arXiv subjects
Publications and source records attributed to P. Levai.
Recent RHIC results on pion production in AuAu collision at sqrt(s)=130 and 200 AGeV display a strong suppression effect at high p_T. This suppression can be connected to final state effects, namely jet energy loss induced by the produced dense colored matter. Applying our pQCD-based parton model we perform a quantitative analysis of the measured suppression pattern and determine the opacity of the produced deconfined matter.
We study pion production in proton-proton collisions within a pQCD-improved parton model in next-to-leading order augmented by intrinsic transverse momentum ($k_\perp$) of the partons. We find the introduction of $k_\perp$ necessary to reproduce the experimental data in the CERN SPS to RHIC energy range. We study the influence of intrinsic $k_\perp$ on the so-called $K$ factor, the ratio of the NLO cross section to the Born term. A strong $p_T$ dependence is seen, especially in the 3-6 GeV transverse momentum region of the outgoing pion, where nuclear effects (e.g. the Cronin effect) play an important role.
The influence of nuclear multiscattering and shadowing on pion spectra is investigated in $pA$ collisions from CERN SPS to RHIC energies. The calculations are performed in a next-to-leading order (NLO) pQCD-improved parton model, including intrinsic partonic transverse momentum distributions. The nuclear modification of the pion spectra (Cronin effect) is considered at different targets in a wide energy range. Theoretical predictions are displayed for planned $pA$ experiments at CERN SPS and recent $dAu$ experimental results are analysed at RHIC.
Using a covariant coalescence model, we study hadron production in relativistic heavy ion collisions from both soft partons in the quark-gluon plasma and hard partons in minijets. Including transverse flow of soft partons and independent fragmentation of minijet partons, the model is able to describe available experimental data on pion, kaon, and antiproton spectra. The resulting antiproton to pion ratio is seen to increase at low transverse momenta and reaches a value of about one at intermediate transverse momenta, as observed in experimental data at RHIC. A similar dependence of the antikaon to pion ratio on transverse momentum is obtained, but it reaches a smaller value at intermediate transverse momenta. At high transverse momenta, the model predicts that both the antiproton to pion and the antikaon to pion ratio decrease and approach those given by the perturbative QCD. Both collective flow effect and coalescence of minijet partons with partons in the quark-gluon plasma affect significantly the spectra of hadrons with intermediate transverse momenta. Elliptic flows of protons, Lambdas, and Omegas have also been evaluated from partons with elliptic flows extracted from fitting measured pion and kaon elliptic flows, and they are found to be consistent with available experimental data.
Perturbative QCD results on pion production are presented in proton-proton, proton-nucleus and nucleus-nucleus collisions from CERN SPS up to RHIC energy. A K_{jet}(s, p_T, Q) factor obtained from jet production is applied to perform next-to-leading order calculations. Using the intrinsic transverse momentum (k_T) we determined transverse momentum spectra for pions in wide energy region. We have investigated nuclear multiscattering and the Cronin effect at RHIC energies.
Based on an analysis in the framework of a coalescence hadronization model (ALCOR) we conclude that in heavy ion collisions at CERN SPS and RHIC energies a new type of matter, the massive quark-antiquark matter is produced.
We summarize new pQCD results on pion production in proton-proton (pp), proton-nucleus (pA) and nucleus-nucleus (AA) collisions. Our calculation introduces intrinsic parton transverse momentum (k_T) and is performed effectively at next-to-leading order (NLO), applying a K factor extracted for jet events. Two different factorization scales, Q=p_{T,jet}/2 and p_{T,jet} are used. Experimental data in pA collisions imply a preference for the latter choice at NLO level. We display our results at CERN SPS for AA collisions.
We discuss the the GLV Reaction Operator formalism for computing the non-abelian energy loss of jets propagating through hot and dense matter. We incorporate our results on induced gluon bremsstrahlung together with the effects of nuclear shadowing and multiple scattering in a two component soft+hard description of heavy ion reactions at RHIC energies. We demonstrate that good agreement between data and theory can be achieved in the measured moderate p_T <= 5 GeV window at root(s)=130 AGeV and we extend our predictions for the high p_T part of the hadronic spectra. We focus on the perspectives of using jet tomographic methods for probing not only the density and geometry of the quark-gluon plasma but also its parton composition at various center of mass energies.
