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Raju Venugopalan

Publications and source records attributed to Raju Venugopalan.

At least 163 records · Page 9Linked to original sources
arXiv2003

Finite-Size Effects on Nucleation in a First-Order Phase Transition

We discuss finite-size effects on homogeneous nucleation in first-order phase transitions. We study their implications for cosmological phase transitions and to the hadronization of a quark-gluon plasma generated in high-energy heavy ion collisions. Very general arguments allow us to show that the finite size of the early universe has virtually no relevance in the process of nucleation and in the growth of cosmological bubbles during the primordial quark-hadron and the electroweak phase transitions. In the case of high-energy heavy ion collisions, finite-size effects play an important role in the late-stage growth of hadronic bubbles.

hep-ph↗
arXiv2003

The Color Glass Condensate and High Energy Scattering in QCD

At very high energies or small values of Bjorken x, the density of partons, per unit transverse area, in hadronic wavefunctions becomes very large leading to a saturation of partonic distributions. When the scale corresponding to the density per unit transverse area, the saturation scale Q_s, becomes large (Q_s\gg Λ_{QCD}), the coupling constant becomes weak (α_S(Q_s)\ll 1) which suggests that the high energy limit of QCD may be studied using weak coupling techniques. This simple idea can be formalized in an effective theory, the Color Glass Condensate (CGC), which describes the behavior of the small x components of the hadronic wavefunction in QCD. The Green functions of the theory satisfy Wilsonian renormalization group equations which reduce to the standard linear QCD evolution equations in the limit of low parton densities. The effective theory has a rich structure that has been explored using analytical and numerical techniques. The CGC can be applied to study a wide range of high energy scattering experiments from Deep Inelastic Scattering at HERA and the proposed Electron Ion Collider (EIC) to proton/deuterium-nucleus and nucleus-nucleus experiments at the RHIC and LHC colliders.

hep-ph↗
arXiv2002

Elliptic Flow from Color Glass Condensate

We show that an observable fraction of the measured elliptic flow may originate in classical gluon fields at the initial stage of a peripheral high-energy nuclear collision. This mechanism complements the contribution of late stage mechanisms, such as those described by hydrodynamics, to the observed elliptic flow.

hep-ph↗
arXiv2002

Gluon production in the Color Glass Condensate model of collisions of ultrarelativistic finite nuclei

We extend previous work on high energy nuclear collisions in the Color Glass Condensate model to study collisions of finite ultrarelativistic nuclei. The changes implemented include a) imposition of color neutrality at the nucleon level and b) realistic nuclear matter distributions of finite nuclei. The saturation scale characterizing the fields of color charge is explicitly position dependent, $Λ_s=Λ_s(x_T)$. We compute gluon distributions both before and after the collisions. The gluon distribution in the nuclear wavefunction before the collision is significantly suppressed below the saturation scale when compared to the simple McLerran-Venugopalan model prediction, while the behavior at large momentum $p_T\gg Λ_s$ remains unchanged. We study the centrality dependence of produced gluons and compare it to the centrality dependence of charged hadrons exhibited by the RHIC data. We demonstrate the geometrical scaling property of the initial gluon transverse momentum distributions for different centralities. Classical Yang-Mills results for $p_T < Λ_s$ are simply matched to perturbative QCD computations for $p_T > Λ_s$-the resulting energy per particle is significantly lower than the purely classical estimates. Our results for nuclear collisions can be used as initial conditions for quantitative studies of the further evolution and possible equilibration of hot and dense gluonic matter produced in heavy ion collisions. Finally, we study $pA$ collisions within the classical framework. Our results agree well with previously derived analytical results in the appropriate kinematical regions.

hep-ph↗
arXiv2002

Elliptic flow of colored glass in high energy heavy ion collisions

We compute the elliptic flow generated by classical gluon fields in a high energy nuclear collision. The classical gluon fields are described by a typical momentum scale, the saturation scale Lambda_s, which is, for RHIC energies, of the order of 1-2 GeV. A significant elliptic flow is generated only over time scales on the order of the system size R. The flow is dominated by soft modes $p_{T} \sim Lambda_s / 4$ which linearize at very late times $tau \sim R \gg 1/Lambda_s$. We discuss the implications of our result for the theoretical interpretation of the RHIC data.

hep-ph↗
arXiv2002

Classical Computation of Elliptic Flow at Large Transverse Momentum

We compute the contribution of classical fields to the second Fourier coefficient (v_2) of the azimuthal gluon distribution at large transverse momentum in heavy ion collisions. We find that the classical contribution to the flow alone cannot account for the experimentally observed behavior of v_2(p_t) at large transverse momentum p_t.

hep-ph↗
arXiv2002

Scaling properties of the transverse mass spectra

Motivated from the formation of an initial state of gluon-saturated matter, we discuss scaling relations for the transverse mass spectra at BNL's Relativistic Heavy-Ion Collider (RHIC). We show on linear plots, that the transverse mass spectra for various hadrons can be described by an universal function. The transverse mass spectra for different centralities can be rescaled into each other. Finally, we demonstrate that scaling is also present in proton-antiproton collider data and compare it to scaling at RHIC.

