arXiv subjects

Kevin Dusling

Publications and source records attributed to Kevin Dusling.

34 records · Page 2Linked to original sources

arXiv2011

The initial spectrum of fluctuations in the little bang

High parton densities in ultra-relativistic nuclear collisions suggest a description of these collisions wherein the high energy nuclear wavefunctions and the initial stages of the nuclear collision are dominated by classical fields. This underlying paradigm can be significantly improved by including quantum fluctuations around the classical background fields. One class of these contributes to the energy evolution of multi-parton correlators in the nuclear wavefunctions. Another dominant class of unstable quantum fluctuations grow rapidly with proper time $τ$ after the collision. These secular terms appear at each loop order; the leading contributions can be resummed to all loop orders to obtain expressions for final state observables. The all-order result can be expressed in terms of the spectrum of fluctuations on the initial proper time surface. We compute, in $A^τ=0$ gauge, the essential elements in this fluctuation spectrum--the small quantum fluctuation modes in the classical background field. With our derivation in QCD, we have all the ingredients to compute inclusive quantities in heavy ion collisions at early times including i) all--order leading logs in Bjorken $x_{1,2}$ of the two nuclei, ii) all strong multiple scattering contributions, and iii) all-order leading secular terms. In the simpler analogous formalism for a scalar $ϕ^4$ theory, numerical analysis of the behavior of the energy-momentum tensor is strongly suggestive of early hydrodynamic flow in the system. In QCD, in addition to studying the possible early onset of hydrodynamic behavior, additional important applications of our results include a) the computation of sphaleron transitions off-equilibrium, and b) "jet quenching", or medium modification of parton spectra, in strong color fields at early times.

nucl-th↗

arXiv2011

Ekpyrosis and inflationary dynamics in heavy ion collisions: the role of quantum fluctuations

We summarize recent significant progress in the development of a first-principles formalism to describe the formation and evolution of matter in very high energy heavy ion collisions. The key role of quantum fluctuations both before and after a collision is emphasized. Systematic computations are now feasible to address early time dynamics essential to quantifying properties of strongly interacting quark-gluon matter.

hep-ph↗

arXiv2011

Elliptic flow of the dilute Fermi gas: From kinetics to hydrodynamics

We use the Boltzmann equation in the relaxation time approximation to study the expansion of a dilute Fermi gas at unitarity. We focus, in particular, on the approach to the hydrodynamic limit. Our main finding are: i) In the regime that has been studied experimentally hydrodynamic effects beyond the Navier-Stokes approximation are small, ii) mean field corrections to the Boltzmann equation are not important, iii) experimental data imply that freezeout occurs very late, that means that the relaxation time remains smaller than the expansion time during the entire evolution of the system, iv) the experimental results also imply that the bulk viscosity is significantly smaller than the shear viscosity of the system.

cond-mat.stat-mech↗

arXiv2011

Role of quantum fluctuations in a system with strong fields

In this work we study how quantum fluctuations modify the quantum evolution of an initially classical field theory. We consider a scalar $ϕ^4$ theory coupled to an external source as a toy model for the Color Glass Condensate description of the early time dynamics of heavy-ion collisions. We demonstrate that quantum fluctuations considerably modify the time evolution driving the system to evolve in accordance with ideal hydrodynamics. We attempt to understand the mechanism behind this relaxation to ideal hydrodynamics by using modified initial spectra and studying the particle content of the theory.

nucl-th↗

arXiv2010

The ridge in proton-proton collisions at the LHC

We show that the key features of the CMS result on the ridge correlation seen for high multiplicity events in sqrt(s)=7TeV proton-proton collisions at the LHC can be understood in the Color Glass Condensate framework of high energy QCD. The same formalism underlies the explanation of the ridge events seen in A+A collisions at RHIC, albeit it is likely that flow effects may enhance the magnitude of the signal in the latter.

