arXiv subjects

J. Noronha

Publications and source records attributed to J. Noronha.

At least 19 recordsLinked to original sources

arXiv2024

Universality Classes of Relativistic Fluid Dynamics: Applications

Using a formalism that was recently developed in a companion paper, we rigorously prove the equivalence, in the linear regime, of a number of apparently different relativistic hydrodynamic theories proposed in the literature. In particular, we show that Hydro+ is indistinguishable from the Israel-Stewart theory for bulk viscosity, which in turn is indistinguishable from a reacting mixture. The two-fluid model for superfluidity coincides with the Israel-Stewart theory for heat conduction in the limit of infinite conductivity, and this explains why the latter has a second sound. Also, MIS$^*$ is equivalent to the Burgers model for viscoelasticity, and this implies that the former must exhibit an elastic behavior at high frequencies. Additionally, we show that if the degrees of freedom and the conservation laws of a hydrodynamic theory have the same geometric character as those of the Israel-Stewart theory, then such theory must be indistinguishable from the Israel-Stewart theory in the linear regime. This explains why all second-order theories turn out to be identical near equilibrium. Finally, we construct the first linearized model for a relativistic supersolid that is proven to be causal, stable, and strongly hyperbolic.

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arXiv2023

Assessing the ultracentral flow puzzle in hydrodynamic modeling of heavy-ion collisions

An outstanding problem in heavy-ion collisions is the inability for models to accurately describe ultra-central experimental flow data, despite that being precisely the regime where a hydrodynamic description should be most applicable. We reassess the status of this puzzle by computing the flow in ultra-central collisions obtained from multiple recent Bayesian models that were tuned to various observables in different collision systems at typical centralities. While central data can now be described with better accuracy than in previous calculations, tension with experimental observation remains and worsens as one goes to ultra-central collisions. Tuning the model parameters cannot remove this tension without destroying the fit at other centralities. As such, new elements are likely needed in the standard modeling of heavy-ion collisions.

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arXiv2023

Hot QCD White Paper

Hot QCD physics studies the nuclear strong force under extreme temperature and densities. Experimentally these conditions are achieved via high-energy collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). In the past decade, a unique and substantial suite of data was collected at RHIC and the LHC, probing hydrodynamics at the nucleon scale, the temperature dependence of the transport properties of quark-gluon plasma, the phase diagram of nuclear matter, the interaction of quarks and gluons at different scales and much more. This document, as part of the 2023 nuclear science long range planning process, was written to review the progress in hot QCD since the 2015 Long Range Plan for Nuclear Science, as well as highlight the realization of previous recommendations, and present opportunities for the next decade, building on the accomplishments and investments made in theoretical developments and the construction of new detectors. Furthermore, this document provides additional context to support the recommendations voted on at the Joint Hot and Cold QCD Town Hall Meeting, which are reported in a separate document.

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arXiv2022

QCD Phase Structure and Interactions at High Baryon Density: Continuation of BES Physics Program with CBM at FAIR

We advocate for an active US participation in the international collaboration of the CBM experiment that will allow the US nuclear physics program to build on its successful exploration of the QCD phase diagram, use the expertise gained at RHIC to make complementary measurements at FAIR, and contribute to achieving the scientific goals of the beam energy scan (BES) program.

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arXiv2018

Resummed hydrodynamic expansion for a plasma of particles interacting with fields

A novel description of kinetic theory dynamics is proposed in terms of resummed moments that embed information of both hydrodynamic and non-hydrodynamic modes. The resulting expansion can be used to extend hydrodynamics to higher orders in a consistent and numerically efficient way; at lowest order it reduces to an Israel-Stewart-like theory. This formalism is especially suited to investigate the general problem of particles interacting with fields. We tested the accuracy of this approach against the exact solution of the coupled Boltzmann-Vlasov-Maxwell equations for a plasma in an electromagnetic field undergoing Bjorken-like expansion, including extreme cases characterized by large deviations from local equilibrium and large electric fields. We show that this new resummed method maintains the fast convergence of the traditional method of moments. We also find a new condition, unrelated to Knudsen numbers and pressure corrections, that justifies the truncation of the series even in situations far from local thermal equilibrium.

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arXiv2016

Nonlinear dynamics from the relativistic Boltzmann equation in the Friedmann-Lemaître-Robertson-Walker spacetime

The dissipative dynamics of an expanding massless gas with constant cross section in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe is studied. The mathematical problem of solving the full nonlinear relativistic Boltzmann equation is recast into an infinite set of nonlinear ordinary differential equations for the moments of the one-particle distribution function. Momentum-space resolution is determined by the number of non-hydrodynamic modes included in the moment hierarchy, i.e., by the truncation order. We show that in the FLRW spacetime the non-hydrodynamic modes decouple completely from the hydrodynamic degrees of freedom. This results in the system flowing as an ideal fluid while at the same time producing entropy. The solutions to the nonlinear Boltzmann equation exhibit transient tails of the distribution function with nontrivial momentum dependence. The evolution of this tail is not correctly captured by the relaxation time approximation nor by the linearized Boltzmann equation. However, the latter probes additional high-momentum details unresolved by the relaxation time approximation. While the expansion of the FLRW spacetime is slow enough for the system to move towards (and not away from) local thermal equilibrium, it is not sufficiently slow for the system to actually ever reach complete local equilibrium. Equilibration is fastest in the relaxation time approximation, followed, in turn, by kinetic evolution with a linearized and a fully nonlinear Boltzmann collision term.

