Publications (67)18.92 Total impact
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Article: Initial state geometry and the role of hydrodynamics in proton-proton, proton-nucleus and deuteron-nucleus collisions
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ABSTRACT: We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.04/2013; -
Article: Comparison of the Color Glass Condensate to di-hadron correlations in proton-proton and proton-nucleus collisions
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ABSTRACT: We perform a detailed comparison of long range rapidity correlations in the Color Glass Condensate framework to high multiplicity di-hadron data in proton-proton and proton-lead collisions from the CMS, ALICE and ATLAS experiments at the LHC. The overall good agreement thus far of the non-trivial systematics of theory with data is strongly suggestive of gluon saturation and the presence of subtle quantum interference effects between rapidity separated gluons. In particular, the yield of pairs collimated in their relative azimuthal angle $\Delta\phi\sim 0$, is sensitive to the shape of unintegrated gluon distributions in the hadrons that are renormalization group evolved in rapidity from the beam rapidities to those of the measured hadrons. We present estimates for the collimated di-hadron yield expected in central deuteron-gold collisions at RHIC.02/2013; -
Article: Event-by-Event Anisotropic Flow in Heavy-ion Collisions from Combined Yang-Mills and Viscous Fluid Dynamics.
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ABSTRACT: Anisotropic flow coefficients v_{1}-v_{5} in heavy ion collisions are computed by combining a classical Yang-Mills description of the early time Glasma flow with the subsequent relativistic viscous hydrodynamic evolution of matter through the quark-gluon plasma and hadron gas phases. The Glasma dynamics, as realized in the impact parameter dependent Glasma (IP-Glasma) model, takes into account event-by-event geometric fluctuations in nucleon positions and intrinsic subnucleon scale color charge fluctuations; the preequilibrium flow of matter is then matched to the music algorithm describing viscous hydrodynamic flow and particle production at freeze-out. The IP-Glasma+MUSIC model describes well both transverse momentum dependent and integrated v_{n} data measured at the Large Hadron Collider and the Relativistic Heavy Ion Collider. The model also reproduces the event-by-event distributions of v_{2}, v_{3} and v_{4} measured by the ATLAS Collaboration. The implications of our results for better understanding of the dynamics of the Glasma and for the extraction of transport properties of the quark-gluon plasma are outlined.Physical Review Letters 01/2013; 110(1):012302. · 7.37 Impact Factor -
Article: Explanation of systematics of CMS p+Pb high multiplicity di-hadron data at $\sqrt{s}_{\rm NN} = 5.02$ TeV
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ABSTRACT: In a recent article (arXiv:1210.3890), we showed that high multiplicity di-hadron proton- proton (p+p) data from the CMS experiment are in excellent agreement with computations in the Color Glass Condensate (CGC) Effective Field Theory (EFT). This agreement of the theory with several hundred data points provides a non-trivial description of both nearside ("ridge") and away-side azimuthal collimations of long range rapidity correlations in p+p collisions. Our prediction in arXiv:1210.3890 for proton-lead (p+Pb) collisions is consistent with results from the recent CMS p+Pb run at $\sqrt{s}_{\rm NN} = 5.02$ TeV for the largest track multiplicity $N_{\rm track}\sim 40$ we considered. The CMS p+Pb data shows the following striking features: i) a strong dependence of the ridge yield on $N_{\rm track}$, with a significantly larger signal than in p+p for the same $N_{\rm track}$, ii) a stronger $p_T$ dependence than in p+p for large $N_{\rm track}$, and iii) a nearside collimation for large $N_{\rm track}$ comparable to the awayside for the lower $p_T = p_{T}^{\rm trig.}=p_{T}^{\rm assoc.}$ di-hadron windows. We show here that these systematic features of the CMS p+Pb di-hadron data are all described by the CGC (with parameters fixed by the p+p data) when we extend our prediction in arXiv:1210.3890 to rarer high multiplicity events. We also predict the azimuthally collimated yield for yet unpublished windows in the $p_{T}^{\rm trig.}$ and $p_{T}^{\rm assoc.}$ matrix.11/2012; -
Article: Thermalization and Bose-Einstein Condensation in Overpopulated Glasma
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ABSTRACT: We report recent progress on understanding the thermalization of the quark-gluon plasma during the early stage in a heavy ion collision. The initially high overpopulation in the far-from-equilibrium gluonic matter ("Glasma") is shown to play a crucial role. The strongly interacting nature (and thus fast evolution) naturally arises as an {\em emergent property} of this pre-equilibrium matter where the intrinsic coupling is weak but the highly occupied gluon states coherently amplify the scattering. A possible transient Bose-Einstein Condensate is argued to form dynamically on a rather general ground. We develop a kinetic approach for describing its evolution toward thermalization, and based on that we find approximate scaling solutions as well as numerically study the onset of condensation.10/2012; -
