arXiv subjects

Jorge Noronha

Publications and source records attributed to Jorge Noronha.

At least 163 records · Page 9Linked to original sources

arXiv2014

Jet quenching effects on the anisotropic flow at RHIC

In this article we investigate how the energy and momentum deposited by partonic dijets in the quark-gluon plasma may perturb the geometry-induced hydrodynamic expansion of the bulk nuclear matter created in heavy ion collisions at RHIC. The coupling between the jets and the medium is done through a source term in the energy-momentum conservation equations for ideal hydrodynamics. We concentrate our attention at mid-rapidity and solve the equations event-by-event imposing boost-invariance. For $p_T \gtrsim 1$ GeV the anisotropic flow is found to be considerably enhanced, if the dijets deposit on average more than 12 GeV in the medium (or equivalently 6 GeV for each jet of the pair), which corresponds, in our model, to an average suppression greater than $65\%$ of the initial jet transverse energy.

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arXiv2014

Elliptic Flow Suppression due to Hadron Mass Spectrum

Hadron resonance gas models provide a good description of the equation of state of quantum chromodynamics determined by lattice QCD calculations at temperatures $T \sim 100-155$ MeV. In this paper we investigate the effects of an exponentially increasing hadron mass spectrum (Hagedorn spectrum) on the azimuthal anisotropy of the rapidly expanding matter formed in ultrarelativistic heavy ion collisions. If the temperature at which the conversion from fluid degrees of freedom to hadrons is sufficiently close to the Hagedorn temperature, the production of Hagedorn resonances suppresses the differential elliptic flow of all hadron species.

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arXiv2014

Azimuthal asymmetries in high-energy collisions of protons with holographic shockwaves

Large azimuthal quadrupole and octupole asymmetries have recently been found in p+Pb collisions at the LHC. We argue that these might arise from a projectile dipole scattering off fluctuations in the target with a size on the order of the dipole. In a holographic scenario, parity even angular moments $v_{2n}$ are generated by the real part of the light-like Wilson loop due to the contribution from the background metric to the Nambu-Goto action. On the other hand, parity odd moments $v_{2n+1}$ must arise from the imaginary part of the light-like Wilson loop, which is naturally induced by a fluctuating Neveu-Schwarz 2-form.

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arXiv2014

Jet quenching effects on the direct, elliptic, and triangular flow at RHIC

In this paper we investigate how the energy and momentum deposited by partonic dijets in the quark-gluon plasma may affect the direct, elliptic and triangular flow of low (and intermediate) $p_T$ hadrons in central Au+Au collisions at RHIC. The dijets are modeled as external sources in the energy-momentum conservation equations for hydrodynamics, which are solved on an event-by-event basis within the ideal fluid approximation. We focus our investigation at mid-rapidity and solve the hydrodynamic equations imposing boost invariance. Differential anisotropic flow coefficients for $p_T \gtrsim 1$ GeV are found to be significantly enhanced if the dijets deposit on average more than 12 GeV in the QGP (or more than 6 GeV per jet). Because this jet-induced extra anisotropic flow is not related to the fluctuations of the initial geometry of the collision, the correlation between the $v_2$ and $v_3$ coefficients and their corresponding eccentricities is considerably weakened. In addition, we argue that the extra amount of direct flow induced by dijets may be quantified by comparing the azimuthal dependence of dihadron correlations in dijet events with the corresponding quantity obtained in events without dijets. This comparison could be used to give a rough estimate of the magnitude of the effective coupling between the jets and the medium.

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arXiv2014

Exact analytical solutions of second-order conformal hydrodynamics

We present some exact solutions of relativistic second-order hydrodynamic equations in theories with conformal symmetry. Starting from a spherically expanding solution in ideal hydrodynamics, we take into account general conformal second-order corrections, and construct, for the first time, fully analytical axisymmetric exact solutions including the case with nonzero vorticity. These solutions are time-reversible despite having a nonvanishing shear stress tensor and provide a useful quantitative measure of the second-order effects in relativistic hydrodynamics.

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arXiv2013

A holographic calculation of the electric conductivity of the strongly coupled quark-gluon plasma near the deconfinement transition

The frequency dependent conductivity $σ(ω)$ of the strongly coupled Quark-Gluon Plasma (QGP) is estimated using a bottom up holographic model that can adequately describe recent lattice data for QCD thermodynamics at zero chemical potential. Different choices for the coupling between the bulk gauge field and the other bulk fields that define the background (the metric and a scalar field) are used in order to fit the lattice data for the electric charge susceptibility $χ_2^Q/T^2$. The ratio $σ_{DC}/T$ is found to vary near the deconfinement transition in a way that is similar to recent lattice results. This model is used to compute the charge diffusion coefficient $D$ of the strongly coupled plasma. We find that the dimensionless combination $DT$ has the same type of temperature dependence displayed by $σ_{DC}/T$ and, thus, charge diffusion is suppressed at low temperatures. The frequency dependent conductivity $σ(ω)$ reveals some nontrivial structure for values of the temperature near the phase transition. None of these structures appear in the associated Euclidean correlator, which we also compute. Our results suggest that the conformal invariance violation near the QCD deconfinement phase transition may be seen in the Euclidean correlator through a downward shift of its value at the minimum, which gives a rough estimate of the temperature dependence of the DC conductivity in the plasma.

