scrutinized in an effort to quantify pertinent uncertainties in the calculations of nuclear modification factors and elliptic flow of open heavy-flavor particles in nuclear collisions. We develop procedures for error assessments and criteria for common model components to improve quantitative estimates for the (low-momentum) heavy-flavor diffusion coefficient as a long-wavelength characteristic of QCD matter as a function of temperature, and for energy loss coefficients of high-momentum heavy-flavor particles.

nucl-th↗

arXiv2018

Addendum to Strangeness Production and Color Deconfinement

Recent extensive data from the beam energy scan of the STAR collaboration at BNL-RHIC provide the basis for a detailed update for the universal behavior of the strangeness suppression factor gamma_s as function of the initial entropy density, as proposed in our recent paper [1]. [1] P. Castorina, S. Plumari and H. Satz, Int. J. Mod. Phys. E26 (2017) 1750081 (arXiv:1709.02706)

nucl-th↗

arXiv2017

Universal strangeness production and size fluctuactions in small and large systems

Strangeness production in high multiplicity events gives indications on the transverse size fluctuactions in nucleus-nucleus ($AA$), proton-nucleus ($pA$) and proton-proton ($pp$) collisions. In particular the behavior of strange particle hadronization in "small" ($pp,pA$) and "large" ($AA$) initial configurations of the collision can be tested for the specific particle species, for different centralities and for large fluctuations of the transverse size in $pA$ and $pp$ by using the recent ALICE data. A universality of strange hadron production emerges by introducing a dynamical variable proportional to the initial parton density in the transverse plane.

nucl-th↗

arXiv2017

Photons from the Early Stages of Relativistic Heavy Ion Collisions

We present results about photons production in relativistic heavy ion collisions. The main novelty of our study is the calculation of the contribution of the early stage photons to the photon spectrum. The initial stage is modeled by an ensemble of classical gluon fields which decay to a quark-gluon plasma via the Schwinger mechanism, and the evolution of the system is studied by coupling classical field equations to relativistic kinetic theory; photons production is then computed by including the pertinent collision processes into the collision integral. We find that the contribution of the early stage photons to the direct photon spectrum is substantial for $p_T \approx 2$ GeV and higher, the exact value depending on the collision energy; therefore we identify this part of the photon spectrum as the sign of the early stage. Moreover, the amount of photons produced during the early stage is not negligible with respect to those produced by a thermalized quark-gluon plasma: we support the idea that there is no dark age in relativistic heavy ion collisions.

nucl-th↗

arXiv2016

Universal Strangeness Production in Hadronic and Nuclear Collisions

We show that strangeness suppression in hadronic and nuclear collisions is fully determined by the initial energy density of the collision. The suppression factor $γ_s(s)$, with $\sqrt s$ denoting the collision energy, can be expressed as a universal function of the initial energy density $ε_0(s)$, and the resulting pattern is in excellent agreement with data from $p-p,~p-Pb,~Cu-Cu,~Au-Au$ and $Pb-Pb$ data over a wide range of energies and for different centralities.

hep-ph↗

arXiv2016

Modelling Early Stages of Relativistic Heavy Ion Collisions: Coupling Relativistic Transport Theory to Decaying Color-electric Flux Tubes

In this study we model early times dynamics of the system produced in relativistic heavy ion collisions by an initial color electric field which then decays to a plasma by the Schwinger mechanism, coupling the dynamical evolution of the initial color field to the dynamics of the many particles system produced by the decay. The latter is described by relativistic kinetic theory in which we fix the ratio $η/s$ rather than insisting on specific microscopic processes. We study isotropization and thermalization of the system produced by the field decay for a static box and for a $1+1$D expanding geometry. We find that regardless of the viscosity of the produced plasma, the initial color electric field decays within $1$ fm/c; however in the case $η/s$ is large, oscillations of the field are effective along all the entire time evolution of the system, which affect the late times evolution of the ratio between longitudinal and transverse pressure. In case of small $η/s$ ($η/s\lesssim0.3$) we find $τ_{isotropization}\approx 0.8$ fm/c and $τ_{thermalization}\approx 1$ fm/c in agreement with the common lore of hydrodynamics. Moreover we have investigated the effect of turning from the relaxation time approximation to the Chapman-Enskog one: we find that this improvement affects mainly the early times evolution of the physical quantities, the effect being milder in the late times evolution.

hep-ph↗

arXiv2015

Shear Viscosity of a strongly interacting system: Green-Kubo vs. Chapman-Enskog and Relaxation Time Approximation

The shear viscosity $η$ has been calculated by using the Green-Kubo relation in the framework of a partonic transport approach solved at cascade level. We compare the numerical results for $η$ obtained from the Green-Kubo correlator with the analytical formulas in both the Relaxation Time Approximation (RTA) and the Chapman-Enskog approximation (CE). We investigate and emphasize the differences between the isotropic and anisotropic cross sections and between the massless and massive particles. We show that in the range of temperature explored in a Heavy Ion collision and for pQCD-like cross section the RTA significantly underestimates the viscosity by about a factor of 2-3, while a good agreement is found between the CE approximation and Gree-Kubo relation already at first order of approximation. The agreement with the CE approximation supplies an analytical formula that allows to develop kinetic transport theory at fixed shear viscosity to entropy density ratio, $η/s$. This open the possibility to explore dissipative non-equilibrium evolution of the distribution functions vs T-dependent $η/s$ and particle momenta in the dynamics of the Quark-Gluon Plasma created in ultra-relativistic heavy-ion collisions.

nucl-th↗

arXiv2015

Shear viscosity $η$ to electric conductivity $σ_{el}$ ratio for the Quark-Gluon Plasma