Article: Quantum chaos in the perfect fluid: spectrum of initial fluctuations in the little bang
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ABSTRACT: We outline how unstable quantum fluctuations decohere classical fields in heavy ion collisions, leading to an equation of state and hydrodynamics. Explicit numerical realization of this framework in a scalar $\phi^4$ theory demonstrates that anomalously low values of $\eta/s$ can be generated.10/2012; -
Article: Initial state fluctuations and higher harmonic flow in heavy-ion collisions
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ABSTRACT: A framework combining Yang-Mills dynamics of the pre-equilibrium glasma with relativistic viscous hydrodynamic evolution of the quark-gluon plasma and hadron gas phases is presented. Event-by-event fluctuations of nucleon positions and color charges are taken into account, leading to negative binomial fluctuations of gluon multiplicities. Experimental anisotropic flow coefficients v2-v5 of charged hadron distributions in heavy-ion collisions at the Large Hadron Collider are well described. Furthermore, event-by-event distributions of v2, v3 and v4 measured by the ATLAS collaboration are reproduced.10/2012; -
Article: Evidence for BFKL and saturation dynamics from di-hadron spectra at the LHC
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ABSTRACT: We demonstrate that rapidity separated di-hadron spectra in high multiplicity proton-proton collisions at the LHC can be quantitatively described by a combination of BFKL and saturation dynamics. Based on these results, we predict the systematics of di-hadron spectra in proton-nucleus collisions at the LHC.10/2012; -
Article: Event-by-event gluon multiplicity, energy density, and eccentricities in ultrarelativistic heavy-ion collisions
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ABSTRACT: The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments εn (up to n=6) at early times in heavy-ion collisions at both the BNL Relativistic Heavy Ion Collider (RHIC) (√s=200 GeV) and the CERN Large Hardron Collider (LHC) (√s=2.76 TeV) are computed in the IP-Glasma model. This framework combines the impact parameter dependent saturation model (IP-Sat) for nucleon parton distributions (constrained by HERA deeply inelastic scattering data) with an event-by-event classical Yang-Mills description of early-time gluon fields in heavy-ion collisions. The model produces multiplicity distributions that are convolutions of negative binomial distributions without further assumptions or parameters. In the limit of large dense systems, the n-particle gluon distribution predicted by the Glasma-flux tube model is demonstrated to be nonperturbatively robust. In the general case, the effect of additional geometrical fluctuations is quantified. The eccentricity moments are compared to the MC-KLN model; a noteworthy feature is that fluctuation dominated odd moments are consistently larger than in the MC-KLN model.Phys. Rev. C. 09/2012; 86(3). -
Article: Event-by-event gluon multiplicity, energy density and eccentricities at RHIC and LHC
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ABSTRACT: The event-by-event multiplicity distribution, the energy densities and energy density weighted eccentricity moments epsilon_n (up to n=6) at early times in heavy-ion collisions at both RHIC (root-s=200 GeV) and LHC (root-s=2.76 TeV) are computed in the IP-Glasma model. This framework combines the impact parameter dependent saturation model (IP-Sat) for nucleon parton distributions (constrained by HERA deeply inelastic scattering data) with an event-by-event classical Yang-Mills description of early-time gluon fields in heavy-ion collisions. The model produces multiplicity distributions that are convolutions of negative binomial distributions without further assumptions or parameters. The eccentricity moments are compared to the MC-KLN model; a noteworthy feature is that fluctuation dominated odd moments are consistently larger than in the MC-KLN model.06/2012; -
Article: Instability induced pressure isotropization in a longitudinally expanding system
