From leading hadron suppression to jet quenching at RHIC and at the LHC

In nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC), one generically observes a strong medium-induced suppression of high- p_T hadron production. This suppression is accounted for in models which assume a significant medium-induced radiative energy loss of high- p_T parent partons produced in the collision. How can we further test the microscopic dynamics conjectured to underlie this abundant high- p_T phenomenon? What can we learn about the dynamics of parton fragmentation, and what can we learn about the properties of the medium which modifies it? Given that inelastic parton scattering is expected to be the dominant source of partonic equilibration processes, can we use hard processes as an experimentally well-controlled window into QCD non-equilibrium dynamics? Here I review what has been achieved so far, and which novel opportunities open up with higher luminosity at RHIC, and with the wider kinematical range accessible soon at the LHC.Received: 15 February 2005, Published online: 3 June 2005PACS: 12.38.Mh, 24.85. + p     

Low- x QCD physics from RHIC and HERA to the LHC

We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of the parton momentum fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced.     

High-p t and jet physics from RHIC to LHC

The observation of the strong suppression of high-p t hadrons in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at BNL has motivated a large experimental program using hard probes to characterize the deconfined medium created. However, what can be denoted as “leading particle” physics accessible at RHIC presents some limitations which motivate at higher energy the study of much more penetrating objects: jets. The gain in center-of-mass energy expected at the Large Hadron Collider (LHC) at CERN will definitively improve our understanding on how the energy is lost in the system, opening a major new window of study: the physics of jets on an event-by-event basis. We will concentrate on the expected performance for jet reconstruction in ALICE using the EMCal calorimeter. (for the ALICE Collaboration) The text was submitted by the author in English.     

Jet quenching and gluon to hadron fragmentation function in non-equilibrium QCD at RHIC and LHC

Theoretical understanding of the observed jet quenching measurements at RHIC and LHC is challenging in QCD because it requires understanding of parton to hadron fragmentation function in non-equilibrium QCD. In this paper, by using closed-time path integral formalism, we derive the gauge invariant definition of the gluon to hadron fragmentation function in non-equilibrium QCD which is consistent with factorization theorem in non-equilibrium QCD from first principles.     

Azimuthal anisotropy of jet quenching at LHC

We analyze the azimuthal anisotropy of jet spectra due to energy loss of hard partons in quark-gluon plasma, created initially in nuclear overlap zone in collisions with non-zero impact parameter. The calculations are performed for semi-central Pb-Pb collisions at LHC energy.     

VISHNU hybrid model for the viscous QCD matter at RHIC and LHC energies

In this proceeding, we briefly describe the viscous hydrodynamics + hadron cascade hybrid model VISHNU for relativistic heavy ion collisions and report the current status on extracting the QGP viscosity from elliptic flow data.     

Global constraints from RHIC and LHC on transport properties of QCD fluids in CUJET/CIBJET framework

We report results of a comprehensive global х~2 analysis of nuclear collision data from RHIC(0.2 ATeV),LHC1(2.76 ATeV), and recent LHC2(5.02 ATeV) energies using the updated CUJET framework. The framework consistently combines viscous hydrodynamic fields predicted by VISHNU2+1(validated with soft p_T﹤2 GeV bulk observables) and the DGLV theory of jet elastic and inelastic energy loss generalized to QGP fluids with an sQGMP color structure, including effective semi-QGP color electric quark and gluon as well as emergent color magnetic monopole degrees of freedom constrained by lattice QCD data. We vary the two control parameters of the model(the maximum value of the running QCD coupling, c, and the ratio cm of color magnetic to electric screening scales) and calculate the global х~2(α_c,c_m) compared with available jet fragment observables( R_(AA),v_2). A global х~2 minimum is found with α_c≈0.9±0.1 and c_m≈0.25±0.03. Using CIBJET, the event-by-event(ebe) generalization of the CUJET framework, we show that ebe fluctuations in the initial conditions do not significantly alter our conclusions(except for v_3). An important theoretical advantage of the CUJET and CIBJET frameworks is not only its global х~2 consistency with jet fragment observables at RHIC and LHC and with non-perturbative lattice QCD data, but also its internal consistency of the constrained jet transport coefficient, ■(E,T)/T~3, with the near-perfect fluid viscosity to entropy ratio(η/s～T~3/■～0.1-0.2) property of QCD fluids near Tc needed to account for the low p_T﹤2 GeV flow observables. Predictions for future tests at LHC with 5.44 ATeV Xe + Xe and 5.02 ATeV Pb + Pb are also presented.     