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arXiv2013

On the fluid behavior of a baryon rich hadron resonance gas

We investigate the effects of finite baryon chemical potential on the transport properties of a hadron resonance gas. We find that a hadron resonance gas with large baryon number density is closer to the ideal fluid limit than the corresponding gas with zero baryon number. This suggests that the system created at the Relativistic Heavy Ion Collider (RHIC) at lower collision energies may behave as a fluid, with an effective fluidity close to the one found at RHIC's highest energy near phase transition. This might explain why the differential elliptic flow coefficient measured at lower collisional energies at RHIC is similar to the one observed at high energies.

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arXiv2013

Solutions of Conformal Israel-Stewart Relativistic Viscous Fluid Dynamics

We use symmetry arguments developed by Gubser to construct the first radially-expanding explicit solutions of the Israel-Stewart formulation of hydrodynamics. Along with a general semi-analytical solution, an exact analytical solution is given which is valid in the cold plasma limit where viscous effects from shear viscosity and the relaxation time coefficient are important. The radially expanding solutions presented in this paper can be used as nontrivial checks of numerical algorithms employed in hydrodynamic simulations of the quark-gluon plasma formed in ultra-relativistic heavy ion collisions. We show this explicitly by comparing such analytic and semi-analytic solutions with the corresponding numerical solutions obtained using the MUSIC viscous hydrodynamics simulation code.

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arXiv2013

Modification of the B Meson Mass in a Magnetic Field from QCD Sum Rules

In this paper we extend the well known QCD sum rules used in the calculation of the mass of heavy mesons to estimate the modification of the charged B meson mass, m_B, in the presence of an external Abelian magnetic field, eB. Two simplifying limits were considered: the weak field limit in which the external field satisfies eB << m^2 (with m being any of the masses involved) and the strong field limit in which the field strength is small in comparison to the bottom quark mass (or the B meson mass) squared but it is large compared to the mass of the light quarks, i.e., m^2_{u,d} << eB << m^2_{b,B}. We found that m_B decreases with the magnetic field in the both of these limits.

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arXiv2013

Estimates for the Thermal Width of Heavy Quarkonia in Strongly Coupled Plasmas from Holography

The gauge/gravity duality is used to investigate the imaginary part of the heavy quark potential (defined via the rectangular Wilson loop) in strongly coupled plasmas. This quantity can be used to estimate the width of heavy quarkonia in a plasma at strong coupling. In this paper the thermal worldsheet fluctuation method, proposed in [J.Noronha and A.Dumitru, Phys.\ Rev.\ Lett.\ {\bf 103}, 152304 (2009)], is revisited and general conditions for the existence of an imaginary part for the heavy quark potential computed within classical gravity models are obtained. We prove a general result that establishes the connection between this imaginary part of the potential determined holographically and the area law displayed by the Wilson loop in the vacuum of confining gauge theories. We also determine the imaginary part of the heavy quark potential in a strongly coupled plasma dual to Gauss-Bonnet gravity. This provides an estimate of how the thermal width of heavy quarkonia changes with the shear viscosity to entropy density ratio, $η/s$, at strong coupling.

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arXiv2013

Large Nc Deconfinement Transition in the Presence of a Magnetic Field

We investigate the effect of a homogeneous magnetic field on the thermal deconfinement transition of QCD in the large $N_c$ limit. First we discuss how the critical temperature decreases due to the inclusion of $N_f \ll N_c$ flavors of massless quarks. Then we study the equivalent correction in the presence of an external Abelian magnetic field. To leading order in $N_{f}/N_{c}$, the fact that the deconfinement critical temperature decreases with the magnetic field depends solely on quarks behaving paramagnetically. Finally, we discuss the effects from a finite quark mass and its competition with magnetic effects.

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arXiv2013

Bulk Viscosity Effects in Event-by-Event Relativistic Hydrodynamics

Bulk viscosity effects on the collective flow harmonics in heavy ion collisions are investigated, on an event by event basis, using a newly developed 2+1 Lagrangian hydrodynamic code named v-USPhydro which implements the Smoothed Particle Hydrodynamics (SPH) algorithm for viscous hydrodynamics. A new formula for the bulk viscous corrections present in the distribution function at freeze-out is derived starting from the Boltzmann equation for multi-hadron species. Bulk viscosity is shown to enhance the collective flow Fourier coefficients from $v_2(p_T)$ to $v_5(p_T)$ when $% p_{T}\sim 1-3$ GeV even when the bulk viscosity to entropy density ratio, $% ζ/s$, is significantly smaller than $1/(4π)$.