hep-ph↗

arXiv2010

Photons as a viscometer of heavy ion collisions

The viscous correction to thermal photon production at leading log order is calculated and integrated over the space-time evolution of a hydrodynamic simulation of heavy-ion collisions. The resulting transverse momentum spectra and elliptic flow can be reliably calculated within a hydrodynamic framework up to transverse momenta of $q_\perp$ = 2.5 GeV and $q_\perp$ = 1.5 GeV respectively. A non-vanishing viscosity leads to a larger thermalization time when extracted from the experimentally measured inverse slope (T_eff) of photon $q_\perp$ spectra. A precise, O(20 MeV), measurement of photon T_eff can place stringent bounds on $τ_0$ and $η/s$.

nucl-th↗

arXiv2010

Radiative energy loss and v2 spectra for viscous hydrodynamics

This work investigates the first correction to the equilibrium phase space distribution and its effects on spectra and elliptic flow in heavy ion collisions. We show that the departure from equilibrium on the freezeout surface is the largest part of the viscous corrections to $v_2(p_T)$. However, the momentum dependence of the departure from equilibrium is not known {\it a priori}, and it is probably not proportional to $p_T^2$ as has been assumed in hydrodynamic simulations. At high momentum in weakly coupled plasmas it is determined by the rate of radiative energy loss and is proportional to $p_T^{3/2}$. The weaker $p_T$ dependence leads to straighter $v_2(p_T)$ curves at the same value of viscosity. Further, the departure from equilibrium is generally species dependent. A species dependent equilibration rate, with baryons equilibrating faster than mesons, can explain ``constituent quark scaling'' without invoking coalescence models.

nucl-th↗

arXiv2010

Quarkonium above deconfinement as an open quantum system

Quarkonium at temperatures above deconfinement is modeled as an open quantum system, whose dynamics is determined not just by a potential energy and mass, but also by a drag coefficient which characterizes its interaction with the medium. The reduced density matrix for a heavy particle experiencing dissipative forces is expressed as an integral over paths in imaginary time and evaluated numerically. We demonstrate that dissipation could affect the Euclidean heavy-heavy correlators calculated in lattice simulations at temperatures just above deconfinement.

nucl-th↗

arXiv2009

Parton energy loss in a classical strongly coupled QGP

We investigate the energy loss of heavy quarks in the gas, liquid and solid phase of a classical quark-gluon plasma (cQGP) using molecular dynamics simulations. The model consists of massive quarks and gluons interacting as a classical non-relativistic colored Coulomb gas. We show that the electric force decorrelates on a short time scale causing the energy loss to be mostly diffusive and langevin-like in the cQGP. We find that the drag coefficient changes with the heavy quark mass, while the diffusion constant does not. The fractional collisional energy loss is much larger than the leading order estimates from a wQGP because of the core repulsion. Following recent suggestions, we show how the cQGP results can be translated to the sQGP results in the $T=(1-3)T_c$ range.

nucl-th↗

arXiv2009

Thermal photons from heavy ion collisions: A spectral function approach

We analyze the photon rates from a hadronic gas in equilibrium using chiral reduction formulas and a density expansion. The chiral reduction is carried to second order in the pion density which in principal includes all kinetic processes of the type $X\to πγ$ and $X\to ππγ$. The resulting rates are encoded in the form of vacuum correlation functions which are amenable to experiment. The hadronic rates computed in this work along with the known perturbative QGP rates are integrated over the space-time evolution of a hydrodynamic model tuned to hadronic observables. The resulting yields are compared to the recent photon and low mass dilepton measurements at the SPS and RHIC. Predictions for the LHC are made.

nucl-th↗

arXiv2009

Effective potentials for heavy quarks above deconfinement

Using methods previously developed by Kelbg and others for creating effective potentials for electron-ion plasmas, we investigate quarkonium potentials above deconfinement. Using results for the internal energy of a static quark-antiquark pair, a set of effective potentials are constructed that take into account quantum effects and symmetry conditions. Bound state effects are explicitly included in order to account for the strongly coupled nature of plasmas. It is hoped that these effective potentials will be useful in simulations of heavy quarks or plasma quasiparticles when the dynamics is treated classically.