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arXiv2015

Analytic solution of the Boltzmann equation in an expanding system

For a massless gas with constant cross section in a homogeneous, isotropically expanding spacetime we reformulate the relativistic Boltzmann equation as a set of non-linear coupled moment equations. For a particular initial condition this set can be solved exactly, yielding the first analytical solution of the Boltzmann equation for an expanding system. The non-equilibrium behavior of this relativistic gas can be mapped onto that of a homogeneous, static non-relativistic gas of Maxwell molecules.

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arXiv2015

Exact solutions of the Boltzmann equation and optimized hydrodynamic approaches for relativistic heavy-ion collisions

Several recent results are reported from work aiming to improve the quantitative precision of relativistic viscous fluid dynamics for relativistic heavy-ion collisions. The dense matter created in such collisions expands in a highly anisotropic manner. Due to viscous effects this also renders the local momentum distribution anisotropic. Optimized hydrodynamic approaches account for these anisotropies already at leading order in a gradient expansion. Recently discovered exact solutions of the relativistic Boltzmann equation in anisotropically expanding systems provide a powerful testbed for such improved hydrodynamic approximations. We present the latest status of our quest for a formulation of relativistic viscous fluid dynamics that is optimized for applications to relativistic heavy-ion collisions.

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arXiv2015

Anisotropic heavy quark potential in strongly-coupled $\mathcal{N}=4$ SYM in a magnetic field

In this work we use the gauge/gravity duality to study the anisotropy in the heavy quark potential in strongly coupled $\mathcal{N}=4$ Super-Yang Mills (SYM) theory (both at zero and nonzero temperature) induced by a constant and uniform magnetic field $\mathcal{B}$. At zero temperature, the inclusion of the magnetic field decreases the attractive force between heavy quarks with respect to its $\mathcal{B}=0$ value and the force associated with the parallel potential is the least attractive force. We find that the same occurs at nonzero temperature and, thus, at least in the case of strongly coupled $\mathcal{N}=4$ SYM, the presence of a magnetic field generally weakens the interaction between heavy quarks in the plasma.

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arXiv2015

Debye screening mass near deconfinement from holography

In this paper the smallest thermal screening mass associated with the correlator of the $CT$-odd operator, $\sim {\rm Tr}F_{μν}\tilde{F}^{μν}$, is determined in strongly coupled non-Abelian gauge plasmas which are holographically dual to non-conformal, bottom-up Einstein+scalar gravity theories. These holographic models are constructed to describe the thermodynamical properties of $SU(N_c)$ plasmas near deconfinement at large $N_c$ and we identify this thermal mass with the Debye screening mass $m_D$. In this class of non-conformal models with a first order deconfinement transition at $T_c$, $m_D/T$ displays the same behavior found for the expectation value of the Polyakov loop (which we also compute) jumping from zero below $T_c$ to a nonzero value just above the transition. In the case of a crossover phase transition, $m_D/T$ has a minimum similar to that found for the speed of sound squared $c_s^2$. This holographic framework is also used to evaluate $m_D$ as a function of $η/s$ in a strongly coupled conformal gauge plasma dual to Gauss-Bonnet gravity. In this case, $m_D/T$ decreases with increasing $η/s$ in accordance with extrapolations from weak coupling calculations.

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arXiv2014

Nonlinear waves in second order conformal hydrodynamics

In this work we study wave propagation in dissipative relativistic fluids described by a simplified set of the 2nd order viscous conformal hydrodynamic equations. Small amplitude waves are studied within the linearization approximation while waves with large amplitude are investigated using the reductive perturbation method. Our results indicate the presence of a "soliton-like" wave solution in 2nd order conformal hydrodynamics despite the presence of dissipation and relaxation effects.

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arXiv2014

Anisotropic shear viscosity of a strongly coupled non-Abelian plasma from magnetic branes

Recent estimates for the electromagnetic fields produced in the early stages of non-central ultra-relativistic heavy ion collisions indicate the presence of magnetic fields $B\sim \mathcal{O}(0.1-15\,m_π^2)$, where $m_π$ is the pion mass. It is then of special interest to study the effects of strong (Abelian) magnetic fields on the transport coefficients of strongly coupled non-Abelian plasmas, such as the quark-gluon plasma formed in heavy ion collisions. In this work we study the anisotropy in the shear viscosity induced by an external magnetic field in a strongly coupled $\mathcal{N} = 4$ SYM plasma. Due to the spatial anisotropy created by the magnetic field, the most general viscosity tensor of a magnetized plasma has 5 shear viscosity coefficients and 2 bulk viscosities. We use the holographic correspondence to evaluate two of the shear viscosities, $η_{\perp} \equiv η_{xyxy}$ (perpendicular to the magnetic field) and $η_{\parallel} \equiv η_{xzxz}=η_{yzyz}$ (parallel to the field). When $B\neq 0$ the shear viscosity perpendicular to the field saturates the viscosity bound $η_{\perp}/s = 1/(4π)$ while in the direction parallel to the field the bound is violated since $η_{\parallel}/s < 1/(4π)$. However, the violation of the bound in the case of strongly coupled SYM is minimal even for the largest value of $B$ that can be reached in heavy ion collisions.