We apply the GLV Reaction Operator formalism to compute the effects of multiple elastic scatterings of jets propagating through dense matter. We derive the elastic Reaction Operator and demonstrate that the recursion relations have a closed form solution that reduces to the familiar Glauber form. We also investigate the accuracy of the Gaussian dipole approximation for jet transverse momentum broadening.
Jet tomography is the analysis of the attenuation pattern of high transverse momentum hadrons to determine certain line integral transforms of the density profile of the QCD matter produced in ultra-relativistic nuclear collisions. In this letter, we calculate the distortion of jet tomography due to multi-gluon fluctuations within the GLV radiative energy loss formalism. We find that fluctuations of the average gluon number, ~ 3 for RHIC initial conditions, reduce the attenuation of pions by approximately a factor Z ~ 0.4-0.5. Therefore the plasma density inferred from jet tomography without fluctuations must be enhanced by a factor 1/Z ~ 2.
Non-abelian energy loss in quark gluon plasma is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. We apply recent (GLV) estimates for the gluon radiative energy loss, which increases linearly with the jet energy up to E<20 GeV and depends quadratically on the nuclear radius, R. The K+/π+ ratio is found to be most sensitive to the initial density of the plasma.
We utilize the ALCOR model for mid-rapidity hadron number predictions at AGS, SPS and RHIC energies. We present simple fits for the energy dependence of stopping and quark production.
We study, in a pQCD calculation augmented by nuclear effects, the jet energy loss needed to reproduce the pi^0 spectra in Au+Au collisions at large p_T, measured by PHENIX at RHIC. The transverse width of the parton momentum distributions (intrinsic-k_T) is used phenomenologically to obtain a reliable baseline pp result. Jet quenching is applied to the nuclear spectra (including shadowing and multiscattering) to fit the data.
High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.
In ultrarelativistic heavy-ion collisions, in the 2 GeV$<p_\perp<$ 6 GeV transverse momentum region, the soft and semi-hard multiple scattering of the incoming nucleons in the nuclear medium results in the broadening of the expected hadronic (e.g. pion) $p_\perp$ spectra relative to proton-proton ($pp$) collisions. Thus, higher transverse-momentum regions are populated than in $pp$ collisions. In a perturbative QCD based calculation we include the intrinsic transverse momentum ($k_\perp$) of the partons in the nucleon (determined from $pp$ collisions), augmented by the extra broadening obtained via a systematic analysis of proton-nucleus ($pA$) collisions in the energy range 17$<\sqrt{s}<$ 39 AGeV. The original polynomial spectra are modified, and a nearly exponential spectrum appears in the region 2$\lesssim p_\perp\lesssim 3.5$ GeV. At present RHIC energies ($\sqrt{s}=$130 AGeV), the slope of the calculated spectra is reminiscent of that of fluid-dynamical descriptions, but lacks any thermal origin. We determine and discuss the size of the modifications originating in multiple scattering, which lead to this state of affairs.
The predicted quenching of jets in A+A at RHIC energies has been discovered by STAR and PHENIX in preliminary data reported at this conference. We apply the GLV theory of QCD radiative energy loss to estimate the opacity, L/λ_g, of the gluon plasma produced in Au+Au collisions at 130 AGeV. We show that (in contrast to the factor of two Cronin enhancement of π^0 found at the SPS by WA98) the factor of 5-8 suppression of p_T \sim 2-4 GeV π^0 reported by PHENIX can be accounted for with an effective static plasma opacity L/λ_g \approx 3-4.
Quark coalescence predictions for various multi-strange baryon to anti-baryon ratios for central Au+Au collisions at RHIC energies are compared to preliminary data of the STAR collaboration. The formation of Quark Matter and the sudden recombination of its constituent quarks into hadrons is found to be in agreement with these preliminary data. It seems that strange hadron and antihadron production in Au+Au collisions at RHIC is similar to that in Pb+Pb collisions at CERN SPS.
We study the production of charm mesons and other charm baryons from quark matter at CERN SPS and RHIC energies. Using quark coalescence models as hadronization mechanism, we predict particle ratios, absolute yields and transverse momentum spectra.