arXiv subjects
Publications and source records attributed to R. L. Thews.
The contrast between model predictions for the transverse momentum spectra of J/Psi observed in Au-Au collisions at RHIC is extended to include effects of nuclear absorption. We find that the difference between initial production and recombination is enhanced in the most central collisions. Models utilizing a combination of these sources may eventually be able to place constraints on their relative magnitudes.
We confront preliminary RHIC data on J/Psi production in nuclear interactions with expectations which follow in scenarios involving charm quark recombination in a region of color deconfinement. The focus is on transverse momentum and rapidity spectra of the J/Psi, which carry a memory of the spectra of the charm quarks. In such a scenario, one predicts that both spectra will be narrower than those expected without recombination. Preliminary results for the transverse momentum spectra point toward a preference for the recombination picture, while the rapidity spectra do not exhibit any narrowing within present large uncertainties. We present new calculations in the recombination model for the centrality behavior of these signals, which map out the necessary experimental precision required for a definitive test.
We examine the possibility to utilize in-medium charmonium formation in heavy ion interactions at collider energy as a probe of the properties of the medium. This is possible because the formation process involves recombination of charm quarks which imprints a signal on the resulting normalized transverse momentum distribution containing information about the momentum distribution of the quarks. We have contrasted the transverse momentum spectra of J/Psi, characterized by , which result from the formation process in which the charm quark distributions are taken at opposite limits with regard to thermalization in the medium. The first uses charm quark distributions unchanged from their initial production in a pQCD process, appropriate if their interaction with the medium is negligible. The second uses charm quark distributions which are in complete thermal equilibrium with the transversely expanding medium, appropriate if a very strong interaction between charm quarks and medium exists. We find that the resulting of the formed J/Psi should allow one to differentiate between these extremes, and that this differentiation is not sensitive to variations in the detailed dynamics of in-medium formation. We include a comparison of predictions of this model with preliminary PHENIX measurements, which indicates compatibility with a substantial fraction of in-medium formation.
We calculate rapidity and transverse momentum distributions of charmonium formed in high energy heavy ion collsions from incoherent recombination of charm quarks. The results are very sensitive to the corresponding distributions of the charm quarks, and thus can serve as a probe of the state of matter produced in the heavy ion collision. At one extreme we generate a set of charm pair momenta directly from pQCD amplitudes, which are appropriate if one can neglect interaction of the quarks with the medium. At the other extreme we generate momenta of charm quarks in thermal equilibrium with the expanding medium, appropriate for an extremely strong interaction. Explicit predictions are made for J/Psi formation in Au-Au interactions at RHIC. We find that for the case in which charm quark momenta are unchanged from the pQCD production calculation, both the rapidity and transverse momentum spectra of the formed J/Psi are substantially narrower than would be anticipated in scenarios which do not include the in-medium formation. In particular, the average transverse momentum of the J/Psi will exhibit a non-monotonic behavior in the progression from p-p to p-A to A-A interactions.
Charmonium formation via charm quark in-medium recombination in heavy ion interactions at collider energies has the potential to probe some properties of the medium by utilizing the sensitivity of the recombination process to the momentum distribution of the quarks. We have examined the transverse momentum spectra of J/Psi, characterized by , which result from the formation process in which the charm quark distributions are unchanged from their initial production in a pQCD process. This is contrasted with the case in which the charm quarks have completely come into thermal equilibrium with an expanding medium whose properties are determined by the spectra of produced light hadrons. We find that the resulting of the formed J/Psi provide a distinct signature of the underlying charm quark spectra, and that signature is essentially independent of the detailed dynamics of the in-medium formation reaction. In addition, both of these signatures are sufficiently separated from the case in which no in-medium formation takes place. Finally, utilizing a model for the fraction of J/Psi which originate from in-medium formation, we predict the centrality behavior of these signatures.
A brief survey of the role of heavy flavors as a probe of the state of matter produced by high energy heavy ion collisions is presented. Specific examples include energy loss, initial state gluon saturation, thermalization and flow. The formation of quarkonium bound states from interactions in which multiple heavy quark-antiquark pairs are initially produced is examined in general. Results from statistical hadronization and kinetic models are summarized. New predictions from the kinetic model for J/Psi at RHIC are presented.
I review the present status of two related models addressing scenarios in which the formation of heavy quarkonium states in high energy heavy ion collisions proceed via "off-diagonal" combinations of a quark and an antiquark. The physical process involved belongs to a general class of quark "recombination", although technically the recombining quarks here were never previously bound in a quarkonium state. Features of these processes relevant as a signature of color deconfinement are discussed.
We present the results from the heavy quarks and quarkonia working group. This report gives benchmark heavy quark and quarkonium cross sections for $pp$ and $pA$ collisions at the LHC against which the $AA$ rates can be compared in the study of the quark-gluon plasma. We also provide an assessment of the theoretical uncertainties in these benchmarks. We then discuss some of the cold matter effects on quarkonia production, including nuclear absorption, scattering by produced hadrons, and energy loss in the medium. Hot matter effects that could reduce the observed quarkonium rates such as color screening and thermal activation are then discussed. Possible quarkonium enhancement through coalescence of uncorrelated heavy quarks and antiquarks is also described. Finally, we discuss the capabilities of the LHC detectors to measure heavy quarks and quarkonia as well as the Monte Carlo generators used in the data analysis.