nucl-th↗

arXiv2009

Three-particle correlation from glasma flux tubes

We compute three particle correlations in the Glasma flux tube model of high energy heavy ion collisions. We obtain a simple geometrical picture of these correlations; when convoluted with final state radial flow, it results in distinct predictions for the near side three particle correlation in central heavy ion collisions.

nucl-th↗

arXiv2009

Hydrodynamic Description of Dilepton Production

The first part of this thesis focuses on the production of thermal dileptons from a hadronic gas at finite temperature. The rates are calculated by an expansion in Pion density and constrained by broken chiral symmetry and vacuum correlation functions, many of which have been measured by experiment. We focus on emission processes having two Pions in the final state. Next, follows a separate discussion on viscous hydrodynamics and its effect on $p_\perp$ spectra and elliptic flow. A non-central hydrodynamic model of Au-Au collisions in 2+1 dimensions is simulated.Off-equilibrium corrections to the distribution can bring about large changes in the differential elliptic flow, especially at higher $p_\perp$. Also discussed is the shear viscous correction to dilepton production in a quark-gluon plasma (QGP) emanating from $q\overline{q}$ annihilation in the Born approximation. It is argued that a thermal description is reliable for invariant masses less than $M_{max} \approx (2τ_0 T_0^2)/(η/s)$. Shear viscosity leads to qualitative differences in dilepton $p_\perp$ spectrum, which could be used to extract information on the thermalization time, viscosity to entropy ratio and possibly the thermalization mechanism in heavy-ion collisions. Finally, the dilepton rates used in this work are integrated over the space-time evolution of the collision region and compared to the recent results from the NA60 experiment at CERN and the PHENIX experiment at RHIC. The role played by chiral symmetry restoration in the hadronic phase and viscosity in the QGP phase is discussed.

nucl-th↗

arXiv2008

Quarkonium transport in thermal AdS/CFT

We consider a heavy meson moving slowly through high temperature non-abelian plasmas. Using a simple dipole effective Lagrangian, we calculate the in-medium mass shift and the drag coefficient of the meson in $\N=4$ Super Yang Mills theory at weak and strong coupling. As anticipated, in the large $N$ limit the mass shift is finite while the drag is suppressed by $1/N^2$ . After comparing results to perturbative QCD estimates (which are also calculated), we reach the conclusion that relative to weak coupling expectations the effect of strong coupling is to reduce the momentum diffusion rate and to {\it increase} the relaxation time by up to a factor of four.

hep-th↗

arXiv2008

Dilepton production from a viscous QGP

This work calculates the first correction to the leading order qq dilepton production rates due to shear viscosity in an expanding gas. The modified rates are integrated over the space-time history of a viscous hydrodynamic simulation of RHIC collisions. The net result is a {\em hardening} of $q_\perp$ spectrum with the magnitude of the correction increasing with invariant mass. We argue that a thermal description is reliable for invariant masses less than $M_{max}\approx(2τ_0 T_0^2)/(η/s)$. For reasonable values of the shear viscosity and thermalization time $M_{max}\approx 4.5$ GeV. Finally, the early emission from a viscous medium is compared to emission from a longitudinally free streaming plasma. Qualitative differences in $q_\perp$ spectrum are seen which could be used to extract information on the thermalization time, viscosity to entropy ratio and possibly the thermalization mechanism in heavy-ion collisions.

nucl-th↗

arXiv2007

Transverse momentum spectra of dileptons at NA60

Recently the NA60 collaboration has reported the transverse mass spectra of dimuons coming from In-In collisions at 158 GeV/A. The measured yields display a strong invariant mass dependence not typical of radial flow, suggesting that different sources contribute in different mass regions. We interpret the dimuon transverse mass spectra from an early thermalized partonic phase and hadronic phase constrained by the strictures of broken chiral symmetry. Each phase develops a specific transverse momentum dependence by hydrodynamical expansion. We show that a measurement of the momentum anisotropy at NA60 could provide information on the dominant emission source (hadronic or partonic) in the intermediate mass region 1.5 <= M <= 3.0 GeV.

hep-ph↗