arXiv subjects

Adrian Dumitru

Publications and source records attributed to Adrian Dumitru.

At least 109 records · Page 6Linked to original sources

arXiv2004

The Quark-Mass Dependence of T_C in QCD: Working up from m=0 or down from m=infinity ?

We analyze the dependence of the QCD transition temperature on the quark (or pion) mass. We find that a linear sigma model, which links the transition to chiral symmetry restoration, predicts a much stronger dependence of T_c on m_pi than seen in present lattice data for m_pi >~ 0.4 GeV. On the other hand, working down from m_pi=infinity, an effective Lagrangian for the Polyakov loop requires only small explicit symmetry breaking, b_1 ~ exp(-m_pi), to describe T_c(m_pi) in the above mass range. Physically, this is a consequence of the flat potential (large correlation length) for the Polyakov loop in the three-color pure gauge theory at T_c. We quantitatively estimate the end point of the line of first order deconfining phase transitions: m_pi = 1.8 GeV and Tc = 240 MeV for three flavors and three colors.

hep-ph↗

arXiv2004

Renormalized Polyakov Loops, Matrix Models and the Gross-Witten Point

The values of renormalized Polyakov loops in the three lowest representations of SU(3) were measured numerically on the lattice. We find that in magnitude, condensates respect the large-N property of factorization. In several ways, the deconfining phase transition for N=3 appears to be like that in the N=infinity matrix model of Gross and Witten. Surprisingly, we find that the values of the renormalized triplet loop are described by an SU(3) matrix model, with an effective action dominated by the triplet loop. Future numerical simulations with a larger number of colors should be able to show whether or not the deconfining phase transition is close to the Gross-Witten point.

hep-ph↗

arXiv2004

Deconfining Phase Transition as a Matrix Model of Renormalized Polyakov Loops

We discuss how to extract renormalized from bare Polyakov loops in SU(N) lattice gauge theories at nonzero temperature in four spacetime dimensions. Single loops in an irreducible representation are multiplicatively renormalized without mixing, through a renormalization constant which depends upon both representation and temperature. The values of renormalized loops in the four lowest representations of SU(3) were measured numerically on small, coarse lattices. We find that in magnitude, condensates for the sextet and octet loops are approximately the square of the triplet loop. This agrees with a large $N$ expansion, where factorization implies that the expectation values of loops in adjoint and higher representations are just powers of fundamental and anti-fundamental loops. For three colors, numerically the corrections to the large $N$ relations are greatest for the sextet loop, $\leq 25%$; these represent corrections of $\sim 1/N$ for N=3. The values of the renormalized triplet loop can be described by an SU(3) matrix model, with an effective action dominated by the triplet loop. In several ways, the deconfining phase transition for N=3 appears to be like that in the $N=\infty$ matrix model of Gross and Witten.

hep-th↗

arXiv2003

Polarized Hyperons from pA Scattering in the Gluon Saturation Regime

We study the production of transversely polarized Lambda hyperons in high-energy collisions of protons with large nuclei. The large gluon density of the target at saturation provides an intrinsic semi-hard scale which should naturally allow for a weak-coupling QCD description of the process in terms of a convolution of the quark distribution of the proton with the elementary quark-nucleus scattering cross section (resummed to all twists) and a fragmentation function. In this case of transversely polarized Lambda production we employ a so-called polarizing fragmentation function, which is an odd function of the transverse momentum of the Lambda relative to the fragmenting quark. Due to this k_t-odd nature, the resulting Lambda polarization is essentially proportional to the derivative of the quark-nucleus cross section with respect to transverse momentum, which peaks near the saturation momentum scale. Such processes might therefore provide generic signatures for high parton density effects and for the approach to the ``black-body'' (unitarity) limit of hadronic scattering.