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ABSTRACT: In two previous works [arXiv:1009.4363,arXiv:1107.0668], we studied the time evolution of a system of real scalar fields with quartic coupling which shares important features with the Color Glass Condensate description of heavy ion collisions. Our primary objective was to understand how such a system, when initialized with a non-perturbatively large classical field configuration, reaches thermal equilibrium. An essential goal of these works was to highlight the role played by the quantum fluctuations. However, these studies considered only a system confined within a box of fixed volume. In the present paper, we extend this work to a system that expands in the longitudinal direction thereby more closely mimicking a heavy ion collision. We conclude that the microscopic processes that drive the system towards equilibrium are able to keep up with the expansion of the system; the pressure tensor becomes isotropic despite the anisotropic expansion.06/2012; -
Article: Fluctuating Glasma initial conditions and flow in heavy ion collisions
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ABSTRACT: We compute initial conditions in heavy-ion collisions within the Color Glass Condensate (CGC) framework by combining the impact parameter dependent saturation model (IP-Sat) with the classical Yang-Mills description of initial Glasma fields. In addition to fluctuations of nucleon positions, this IP-Glasma description includes quantum fluctuations of color charges on the length-scale determined by the inverse nuclear saturation scale Q_s. The model naturally produces initial energy fluctuations that are described by a negative binomial distribution. The ratio of triangularity to eccentricity is close to that in a model tuned to reproduce experimental flow data. We compare transverse momentum spectra and v_(2,3,4)(p_T) of pions from different models of initial conditions using relativistic viscous hydrodynamic evolution.02/2012; -
Article: Azimuthal collimation of long range rapidity correlations by strong color fields in high multiplicity hadron-hadron collisions
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ABSTRACT: The azimuthal collimation of di-hadrons with large rapidity separations in high multiplicity p+p collisions at the LHC is described in the Color Glass Condensate (CGC) effective theory [1] by N_c^2 suppressed multi-ladder QCD diagrams that are enhanced \alpha_S^(-8) due to gluon saturation in hadron wavefunctions. We show that quantitative computations in the CGC framework are in good agreement with data from the CMS experiment on per trigger di-hadron yields and predict further systematics of these yields with varying trigger pT and charged hadron multiplicity. Radial flow generated by re-scattering is strongly limited by the structure of the p+p di-hadron correlations. In contrast, radial flow explains the systematics of identical measurements in heavy ion collisions.01/2012; -
Article: Renormalization group evolution of multi-gluon correlators in high energy QCD
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ABSTRACT: Many-body QCD in leading high energy Regge asymptotics is described by the Balitsky-JIMWLK hierarchy of renormalization group equations for the x evolution of multi-point Wilson line correlators. These correlators are universal and ubiquitous in final states in deeply inelastic scattering and hadronic collisions. For instance, recently measured di-hadron correlations at forward rapidity in deuteron-gold collisions at the Relativistic Heavy Ion Collider (RHIC) are sensitive to four and six point correlators of Wilson lines in the small x color fields of the dense nuclear target. We evaluate these correlators numerically by solving the functional Langevin equation that describes the Balitsky-JIMWLK hierarchy. We compare the results to mean-field Gaussian and large N_c approximations used in previous phenomenological studies. We comment on the implications of our results for quantitative studies of multi-gluon final states in high energy QCD.08/2011; -
Article: Bose--Einstein Condensation and Thermalization of the Quark Gluon Plasma
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ABSTRACT: In ultra-relativistic heavy ion collisions, the matter formed shortly after the collision is a dense, out of equilibrium, system of gluons characterized by a semi-hard momentum scale $Q_{\rm s}$. Simple power counting arguments indicate that this system is over-occupied: the gluon occupation number is parametrically large when compared to a system in thermal equilibrium with the same energy density. On short time scales, soft elastic scatterings tend to drive the system towards the formation of a Bose--Einstein condensate that contains a large fraction of the gluons while contributing little to the energy density. The lifetime and existence of this condensate depends on whether inelastic processes, that occur on the same time scale as the elastic ones, preferably increase or decrease the number of gluons. During this overpopulated stage, and all the way to thermalization, the system behaves as a strongly interacting fluid, even though the elementary coupling constant is small. We argue that while complete isotropization may never be reached, the system may yet evolve for a long time with a fixed anisotropy between average longitudinal and transverse momenta.07/2011; -