Predictions of J/Psi formation at RHIC via "off-diagonal" combinations of charm and anticharm quarks in a region of color deconfinement are confronted with initial data from the PHENIX collaboration. We find that the measured centrality behavior places significant constraints on the various parameters which control model calculations of J/Psi formation. Within present statistical and systematic uncertainties, one can map out a region of parameter space within which the contribution of formation in a deconfined phase is allowed. As these uncertainties decrease and new data from d-Au interactions becomes available, it is expected that definitive tests for the presence of this formation mechanism will be possible. We anticipate that the rapidity and transverse momentum spectra will prove decisive for a final determination.
The goal of this section is to assess the possibility that quarkonium production rates may be enhanced in nucleus-nucleus interactions at the LHC relative to that predicted by extrapolation of processes thought to be dominant at lower energy. This enhancement could follow from the effects of incoherent recombination mechanisms involving uncorrelated pairs of heavy quarks and antiquarks which result from multiple pair production. Two different approaches have been considered: statistical hadronization and kinetic formation. Updated predictions relevant to Pb+Pb collisions at the LHC are given.
We anticipate new features of quarkonium production in heavy ion collisions at RHIC and LHC energies which differ from a straightforward extrapolation of results at CERN SPS energy. General arguments indicate that one may expect quarkonium formation rates to increase more rapidly with energy and centrality than the production rate of the heavy quarks which they contain. This is due to new formation mechanisms in which independently-produced quarks and antiquarks form a bound quarkonium state. This mechanism will depend quadratically on the total number of initially-produced heavy quark pairs, and becomes numerically significant only at RHIC and LHC energy. When viewed as a signal of color deconfinement, a transition from suppression to enhancement may be observed. Explicit model calculations are presented, in which one can follow striking variations of final quarkonium production within a range of parameter space.
The production of quarkonium in heavy ion collisions is studied at RHIC and LHC energies. General arguments indicate that, due to initial production of multiple quark pairs in each central collision, the final population of quarkonium may exhibit significant enhancements over straightforward extrapolation of behavior at SPS energy. Explicit calculations based on both a statistical hadronization picture and a kinetic formation mechanism in a deconfined state verify these general expectations. Such enhancements will alter the nature of how quarkonium yields may be used as a signature of deconfinement.
We compute cross sections for the Drell-Yan process in nuclear collisions at next-to-leading order (NLO) in α_s. The effects of shadowing on the normalization and on the mass and rapidity dependence of these cross sections are presented. An estimate of higher order corrections is obtained from next-to-next-to-leading order (NNLO) calculation of the rapidity-integrated mass distribution. Variations in these predictions resulting from choices of parton distribution sets are discussed. Numerical results for mass distributions at NLO are presented for RHIC and LHC energies, using appropriate rapidity intervals. The shadowing factors in the dilepton mass range 2 < M < 10 GeV are predicted to be substantial, typically 0.5 - 0.7 at LHC, 0.7 - 0.9 at RHIC, and approximately independent of the choice of parton distribution sets and the order of calculation.
Nuclear modifications to the Drell-Yan dilepton production cross sections in p+A and A+A collisions in the leading twist approximation are caused by nuclear effects in the parton distributions of bound nucleons. For non-isoscalar nuclei, isospin corrections must also be considered. We calculate these effects for p+A and Pb+Pb collisions at CERN SPS energies. Our goal is to place constraints on nuclear effects in sea quark distributions in the region x > 0.2. We show that the net nuclear corrections remain small for p+A collisions at E_lab=450 GeV. However, in Pb+Pb collisions at E_lab=158 AGeV, effects of > 20% are predicted at large M. The data collected by the NA50 collaboration could thus be used to constrain the nuclear effects in the sea quark distributions in the region of the EMC effect, x > 0.3.
We use dispersion relations to reconstruct, in a model-independent way, the formation dynamics of heavy quarkonium from the experimental data on e+ e- annihilation to Q-bar Q. We extract a distribution of formation times with a mean value for the J/psi, tau{J/psi} = 0.44 fm; and for the Upsilon, tau{Upsilon} = 0.32 fm. The corresponding widths of these distributions are given by Delta-tau{J/psi} = 0.31 fm and Delta-tau{Upsilon} = 0.28 fm. This information can be used as an input in modeling of heavy quarkonium production on nuclear targets.
We investigate the fate of bottom quarks produced in heavy-ion collisions at RHIC. Examining both the direct capture of a charmed quark, and multi-step processes, where the B_c meson is formed in a sequence of quark capture and exchange reactions, we find: a) that a sufficiently high number of B_c's will be produced to generate a detectable tri-lepton signal, and b) that the production rate of B_c's is highly sensitive to the properties of the deconfined source. A flavor-independent potential model, which includes color screening effects, is used to study the propagation of a B_c in a quark-gluon fireball and to compare this behavior with that of J/Psi mesons.
The possible chiral phase transition in high energy heavy-ion collisions may lead to the formation of a disoriented chiral condensate (DCC). However, the existence of many uncorrelated small domains in the rather large interaction volume and the huge combinatorial background make the experimental search for the DCC signal a rather difficult task. We present a novel method for studying the formation of a DCC in high energy heavy-ion collisions utilizing a discrete wavelet transformation. We find the wavelet power spectrum for a DCC to exhibit a strong dependence on the scale while an equilibrium system and the standard dynamical models such as HIJING have a flat spectrum.
The most abundantly produced hadron species in $Si\!-\!Au$ collisions at the BNL-AGS (nucleons, pions, kaons, antikaons and hyperons) are shown to be in accord with emission from a thermal resonance gas source of temperature $T\simeq 110$ MeV and baryochemical potential $μ_B \simeq 540$ MeV, corresponding to about 1/3 standard nuclear density. Our analysis takes the isopin asymmetry of the initial state fully into account.