hep-ph↗

arXiv2003

Proton breakup in high-energy pA collisions from perturbative QCD

We argue that the distribution of hadrons near the longitudinal light-cone in central high-energy pA collisions is computable in weak coupling QCD. This is because the density of gluons per unit transverse area in the dense target at saturation provides an intrinsic semi-hard momentum scale, Qs. We predict that the longitudinal distribution of (anti-)baryons and mesons steepens with increasing energy and atomic number of the target, and that the transverse momentum distribution broadens. We show that the evolution of high moments of the longitudinal net baryon distribution with Qs is determined by the anomalous dimension gamma_qq.

hep-ph↗

arXiv2002

Scattering of Gluons from the Color Glass Condensate

We prove that the inclusive single-gluon production cross section for a hadron colliding with a high-density target factorizes into the gluon distribution function of the projectile, defined as usual within the DGLAP collinear approximation, times the cross section for scattering of a single gluon on the strong classical color field of the target. We then derive the gluon-proton (nucleus) inelastic cross section and show that it is (up to logarithms) infrared safe and that it grows slowly with center of mass energy. Furthermore, we discuss jet transverse momentum broadening for the case of nuclear targets. We show that in the saturation regime, in contrast to the perturbative regime, the width of the transverse momentum distribution is infrared finite and grows rapidly with energy and rapidity. In both regimes, however, transverse momentum broadening exibits the same dependence on atomic number A.

hep-ph↗

arXiv2002

How the HBT-Puzzle at RHIC might dissipate

I compute the first-order corrections to two-particle Bose-Einstein correlations due to deviations from equilibrium. Based on that result I argue that for nearly perfect fluids, the HBT radii ``freeze out'' much later than the single-inclusive distribution. Therefore, to prevent a big longitudinal homogeneity length, the QGP presumably should hadronize not into a nearly perfect fluid of hadrons but rather into a very dissipative one. This could be achieved by a hadronization phase transition away from equilibrium.

nucl-th↗

arXiv2002

Two-point functions for SU(3) Polyakov Loops near T_c

We discuss the behavior of two point functions for Polyakov loops in a SU(3) gauge theory about the critical temperature, T_c. From a Z(3) model, in mean field theory we obtain a prediction for the ratio of masses at T_c, extracted from correlation functions for the imaginary and real parts of the Polyakov loop. This ratio is m_i/m_r = 3 if the potential only includes terms up to quartic order in the Polyakov loop; its value changes as pentic and hexatic interactions become important. The Polyakov Loop Model then predicts how m_i/m_r changes above T_c.

hep-ph↗

arXiv2002

Forward Quark Jets from Protons Shattering the Colored Glass

We consider the single-inclusive minijet cross section in pA at forward rapidity within the Color Glass Condensate model of high energy collisions. We show that the nucleus appears black to the incident quarks except for very large impact parameters. A markedly flatter p_t distribution as compared to QCD in the dilute perturbative limit is predicted for transverse momenta about the saturation scale, which could be as large as Q_s^2 ~ 10 GeV^2 for a gold nucleus boosted to rapidity ~10 (as at the BNL-RHIC).

hep-ph↗

arXiv2002

Color Glass Condensate at RHIC ?

Heavy-ion collisions at the BNL-RHIC collider can probe whether gluon saturation effects in nuclei at small x have set in, or whether leading-twist perturbative estimates of particle production are still applicable. I discuss that soon to come data from pA collisions at RHIC may provide more systematic insight into this problem than present data on the energy and centrality dependence of hadron production from Au+Au collisions. Results from pA collisions at RHIC could also help to understand some controversial and puzzling results from ion-ion collisions, for example the large azimuthal asymmetry at high p_t, as measured by the STAR collaboration.

nucl-th↗

arXiv2001

Test of the Polyakov Loop Model

We discuss the two point functions for the real and imaginary parts of the Polyakov loop in a pure SU(3) gauge theory. The behavior of these correlation functions in the Polyakov Loop Model is markedly different from that in perturbation theory.

hep-lat↗

arXiv2001

Degrees of Freedom and the Deconfining Phase Transition

There is a sharp increase in the relative number of degrees of freedom at the deconfining phase transition. Characterizing this increase using the Polyakov Loop model, we find that for a nearly second order deconfining phase transition, the medium-induced energy loss turns on rapidly above T_c, proportional to the relative number of degrees of freedom. Further, energy loss is logarithmically dependent on the screening mass, and thus is sensitive to nearly critical scattering.