An alternative model of jet suppression at RHIC energies

We propose a simple Glauber-type mechanism for the suppression of jet production up to transverse momenta of about 10 at RHIC. For processes in this kinematic region, the formation time is smaller than the interval between two successive hard partonic collisions and the subsequent collision influences the jet production. The number of jets then roughly scales with the number of participants. Proportionality to the number of binary collisions is reco vered for very high transverse momenta. The model predicts suppression of jet production in collisions at RHIC. Received: 20 January 2003 / Published online: 7 March 2003     

Bound-state approach to the QCD coupling constant at low-energy scales

We exploit theoretical results on the meson spectrum within the framework of a Bethe-Salpeter (BS) formalism adjusted for QCD, in order to extract an experimental coupling alpha(s)exp (Q2) below 1 GeV by comparison with the data. Our results for alpha(s)exp (Q2) exhibit a good agreement with the infrared safe analytic perturbation theory (APT) coupling from 1 GeV down to 200 MeV. As a main result, we claim that the combined BS-APT theoretical scheme provides us with a rather satisfactory correlated understanding of very high- and low-energy phenomena.     

A systematic study of QCD coupling constant from deep-inelastic measurements

Physics of Atomic Nuclei - We reanalyze deep-inelastic scattering data of the BCDMS Collaboration by including proper cuts of ranges with large systematic errors. We also perform the fits of...     

Dynamical magnetic enhancement of light and heavy quark jet quenching at RHIC

A new Monte Carlo implementation of Djordjevic?s dynamical scattering generalization of the DGLV radiative energy loss opacity series is used with a hybrid interpolation scheme to compute both light and heavy quark jet quenching up to third order in opacity. The enhancement of the ratio of bottom to charm quark energy loss due to perturbative long range color magnetic effects in nonuniform Bjorken expanding geometries is found to reduce the significance of the heavy quark jet puzzle posed by the observed near equality (within sizeable errors) of pion and nonphotonic electron nuclear modification at RHIC. Jet Flavor Spectroscopy discussed below will be a powerful tool to differentiate competing dynamical models of the QGP produced in ultra-relativistic nuclear collisions.     

Novel jet quenching observables in heavy ion collisions at the LHC

We discuss such novel jet quenching observables at the LHC as direct measurements of medium-modified jet fragmentation function, nuclear modification factor and azimuthal anisotropy for jets with finite cone size and p _T -imbalance for dimuon tagged jets. The corresponding predictions obtained with PYQUEN partonic energy loss model are presented.     

Fluctuations as probe of the QCD phase transition and freeze-out in heavy ion collisions at LHC and RHIC

We discuss the relevance of higher order cumulants of net baryon number fluctuations for the analysis of freeze-out and critical conditions in heavy ion collisions at LHC and RHIC. Using properties of O(4) scaling functions, we discuss the generic structure of these higher cumulants at vanishing baryon chemical potential and apply chiral model calculations to explore their properties at non-zero baryon chemical potential. We show that the ratios of the sixth to second and eighth to second order cumulants of the net baryon number fluctuations change rapidly in the transition region of the QCD phase diagram. Already at vanishing baryon chemical potential they deviate considerably from the predictions of the hadron resonance gas model which reproduce the second and fourth order cumulants of the net proton number fluctuations at RHIC. We point out that the sixth order cumulants of baryon number and electric charge fluctuations remain negative at the chiral transition temperature. Thus, they offer the possibility to probe the proximity of the chemical freeze-out to the crossover line.     

Two-particle correlations from RHIC to LHC,a Monte Carlo approach

The aim of this work is to extend to LHC the results observed for two-particle correlations at RHIC, especially in terms of jet quenching effects. In this study a parton quenching model developed in the BDMPS-Z-SW framework is considered and implemented as an afterburner for PYTHIA and HIJING. A simplified parametrization of the quenching mechanism at the parton level is included in one of the most popular Monte Carlo event generators for AA collisions, HIJING. The simulation method, tuned on the RHIC data, is then used to make predictions for the LHC energy regime in order to probe the scenario we will study in the ALICE experiment.     

Effective method for calculating the analytic QCD coupling constant

The analytic running coupling constant α _an for strong interactions is considered for approximations of standard perturbation theory up to the three-loop level. Nonperturbative contributions are singled out explicitly in α _an. They are represented in the form of an expansion in a series in inverse powers of the Euclidean momentum squared. It is shown that two-and three-loop corrections lead to a partial compensation of the nonperturbative one-loop contribution of order 1/q ², which is leading in the ultraviolet region. An efficient method for calculating the analytic running coupling constant for all q>Λ is developed on the basis of the above expansion. A comparative analysis of perturbative and nonperturbative contributions is performed in the infrared region, where the latter play the most important role. A simultaneous consideration of the momentum dependence of α _an and its perturbative component for one-to three-loop cases leads to the conclusion that the analytic running coupling constant is stable with respect to higher corrections and that it depends only slightly on conditions imposed in matching solutions that involve different numbers n _f of active-quark flavors.