Article: Ekpyrosis and inflationary dynamics in heavy ion collisions: the role of quantum fluctuations
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ABSTRACT: 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.07/2011; -
Article: The initial spectrum of fluctuations in the little bang
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ABSTRACT: 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 $\tau$ 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 $A^\tau=0$ 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 $\phi^4$ 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.06/2011; -
Article: Role of quantum fluctuations in a system with strong fields: Onset of hydrodynamical flow
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ABSTRACT: Quantum fluctuations are believed to play an important role in the thermalization of classical fields in inflationary cosmology but their relevance for isotropization/thermalization of the classical fields produced in heavy ion collisions is not completely understood. We consider a scalar ϕ4 toy model coupled to a strong external source, like in the Color Glass Condensate description of the early time dynamics of ultrarelativistic heavy ion collisions. The leading order classical evolution of the scalar fields is significantly modified by the rapid growth of time-dependent quantum fluctuations, necessitating an all order resummation of such “secular” terms. We show that the resummed expressions cause the system to evolve in accordance with ideal hydrodynamics. We comment briefly on the thermalization of the quantum system and the extension of our results to a gauge theory.Nuclear Physics A. 09/2010; -
Article: Long range two-particle rapidity correlations in A+A collisions from high energy QCD evolution
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ABSTRACT: Long range rapidity correlations in A+A collisions are sensitive to strong color field dynamics at early times after the collision. These can be computed in a factorization formalism (Gelis, Lappi and Venugopalan (2009) [1]) which expresses the n-gluon inclusive spectrum at arbitrary rapidity separations in terms of the multi-parton correlations in the nuclear wavefunctions. This formalism includes all radiative and rescattering contributions, to leading accuracy in αsΔY, where ΔY is the rapidity separation between either one of the measured gluons and a projectile, or between the measured gluons themselves. In this paper, we use a mean field approximation for the evolution of the nuclear wavefunctions to obtain a compact result for inclusive two gluon correlations in terms of the unintegrated gluon distributions in the nuclear projectiles. The unintegrated gluon distributions satisfy the Balitsky–Kovchegov equation, which we solve with running coupling and with initial conditions constrained by existing data on electron–nucleus collisions. Our results are valid for arbitrary rapidity separations between measured gluons having transverse momenta p⊥,q⊥≳Qs, where Qs is the saturation scale in the nuclear wavefunctions. We compare our results to data on long range rapidity correlations observed in the near-side ridge at RHIC and make predictions for similar long range rapidity correlations at the LHC.Nuclear Physics A. 11/2009; -
Article: Glasma flux tubes and the near side ridge phenomenon at RHIC
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ABSTRACT: We investigate the consequences of long range rapidity correlations in the Glasma. Particles produced locally in the transverse plane are correlated by approximately boost invariant flux tubes of longitudinal color electric and magnetic fields that are formed when two sheets of Colored Glass Condensate pass through one another, each acquiring a modified color charge density in the collision. We argue that such long range rapidity correlations persist during the evolution of the Quark–Gluon Plasma formed later in the collision. When combined with transverse flow, these correlations reproduce many of the features of the recently observed ridge events in heavy ion collisions at RHIC.Nuclear Physics A. 05/2008;
Top co-authors
Top Journals
Institutions
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2013
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McGill University
- Department of Physics
Montréal, Quebec, Canada
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2000–2012
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Brookhaven National Laboratory
- Physics Department
New York City, NY, USA
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2005
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The University of Tokyo
Kashiwa, Chiba-ken, Japan
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1994
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University of Minnesota Duluth
Duluth, MN, USA
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