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arXiv2014

Bulk viscosity-driven suppression of shear viscosity effects on the flow harmonics at RHIC

The interplay between shear and bulk viscosities on the flow harmonics, $v_n$'s, at RHIC is investigated using the newly developed relativistic 2+1 hydrodynamical code v-USPhydro that includes bulk and shear viscosity effects both in the hydrodynamic evolution and also at freeze-out. While shear viscosity is known to attenuate the flow harmonics, we find that the inclusion of bulk viscosity decreases the shear viscosity-induced suppression of the flow harmonics bringing them closer to their values in ideal hydrodynamical calculations. Depending on the value of the bulk viscosity to entropy density ratio, $ζ/s$, in the quark-gluon plasma, the bulk viscosity-driven suppression of shear viscosity effects on the flow harmonics may require a re-evaluation of the previous estimates of the shear viscosity to entropy density ratio, $η/s$, of the quark-gluon plasma previously extracted by comparing hydrodynamic calculations to heavy ion data.

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arXiv2013

A Study of the Di-Hadron Angular Correlation Function in Event by Event Ideal Hydrodynamics

The di-hadron angular correlation function is computed within boost invariant, ideal hydrodynamics for Au+Au collisions at $\sqrt{s}_{NN}=200$ GeV using Monte Carlo Glauber fluctuating initial conditions. When $0<p_T< 3$ GeV, the intensity of the flow components and their phases, $\left\{v_n, Ψ_n \right \}$ ($n=2,3$), are found to be correlated on an event by event basis to the initial condition geometrical parameters $\left\{\varepsilon_{2,n}, Φ_{2,n} \right \}$, respectively. Moreover, the fluctuation of the relative phase between trigger and associated particles, $Δ_n =Ψ_n^t - Ψ_n^a$, is found to affect the di-hadron angular correlation function when different intervals of transverse momentum are used to define the trigger and the associated hadrons.

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arXiv2013

Heavy quarkonium production in a strong magnetic field

It is well known that in noncentral heavy-ion collisions a transient strong magnetic field is generated in the direction perpendicular to the reaction plane. The maximal strength of this field is estimated to be $eB \sim m^2_π \sim 0.02 \text{GeV}^2$ at the RHIC and $eB \sim 15 m^2_π \sim 0.3 \text{GeV}^2$ at the LHC. We investigate the effects of a strong magnetic field on $B$ and $D$ mesons, focusing on the changes of the energy levels and the masses of the bound states. Using the Color Evaporation Model we discuss the possible changes in the production of $J/ψ$ and $Υ$.

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arXiv2011

Determination of the Shear Viscosity Relaxation Time at Weak and Strong Coupling

We investigate the microscopic origin of the relaxation time coefficient in relativistic fluid dynamics. We show that the extraction of the shear viscosity relaxation time via the gradient expansion is ambiguous and in general fails to give the correct result. The correct value for the shear viscosity relaxation time is extracted from the slowest non-hydrodynamic pole of the corresponding retarded Green's function, if such a pole is purely imaginary. According to the AdS/CFT correspondence, in strongly-coupled $\mathcal{N}=4$ SYM the non-hydrodynamic poles of the shear stress tensor nearest to the origin have a nonzero real part, which implies that the transient fluid-dynamical equations for this gauge theory are not equivalent to the well-known Israel-Stewart equations.

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arXiv2011

Origin of the Relaxation Time in Dissipative Fluid Dynamics

We show how the linearized equations of motion of any dissipative current are determined by the analytical structure of the associated retarded Green's function. If the singularity of the Green's function, which is nearest to the origin in the complex-frequency plane, is a simple pole on the imaginary frequency axis, the linearized equations of motion can be reduced to relaxation-type equations for the dissipative currents. The value of the relaxation time is given by the inverse of this pole. We prove that, if the relaxation time is sent to zero, or equivalently, the pole to infinity, the dissipative currents approach the values given by the standard gradient expansion.

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arXiv2011

Microscopic Origin of the Shear Relaxation Time in Causal Dissipative Fluid Dynamics

In this paper we show how to compute the shear relaxation time from an underlying microscopic theory. We prove that the shear relaxation time in Israel-Stewart-type theories is given by the inverse of the pole of the corresponding retarded Green's function, which is nearest to the origin in the complex energy plane. Consequently, the relaxation time in such theories is a microscopic, and not a macroscopic, i.e., fluid-dynamical time scale.

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