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arXiv2013

Transient Fluid Dynamics of the Quark-Gluon Plasma According to AdS/CFT

We argue, using the AdS/CFT correspondence, that the transient dynamics of the shear stress tensor in a strongly coupled $\mathcal{N}=4$ SYM plasma is not described by relaxation-type, fluid dynamical equations: at long times the equations of motion should contain a \textit{second-order} comoving derivative of the shear stress tensor. This occurs because in this strongly-coupled system the lowest "non-hydrodynamical" quasinormal modes associated with shear stress possess a nonzero real part at zero wavenumber. We use Weyl invariance to obtain the most general equations of motion containing 2 comoving derivatives of the shear stress tensor in the transient regime that are compatible with the symmetries. We show that the asymptotic solution of this theory valid at times much larger than the timescale associated with the "non-hydrodynamical" modes reproduces the well-known results previously obtained directly from the AdS/CFT correspondence. If the QGP formed in heavy ion collisions can be at least qualitatively understood in terms of strongly-coupled $\mathcal{N}=4$ SYM theory, the second time derivative present in the equations of motion of the fluid may lead to an unexpected dependence on the initial conditions for the shear stress tensor needed in numerical hydrodynamic simulations.

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arXiv2012

Further results on peripheral-tube model for ridge correlation

Peripheral one-tube model has shown to be a nice tool for dynamically understanding several aspects of ridge structures in long-range two-particle correlations, observed experimentally and obtained also in our model calculations using NexSPheRIO code. Here, we study an extension of the model, to initial configurations with several peripheral tubes distributed randomly in azimuth. We show that the two-particle correlation is almost independent of the number of tubes, although the flow distribution becomes indeed strongly event dependent. In our picture, the ridge structures are causally connected not only in the longitudinal direction but also in azimuth.

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arXiv2012

Falling Strings and Light Quark Jet Quenching at LHC

We explore phenomenological signatures of light quark jet quenching within the AdS/CFT correspondence. Numerical studies of the instantaneous energy loss of light quarks, modeled as falling strings, suggest a linear path dependence. We propose a phenomenological model for the energy loss and use it to compute the nuclear modification factor $R_{AA}$ for light quarks in an expanding plasma with Glauber initial conditions. The results are compared to the light hadron $R_{AA}$ data at the LHC and, although they show qualitative agreement, the quantitative disagreement we found motivated the exploration of effects from higher order derivative corrections to $AdS_5$ on this observable.

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arXiv2012

Hadron Mass Spectrum and the Shear Viscosity to Entropy Density Ratio of Hot Hadronic Matter

Lattice calculations of the QCD trace anomaly at temperatures $T<160$ MeV have been shown to match hadron resonance gas model calculations, which include an exponentially rising hadron mass spectrum. In this paper we perform a more detailed comparison of the model calculations to lattice data that confirms the need for an exponentially increasing density of hadronic states. Also, we find that the lattice data is compatible with a hadron density of states that goes as $ρ(m) \sim m^{-a}\exp(m/T_H) $ at large $m$ with $a> 5/2$ (where $T_H \sim 167$ MeV). With this specific subleading contribution to the density of states, heavy resonances are most likely undergo 2-body decay (instead of multi-particle decay), which facilitates their inclusion into hadron transport codes. Moreover, estimates for the shear viscosity and the shear relaxation time coefficient of the hadron resonance model computed within the excluded volume approximation suggest that these transport coefficients are sensitive to the parameters that define the hadron mass spectrum.

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arXiv2011

Jet Quenching in Non-Conformal Holography

We use our non-conformal holographic bottom-up model for QCD described in 1012.0116 to further study the effect of the QCD trace anomaly on the energy loss of both light and heavy quarks in a strongly coupled plasma. We compute the nuclear modification factor $R_{AA}$ for bottom and charm quarks in an expanding plasma with Glauber initial conditions. We find that the maximum stopping distance of light quarks in a non-conformal plasma scales with the energy with a temperature (and energy) dependent effective power.

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arXiv2011

Non-conformal Holography of Heavy Quark Quenching

We develop a holographic (bottom-up) gravity model for QCD to understand the connection between the peak in the trace anomaly and the magnitude of heavy quark energy loss in a strongly-coupled plasma. The potential of the scalar field on the gravity side is constructed to reproduce some properties of QCD at finite temperature and its parameters are constrained by fitting lattice gauge theory results. The energy loss of heavy quarks (as predicted by the holographic model) is found to be strongly sensitive to the medium properties.

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