J/ψJ/\psi production in Au+Au collisions at RHIC and the nuclear absorption

It is shown that a QCD based nuclear absorption model, with few parameters fixed to reproduce experimental $J/\psi$ yield in 200 GeV pp/pA and 450 GeV pA collisions can explain the preliminary PHENIX data on the centrality dependence of $J/\psi$ suppression in Cu+Cu collisions at RHIC energy, $\sqrt{s_{NN}}$=200 GeV. However, the model does not give satisfactory description to the preliminary PHENIX data on the centrality dependence of $J/\psi$ suppression in Au+Au collisions. The analysis suggest that in Au+Au collisions, $J/\psi$ are suppressed in a medium unlike the medium produced in SPS energy nuclear collisions or in RHIC energy Cu+Cu collisions.Comment: Revised version, published in Phys. Rev. C 74(2006)044907. PHENIX preliminary data on the centrality dependence of nuclear modification factor for $J/\psi$ in Cu+Cu and in Au+Au are analysed. 6 pages, 5 figure

Shadowing and Absorption Effects on J/psi Production in dA Collisions

We study medium modifications of J/psi production in cold nuclear media in deuterium-nucleus collisions. We discuss several parameterizations of the modifications of the parton densities in the nucleus, known as shadowing, an initial-state effect. We also include absorption of the produced J/psi by nucleons, a final-state effect. Both spatially homogeneous and inhomogeneous shadowing and absorption are considered. We use the number of binary nucleon-nucleon collisions as a centrality measure. Results are presented for d+Au collisions at sqrt{S_{NN}} = 200 GeV and for d+Pb collisions at sqrt{S_{NN}} = 6.2 TeV. To contrast the centrality dependence in pA and dA collisions, we also present pPb results at sqrt{S_{NN}} = 8.8 TeV.Comment: 22 pages, 11 figures, uses revte

A new approach to evaluate gamma-ray measurements

Misunderstandings about the term random samples its implications may easily arise. Conditions under which the phases, obtained from arrival times, do not form a random sample and the dangers involved are discussed. Watson's U sup 2 test for uniformity is recommended for light curves with duty cycles larger than 10%. Under certain conditions, non-parametric density estimation may be used to determine estimates of the true light curve and its parameters

Color Screening and the Suppression of the Charmonium State Yield in Nuclear Reactions

We discuss the new data for the production of the $\psi'$ meson in pA collisions at 450 GeV at CERN-SPS (of the NA50-collaboration) [1]. We extract from the CERN data $\sigma(\psi' N)\approx 8$ mb under the assumption that the $\psi'$ is produced as a result of the space-time evolution of a point-like $c\bar c$ pair which expands with time to the full size of the charmonium state. In the analysis we assume the existence of a relationship between the distribution of color in a hadron and the cross section of its interaction with a nucleon. However, our result is rather sensitive to the pattern of the expansion of the wave packet and significantly larger values of $\sigma(\psi' N)$ are not ruled out by the data. We show that recent CERN data confirm the suggestion of [2] that color fluctuations of the strengths in charmonium-nucleon interaction are the major source of suppression of the $J/\psi$ yield as observed at CERN in both pA and AA collisions.Comment: 10 pages, 5 figures (one with color

J/\Psi production, χ\chi polarization and Color Fluctuations

The hard contributions to the heavy quarkonium-nucleon cross sections are calculated based on the QCD factorization theorem and the nonrelativistic quarkonium model. We evaluate the nonperturbative part of these cross sections which dominates at $\sqrt{s_{NN}}\approx 20$ GeV at the Cern Super Proton Synchrotron (SPS) and becomes a correction at $\sqrt{s_{NN}}\approx 6$ TeV at the CERN Large Hadron Collider (LHC). \J production at the CERN SPS is well described by hard QCD, when the larger absorption cross sections of the $\chi$ states predicted by QCD are taken into account. We predict an $A$-dependent polarization of the $\chi$ states. The expansion of small wave packets is discussed.Comment: 13 pages REVTEX, 1 table, 2 PostScript, corrected some typo

The charged-hadron/pion ratio at the Relativistic Heavy Ion Collider

The hadron/pion ratio is calculated in 200 GeV AuAu collisions at midrapidity, applying pQCD and non-universal transverse-momentum broadening. Arguments are presented for such non-universality, and the idea is implemented in a model, which explains the enhancement of the hadron/pion ratio in central AuAu collisions. The model also describes the qualitative difference between the recently-measured dAu nuclear enhancement factors for pions and charged hadrons.Comment: 4 pages, 3 figure

Nuclear Broadening Effects on Hard Prompt Photons at Relativistic Energies

We calculate prompt photon production in high-energy nuclear collisions. We focus on the broadening of the intrinsic transverse momenta of the partons in the initial state from nuclear effects, and their influence on the prompt photon p_t distribution. Comparing to WA98 data from Pb+Pb collisions at SPS energy we find evidence for the presence of nuclear broadening at high p_t in this hard process. Below p_t=2.7 GeV the photon distribution is due to small momentum transfer processes. At RHIC energy, the effect of intrinsic transverse momentum on the spectrum of prompt photons is less prominent. The region p_t=3-4 GeV would be the most promising for studying the nuclear broadening effects at that energy. Below p_t=2-3 GeV the contribution from large momentum transfers flattens out, and we expect that region to be dominated by soft contributions.Comment: 19 pages, 3 figures, minor changes, a few references adde

Meson-induced correlations of nucleons in nuclear Compton scattering

The non-resonant (seagull) contribution to the nuclear Compton amplitude at low energies is strongly influenced by nucleon correlations arising from meson exchange. We study this problem in a modified Fermi gas model, where nuclear correlation functions are obtained with the help of perturbation theory. The dependence of the mesonic seagull amplitude on the nuclear radius is investigated and the influence of a realistic nuclear density on this amplitude is dicussed. We found that different form factors appear for the static part (proportional to the enhancement constant $\kappa$) of the mesonic seagull amplitude and for the parts, which contain the contribution from electromagnetic polarizabilities.Comment: 15 pages, Latex, epsf.sty, 9 eps figures

Search for a new gauge boson in the A′A' Experiment (APEX)

We present a search at Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling $\alpha'$ to electrons. Such a particle $A'$ can be produced in electron-nucleus fixed-target scattering and then decay to an $e^+e^-$ pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV, found no evidence for an $A'\to e^+e^-$ reaction, and set an upper limit of $\alpha'/\alpha \simeq 10^{-6}$. Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.Comment: 5 pages, 5 figures, references adde