hep-ph↗

arXiv2001

How Protons Shatter Colored Glass

We consider the implications of the Color Glass Condensate for the central region of p+A collisions. We compute the k_t distribution of radiated gluons and their rapidity distribution dN/dy analytically, both in the perturbative regime and in the region between the two saturation momenta. We find an analytic expression for the number of produced gluons which is valid when the saturation momentum of the proton is much less than that of the nucleus. We discuss the scaling of the produced multiplicity with A. We show that the slope of the rapidity density dN/dy provides an experimental measure for the renormalization-group evolution of the color charge density of the Color Glass Condensate (CGC). We also argue that these results are easily generalized to collisions of nuclei of different A at central rapidity, or with the same A but at a rapidity far from the central region.

hep-ph↗

arXiv2001

The Transverse Energy as a Barometer of a Saturated Plasma

The evolution of the gluon plasma produced with saturation initial conditions is calculated via Boltzmann transport theory for nuclear collisions at high energy. The saturation scale increases with the nuclear size and the beam energy, and thus we find that the perturbative rescattering rate decreases relative to the initial longitudinal expansion rate of the plasma. The effective longitudinal pressure remains significantly below the lattice QCD pressure until the plasma cools to near the confinement scale. Therefore, the transverse energy per unit of rapidity and its dependence on beam energy provides a sensitive test of gluon saturation models: the fractional transverse energy loss due to final state interactions is smaller and exhibits a weaker energy dependence than if ideal (nondissipative) hydrodynamics applied throughout the evolution.

hep-ph↗

arXiv2001

Pion Interferometry at RHIC: Probing a thermalized Quark-Gluon-Plasma?

We calculate the Gaussian radius parameters of the pion-emitting source in high energy heavy ion collisions, assuming a first order phase transition from a thermalized Quark-Gluon-Plasma (QGP) to a gas of hadrons. Such a model leads to a very long-lived dissipative hadronic rescattering phase which dominates the properties of the two-pion correlation functions. The radii are found to depend only weakly on the thermalization time tau_i, the critical temperature T_c (and thus the latent heat), and the specific entropy of the QGP. The dissipative hadronic stage enforces large variations of the pion emission times around the mean. Therefore, the model calculations suggest a rapid increase of R_out/R_side as a function of K_T if a thermalized QGP were formed.

nucl-th↗

arXiv2001

Event-by-Event Fluctuations from Decay of a Polyakov Loop Condensate

A model for particle production at the deconfining phase transition in QCD is developed, as the semi-classical decay of a condensate for the Polyakov loop. In such a model, generically particle production, as measured on an event-by-event basis, exhibits significant deviations from statistical behavior.

hep-ph↗

arXiv2001

Explosive Collisions at RHIC?

Motivated by experimental results from RHIC, we suggest how a condensate for the Polyakov loop might produce explosive behavior at the QCD phase transition. This is due to a rapid rollover of the condensate field below the transition temperature.

hep-ph↗

arXiv2000

Equilibration of the Gluon-Minijet Plasma at RHIC and LHC

We study the production and equilibration of the gluon-minijet plasma expected to be formed in the central region of ultrarelativistic heavy-ion collisions at the BNL-RHIC and the CERN-LHC by solving a self-consistent relativistic transport equation. We compute the minijet production within perturbative QCD. Subsequent collisions among the semi-hard partons are treated by considering the elastic gg -> gg processes with screening of the long wavelength modes taken into account. We determine the time $τ_{eq}$ where close to ideal hydrodynamic flow sets in, and find rather similar numbers for central heavy-ion collisions at BNL-RHIC and CERN-LHC energies. The number densities, energy densities and temperatures of the minijet plasma are found to be different at RHIC and LHC, e.g. $T(τ_{eq})\sim220$ MeV and 380 MeV, respectively.

hep-ph↗