Location: Cyclotron Building (434), Seminar Room
Dr. Daniel Cabrera, Texas A&M University
T-Matrix Approach to Quarkonium Correlation Functions in the QGP
Abstract:
We study the evolution of heavy quarkonium states with temperature in a Quark Gluon Plasma (QGP) by evaluating the in-medium Q anti-Q T-matrix within a reduced Bethe-Salpeter equation in both S- and P-wave channels. The underlying interaction kernel is extracted from recent finite-temperature QCD lattice calculations of the singlet free energy of a Q anti-Q pair. The bound states are found to gradually move above the Q anti-Q threshold after which they rapidly dissolve in the hot system. The T-matrix approach is particularly suited to investigate these mechanisms as it provides a unified treatment of bound and scattering states including threshold effects and the transition to the (perturbative) continuum. The T-matrix is then applied to calculate Q anti-Q spectral functions as well as pertinent Euclidean-time correlation functions which are then compared to results from lattice QCD. The sensitivity to the interplay of bound and scattering states is found to be large. We furthermore investigate the impact of finite-width effects on the single-quark propagators in the QGP as estimated from recent applications of heavy-quark rescattering to RHIC data.
Dr. Hendrik van Hees, Texas A&M University
Heavy-quark energy loss in the QGP and non-photonic single-electron observables at RHIC
Abstract:
I will give a general overview about our current theoretical understanding of the thermalization and flow of c- and b-quarks in a Quark-Gluon Plasma (QGP), as believed to be produced in ultra-relativistic heavy-ion collisions. First I will summarize the assessment of heavy-quark-energy loss in the medium through perturbative QCD. Recently, due to the experimental findings about the transverse-momentum (pT) spectra and elliptic flow (v2) of non-photonic single electrons (e±) at the BNL Relativistic Heavy Ion Collider (RHIC), the importance of elastic quark rescattering in addition to the gluon-radiative processes for parton-energy loss has become evident. However, to explain the e± data the corresponding effects have to be enhanced by either tuning up the transport coefficient for quark-energy loss, qˆ, or the gluon density of the medium.
Thus also non-perturbative effects have to be considered. We evaluate resonant elastic c- and b-quark rescattering as a non-perturbative mechanism for the thermalization of heavy quarks with the QGP. We describe the interactions of heavy quarks with light quarks within a field theory with light and heavy quarks as well as heavy-light meson resonances as effective degrees of freedom within the QGP. The model is based on chiral and heavy-quark symmetry, taking into account pseudo-scalar D (B) and vector D* (B*) mesons and their chiral partners. Within this model, we evaluate drag and diffusion coefficients to assess the flow properties of c- and b-quarks within the QGP, as produced in URHIC's, using a relativistic Langevin simulation. We find that the survival of the resonances at temperatures T≤2 Tc (Tc≈180 MeV: critical temperature for the deconfinement transition) accelerates the equilibration of c- and (to less extent) b-quarks significantly compared to the use of perturbative-QCD elastic scattering processes only. Using the such obtained heavy-quark pT-spectra and elliptic flow, v2, we employ a combined coalescence/fragmentation model for hadronization to D and B mesons for the pertinent non-photonic electron observables and compare to the data from the PHENIX and STAR collaborations at RHIC.
Dr. Hendrik van Hees, Texas A&M University
In-medium Modifications of Hadrons and the NA60 dimuon measurements
Abstract:
The theoretical understanding of dimuon spectra as measured in 158AGeV In-In collisions by the NA60 collaboration at the CERN SPS is summarized. The low-mass region, M≤0.9 GeV, is well described by in-medium modifications of the ρ-meson spectral function within a hadronic many-body approach. To account for the yield in the intermediate-mass region, 0.9 GeV≤M≤1.5 GeV, four-pion contributions in the electromagnetic emission function have to be taken into account. The data are consistent with the assumption of chiral mixing of isovector-vector and -axialvector currents, which could be indicative for the onset of chiral-symmetry restoration in heavy-ion collisions. Our calculation also includes the contribution from the quark-gluon plasma phase which turns out to be small compared to that of the hadronic source. Predictions for the modifications of the ω- and φ-meson spectral shape may be experimentally checked in future experiments.