The transport coefficients of strongly interacting matter are currently subject of intense theoretical and phenomenological studies due to their relevance for the characterization of the quark-gluon plasma produced in ultra relativistic heavy-ion collisions (uRHIC). We discuss the connection between the shear viscosity to entropy density ratio, $η/s$, and the electric conductivity, $σ_{el}$. Once the relaxation time is tuned to have a minimum value of $η/s=1/4π$ near the critical temperature $T_c$, one simultaneously predicts $σ_{el}/T$ very close to recent lQCD data. More generally, we discuss why the ratio of $(η/s)/(σ_{el}/T)$ supplies a measure of the quark to gluon scattering rates whose knowledge would allow to significantly advance in the understanding of the QGP phase. We also predict that $(η/s)/(σ_{el}/T)$, independently on the running coupling $α_s(T)$, should increase up to about $\sim 20$ for $T \rightarrow T_c$, while it goes down to a nearly flat behavior around $\simeq 4$ for $T \geq 4\, T_c$. Therefore we in general predict a stronger T dependence of $σ_{el}/T$ with respect to $η/s$ that in a quasiparticle approach is constrained by lQCD thermodynamics. A conformal theory, instead, predicts a similar T dependence of $η/s$ and $σ_{el}/T$.

hep-ph↗

arXiv2014

Electric Conductivity from the solution of the Relativistic Boltzmann Equation

We present numerical results of electric conductivity $σ_{el}$ of a fluid obtained solving the Relativistic Transport Boltzmann equation in a box with periodic boundary conditions. We compute $σ_{el}$ using two methods: the definition itself, i.e. applying an external electric field, and the evaluation of the Green-Kubo relation based on the time evolution of the current-current correlator. We find a very good agreement between the two methods. We also compare numerical results with analytic formulas in Relaxation Time Approximation (RTA) where the relaxation time for $σ_{el}$ is determined by the transport cross section $σ_{tr}$, i.e. the differential cross section weighted with the collisional momentum transfer. We investigate the electric conductivity dependence on the microscopic details of the 2-body scatterings: isotropic and anisotropic cross-section, and massless and massive particles. We find that the RTA underestimates considerably $σ_{el}$; for example at screening masses $m_D \sim \,T$ such underestimation can be as large as a factor of 2. Furthermore, we study a more realistic case for a quark-gluon system (QGP) considering both a quasi-particle model, tuned to lQCD thermodynamics, as well as the case of a pQCD gas with running coupling. Also for these cases more directly related to the description of the QGP system, we find that RTA significantly underestimate the $σ_{el}$ by about a $60-80\%$.

hep-ph↗

arXiv2014

Relativistic Boltzmann transport approach with Bose-Einstein statistics and the onset of gluon condensation

We study the evolution of a gluon system under conditions of density and temperature similar to those explored in the early stage of ultra-relativistic heavy-ion collisions. We first describe the implementation of Relativistic Boltzmann-Nordheim (RBN) transport approach that includes in the collision integral the quantum effects of Bose-Einstein Statistics. Then, we describe the evolution of a spatially uniform gluon system in a box under elastic collisions solving the RBN for various initial conditions. We discuss the critical phase-space density that leads to the onset of a Bose-Einstein condensate (BEC) and the time scale for this process to occur. In particular, thanks to the fact that RBN allows to relax the small angle approximation, we study the effect at both small and large screening mass $ m_{D} $. For small $ m_{D}\ll T $ we see that our solution of RBN is in agreement with the recent extensive studies within a Fokker-Planck scheme in small angle approximation. For the same total cross section but with large $ m_{D}\simeq 2\, T $ (large angle scatterings), we see a significant time speed-up of the onset of BEC respect to small $m_{D}\ll T$. This further strengthen the possibility that at least a transient BEC is formed in the early stage of ultra-relativistic heavy-ion collisions.

nucl-th↗

arXiv2013

Neutron Star masses from the Field Correlator Method Equation of State

We analyse the hadron-quark phase transition in neutron stars by confronting the hadronic Equation of State (EoS) obtained according to the microscopic Brueckner-Hartree-Fock many body theory, with the quark matter EoS derived within the Field Correlator Method. In particular, the latter EoS is only parametrized in terms of the gluon condensate and the large distance quark-antiquark potential, so that the comparison of the results of this analysis with the most recent measurements of heavy neutron star masses provides some physical constraints on these two parameters.

hep-ph↗

arXiv2013

Quark-to-gluon composition of the quark-gluon plasma in relativistic heavy-ion collisions

We study the evolution of the quark-gluon composition of the plasma created in ultra-Relativistic Heavy-Ion Collisions (uRHIC's) employing a partonic transport theory that includes both elastic and inelastic collisions plus a mean fields dynamics associated to the widely used quasi-particle model. The latter, able to describe lattice QCD thermodynamics, implies a "chemical" equilibrium ratio between quarks and gluons strongly increasing as $T\rightarrow T_c$, the phase transition temperature. Accordingly we see in realistic simulations of uRHIC's a rapid evolution from a gluon dominated initial state to a quark dominated plasma close to $T_c$. The quark to gluon ratio can be modified by about a factor of $\sim 20$ in the bulk of the system and appears to be large also in the high $p_T$ region. We discuss how this aspect, often overflown, can be important for a quantitative study of several key issues in the QGP physics: shear viscosity, jet quenching, quarkonia suppression. Furthermore a bulk plasma made by more than $80\%$ of quarks plus antiquarks provides a theoretical basis for hadronization via quark coalescence.

nucl-th↗