Angular pattern of minijet transverse energy flow in hadron and nuclear collisions

The azimuthal asymmetry of minijet system produced at the early stage of nucleon-nucleon and nuclear collisions in a central rapidity window is studied. We show that in pp collisions the minijet transverse energy production in a central rapidity window is essentially unbalanced in azimuth due to asymmetric contributions in which only one minijet hits the acceptance window. We further study the angular pattern of transverse energy flow generated by semihard degrees of freedom at the early stage of high energy nuclear collisions and its dependence on the number of semihard collisions in the models both including and neglecting soft contributions to the inelastic cross section at RHIC and LHC energies as well as on the choice of the infrared cutoff.Comment: 25 LaTeX pages, 11 figures embedded with epsf; expanded versio

ALICE potential for heavy-flavour physics

The Large Hadron Collider (LHC), where lead nuclei will collide at the unprecedented c.m.s. energy of 5.5 TeV per nucleon-nucleon pair, will offer new and unique opportunities for the study of the properties of strongly interacting matter at high energy density over extended volumes. We will briefly explain why heavy-flavour particles are well-suited tools for such a study and we will describe how the ALICE experiment is preparing to make use of these tools.Comment: 6 pages, 3 figures, prepared for the Proceedings of "Strange Quark Matter 2007", Levoca, Slovaki

CGC and initial state effects in Heavy Ion Collisions

A brief review of the phenomenological studies in the field of heavy ion collisions based on the Color Glass Condensate theory and, in particular, of those relying in the use of the BK equation including running coupling effects is presented.Comment: 6 pages, 5 figures. Contribution to the proceedings of the Hot Quarks 2010 Conference. June 21-26, La Londe Les Maures, Franc

Jets in 200 GeV p+p and d+Au collisions from the STAR experiment at RHIC

Full jet reconstruction in heavy-ion collisions is a promising tool for the quantitative study of properties of the dense medium produced at RHIC. Measurements of d+Au collisions are important to disentangle initial state nuclear effects from medium-induced kT broadening and jet quenching. Study of jet production and properties in d+Au in combination with similar studies in p+p is an important baseline measurement needed to better understand heavy-ion results. We present mid-rapidity inclusive jet pT spectra and di-jet correlations (kT) in 200 GeV p+p and d+Au collisions from the 2007-2008 RHIC run. We discuss the methods used to correct the data for detector effects and for background in d+Au collisions.Comment: 4 pages, 5 figures. To appear in Hot Quarks 2010 conference proceeding

From EMC- and Cronin-effects to signals of quark-gluon plasma

The EMC- and Cronin-effects are explained by a unitarized evolution equation, where the shadowing and antishadowing corrections are dynamically produced by gluon fusions. For this sake, an alternative form of the GLR-MQ-ZRS equation is derived. The resulting integrated and unintegrated gluon distributions in proton and nuclei are used to analyze the contributions of the initial parton distributions to the nuclear suppression factor in heavy ion collisions. A simulation of the fractional energy loss is extracted from the RHIC and LHC data, where the contributions of the nuclear shadowing and antishadowing effects are considered. We find a rapid crossover from week energy loss to strong energy loss at a universal critical energy of gluon jet $E_c\sim 10 GeV$.Comment: 35 pages, 13 figures, to be published in Int. J. Mod. Phys.

Calculating Dilepton Rates from Monte Carlo Simulations of Parton Production

To calculate dilepton rates in a Monte Carlo simulation of ultrarelativistic heavy ion collisions, one usually scales the number of similar QCD processes by a ratio of the corresponding differential probabilities. We derive the formula for such a ratio especially for dilepton bremsstrahlung processes. We also discuss the non-triviality of including higher order corrections to direct Drell-Yan process. The resultant mass spectra from our Monte Carlo simulation are consistent with the semi-analytical calculation using dilepton fragmentation functions.Comment: 14 pages in RevTex, 3 figures in uuencoded files, LBL-3466

Elliptic flow in nuclear collisions at the Large Hadron Collider

We use perfect-fluid hydrodynamical model to predict the elliptic flow coefficients in Pb + Pb collisions at the Large Hadron Collider (LHC). The initial state for the hydrodynamical calculation for central $A + A$ collisions is obtained from the perturbative QCD + saturation (EKRT) model. The centrality dependence of the initial state is modeled by the optical Glauber model. We show that the baseline results obtained from the framework are in good agreement with the data from the Relativistic Heavy Ion Collider (RHIC), and show predictions for the $p_T$ spectra and elliptic flow of pions in Pb + Pb collisions at the LHC. Also mass and multiplicity effects are discussed.Comment: 11 pages, 10 figure

Dynamical freeze-out condition in ultrarelativistic heavy ion collisions

We determine the decoupling surfaces for the hydrodynamic description of heavy ion collisions at RHIC and LHC by comparing the local hydrodynamic expansion rate with the microscopic pion-pion scattering rate. The pion $p_T$ spectra for nuclear collisions at RHIC and LHC are computed by applying the Cooper-Frye procedure on the dynamical-decoupling surfaces, and compared with those obtained from the constant-temperature freeze-out surfaces. Comparison with RHIC data shows that the system indeed decouples when the expansion rate becomes comparable with the pion scattering rate. The dynamical decoupling based on the rates comparison also suggests that the effective decoupling temperature in central heavy ion collisions remains practically unchanged from RHIC to LHC.Comment: 7 pages, 9 figure

On the theoretical and experimental uncertainties in the extraction of the J/psi absorption cross section in cold nuclear matter

We investigate the cold nuclear matter effects on $J/\psi$ production, whose understanding is fundamental to study the quark-gluon plasma. Two of these effects are of particular relevance: the shadowing of the parton distributions and the nuclear absorption of the $c\bar{c}$ pair. If $J/\psi$'s are not produced {\it via} a $2 \to 1$ process as suggested by recent theoretical works, one has to modify accordingly the way to compute the nuclear shadowing. This naturally induces differences in the absorption cross-section fit to the data. A careful analysis of these differences however requires taking into account the experimental uncertainties and their correlations, as done in this work for $d$Au collisions at \sqrtsNN=200\mathrm{GeV}, using several shadowing parametrisations.Comment: 6 pages, 1 table, 3 figures, Submitted to J. Phys. G, talk given at the International Conference on Strangeness in Quark Matter (SQM2009), Buzios, Brasil, Sep. 27 - Oct. 2, 200

Constraining the Physics of Jet Quenching

Hard probes in the context of ultrarelativistic heavy ion collisions represent a key class of observables studied to gain informations about the QCD medium created in such collisions. However, in practice the so-called jet tomography has turned out to be more difficult than expected initially. One of the major obstacles in extracting reliable tomographic information from the data is that neither the parton-medium interaction nor the medium geometry are known with great precision, and thus a difference in model assumptions in the hard perturbative Quantum Choromdynamics (pQCD) modelling can usually be compensated by a corresponding change of assumptions in the soft bulk medium sector and vice versa. The only way to overcome this problem is to study the full systematics of combinations of parton-medium interaction and bulk medium evolution models. This work presents a meta-analysis summarizing results from a number of such systematical studies and discusses in detail how certain data sets provide specific constraints for models. Combining all available information, only a small group of models exhibiting certain characteristic features consistent with a pQCD picture of parton-medium interaction is found to be viable given the data. In this picture, the dominant mechanism is medium-induced radiation combined with a surprisingly small component of elastic energy transfer into the medium.Comment: 22 pages, 3 figures, 2 table