Showing papers by "Hermine Katharina Wöhri published in 2009"

Energy dependence of J/ψ absorption in proton-nucleus collisions

Abstract: Charmonium states are expected to be considerably suppressed in the case of quark-gluon plasma formation in high-energy heavy-ion collisions. However, a robust identification of suppression patterns as signatures of a deconfined QCD medium requires a detailed understanding of the ``normal nuclear absorption'' already present in proton-nucleus collisions, where the charmonium production cross sections increase less than linearly with the number of target nucleons. We analyse the J/ψ production cross sections measured in proton-nucleus collisions in fixed target experiments, with proton beam energies from 200 to 920 GeV, and in d-Au collisions at RHIC, at (sNN)1/2 = 200 GeV, in the framework of the Glauber formalism, using several sets of parton distributions with and without nuclear modifications. The results reveal a significant dependence of the ``absorption cross section'' on the kinematics of the J/ψ and on the collision energy. Extrapolating the observed patterns we derive the level of absorption expected at Elab = 158 GeV, the energy at which the heavy-ion data sets were collected at the CERN SPS.

J / psi polarization from fixed-target to collider energies.

Abstract: The determination of the magnitude and 'sign' of the J/psi polarization crucially depends on the reference frame used in the analysis of the data and a full understanding of the polarization phenomenon requires measurements reported in two 'orthogonal' frames, such as the Collins-Soper and helicity frames. Moreover, the azimuthal anisotropy can be, in certain frames, as significant as the polar one. The seemingly contradictory J/psi polarization results reported by E866, HERA-B and CDF can be consistently described assuming that the most suitable axis for the measurement is along the direction of the relative motion of the colliding partons, and that directly produced J/psi's are longitudinally polarized at low momentum and transversely polarized at high momentum. We make specific predictions that can be tested on existing CDF data and by LHC measurements, which should show a full transverse polarization for direct J/psi mesons of pT > 25 GeV/c.

Highlights from NA60

Abstract: NA60 measured dimuon production in p-A and In-In collisions at the CERN SPS. This paper presents a high statistics measurement of $\phi$ meson production in In-In collisions at 158 AGeV. Both the transverse momentum, rapidity, decay angular distributions and the absolute yield were measured as a function of centrality. The results are compared to previous measurements in order to shed light on the long standing $\phi$ puzzle. In addition, highlights on $\eta$ meson production and on the dimuon excess below the J/$\psi$ mass are presented.

Quarkonium Production and Absorption in Proton-Nucleus collisions

Abstract: Lattice QCD calculations [1] predict that, at sufficiently large-energy densities, hadronic matter undergoes a phase transition to a “plasma” of deconfined quarks and gluons (QGP). Considerable efforts have been invested since 1986 in the study of high-energy heavy-ion collisions to reveal the existence of this phase transition and to study the properties of strongly interacting matter in the new phase, in view of improving our understanding of confinement, a crucial feature of QCD. The study of quarkonium production and suppression is among the most interesting investigations in this field, because the calculations indicate that the QCD binding potential is screened in the QGP phase, the screening level increasing with the energy density of the system. Given the existence of several quarkonium states, of different binding energies, it is expected that they will be consecutively “dissolved” (into open charm or beauty mesons) above certain energy density thresholds [2,3].

Measuring Dimuons Produced in Proton–Nucleus Collisions with the NA60 Experiment at the SPS

Abstract: Considerable efforts are currently being invested in the study of high-energy heavy-ion collisions. The main goals of this experimental programme are the discovery of the phase transition from confined hadronic matter to deconfined partonic matter, predicted by lattice QCD calculations to occur when the system reaches sufficiently high energy densities or temperatures [1] and the study of the physical properties of the new phase. Both goals are fundamental for the understanding of confinement, a crucial feature of QCD.