nucl-th↗
arXiv2002

Generalized scaling of the transverse mass spectrum at the Relativistic Heavy-Ion Collider

We argue that the transverse mass spectra of identified hadrons as measured in gold-gold collisions at BNL's Relativistic Heavy-Ion Collider (RHIC) follows a generalized scaling law. Such a scaling behavior is motivated by the idea of a Color Glass Condensate, or more generally, saturation of the gluon density. In particular, we describe the shapes of transverse mass spectra as a function of centrality. This scaling of the transverse mass spectrum is shown to be consistent with previously observed scaling of multiplicity with centrality.

nucl-th↗
arXiv2001

Coherent gluon production in very high energy heavy ion collisions

The early stages of a relativistic heavy-ion collision are examined in the framework of an effective classical SU(3) Yang-Mills theory in the transverse plane. We compute the initial energy and number distributions, per unit rapidity, at mid-rapidity, of gluons produced in high energy heavy ion collisions. We discuss the phenomenological implications of our results in light of the recent RHIC data.

hep-ph↗
arXiv2001

Small x physics and the initial conditions in heavy ion collisions

At very high energies, the high parton densities (characterized by a semi-hard saturation scale Λ_s) ensure that parton distributions can be described by a classical effective field theory with remarkable properties analogous to those of spin glass systems. This color glass condensate (CGC) of gluons also provides the initial conditions for multi-particle production in high energy nuclear collisions. In this talk, we briefly summarize recent theoretical and phenomenological progress in the CGC approach to small x physics. In particular, we discuss recent numerical work on the real time gluodynamics of partons after a nuclear collision. The implications of this work for the theoretical study of thermalization in nuclear collisions and on the phenomenological interpretation of results of the recent RHIC experiments are also discussed.

hep-ph↗
arXiv2000

Solution of the Boltzmann equation for gluons after a heavy ion collision

A non-linear Boltzmann equation describing the time evolution of a partonic system in the central rapidity region after a heavy ion collision is solved numerically. A particular model of the collinear logarithmic divergences due to small angle scattering is employed in the numerical solution. The system is followed until it reaches kinetic equilibrium where the equilibration time, temperature and chemical potential are determined for both RHIC and LHC.

hep-ph↗
arXiv2000

From colored glass condensate to gluon plasma: equilibration in high energy heavy ion collisions

The initial distribution of gluons at the very early times after a high energy heavy ion collision is described by the bulk scale $Q_s$ of gluon saturation in the nuclear wavefunction. The subsequent evolution of the system towards kinetic equilibrium is described by a non-linear Landau equation for the single particle distributions \cite{Mueller1,Mueller2}. In this paper, we solve this equation numerically for the idealized initial conditions proposed by Mueller, and study the evolution of the system to equilibrium. We discuss the sensitivity of our results on the dynamical screening of collinear divergences. In a particular model of dynamical screening, the convergence to the hydrodynamic limit is seen to be rapid relative to hydrodynamic time scales. The equilibration time, the initial temperature, and the chemical potential are shown to have a strong functional dependence on the initial gluon saturation scale $Q_s$.

hep-ph↗
arXiv2000

Wong's equations and the small x effective action in QCD

We propose a new form for the small x effective action in QCD. This form of the effective action is motivated by Wong's equations for classical, colored particles in non-Abelian background fields. We show that the BFKL equation, which sums leading logarithms in x, is efficiently reproduced with this form of the action. We argue that this form of the action may be particularly useful in computing next-to- leading-order results in QCD at small x.

hep-ph↗
arXiv2000

The initial gluon multiplicity in heavy ion collisions

The initial gluon multiplicity per unit area per unit rapidity, dN/L^2/dη, in high energy nuclear collisions, is equal to f_N (g^2μL) (g^2μ)^2/g^2, with μ^2 proportional to the gluon density per unit area of the colliding nuclei. For an SU(2) gauge theory, we compute f_N (g^2μL)=0.14\pm 0.01 for a wide range in g^2μL. Extrapolating to SU(3), we predict dN/L^2/dηfor values of g^2μL in the range relevant to the Relativistic Heavy Ion Collider and the Large Hadron Collider. We compute the initial gluon transverse momentum distribution, dN/L^2/d^2 k_\perp, and show it to be well behaved at low k_\perp.

hep-ph↗
arXiv2000

Non-perturbative gluodynamics of high energy heavy-ion collisions

The dynamics of low-x partons in the transverse plane of a high-energy nuclear collision is classical, and therefore admits a fully non--perturbative numerical treatment. We report results of a recent study estimating the initial energy density in the central region of a collision. Preliminary estimates of the number of gluons per unit rapidity, and the initial transverse momentum distribution of gluons, are also provided.

hep-ph↗
arXiv1999

Classical methods in DIS and nuclear scattering at small x

In hadrons and nuclei at very small x, parton distributions saturate at a scale Q_s(x). Since the occupation number is large, and $Q_s(x)>>Λ_{QCD}$, classical weak coupling methods may be used to study this novel regime of non-linear classical fields in QCD. In these lectures, we apply these methods to compute structure functions in deeply inelastic scattering (DIS) and the energy density of gluons produced in high energy nuclear collisions.

hep-ph↗