34 research outputs found
Is Rho-Meson Melting Compatible with Chiral Restoration?
Utilizing in-medium vector spectral functions which describe dilepton data in
ultra-relativistic heavy-ion collisions, we conduct a comprehensive evaluation
of QCD and Weinberg sum rules at finite temperature. The starting point is our
recent study in vacuum, where the sum rules have been quantitatively satisfied
using phenomenological axial-/vector spectral functions which describe hadronic
tau-decay data. In the medium, the temperature dependence of condensates and
chiral order parameters is taken from thermal lattice QCD where available, and
otherwise estimated from a hadron resonance gas. Since little is known about
the in-medium axial-vector spectral function, we model it with a Breit-Wigner
ansatz allowing for smooth temperature variations of its width and mass
parameters. Our study thus amounts to testing the compatibility of the
-broadening found in dilepton experiments with (the approach toward)
chiral restoration, and thereby searching for viable in-medium axial-vector
spectral functions.Comment: 8 pages, 4 figures, updated to be consistent with published versio
Charmonium moving through a strongly coupled QCD plasma: a holographic perspective
We study the properties of charmonium in a strongly coupled QCD-like plasma
at finite momentum. As a basis for this study, a "bottom-up" holographic model
is used which has been previously shown to reproduce charmonium phenomenology
in vacuum and give a reasonable dissociation temperature at zero momentum. The
finite momentum spectral functions are presented and found to be consistent
with recent lattice results. The in-medium dispersion relation and momentum
dependence of decay width of J/Psi have also been studied. We find no signature
of a subluminal limiting velocity from the dispersion relation, while we note
that the dissociation temperature decreases with momentum faster than previous
holographic models. Based upon the dissociation temperature, a maximum momentum
for J/Psi in medium is identified and its phenomenological implications on
J/Psi suppression are discussed.Comment: 23 pages, 8 figures. References added. Published versio
Quantitative sum rule analysis of low-temperature spectral functions
We analyze QCD and Weinberg-type sum rules in a low-temperature pion gas
using vector and axial-vector spectral functions following from the
model-independent chiral-mixing scheme. Toward this end we employ recently
constructed vacuum spectral functions with ground and first-excited states in
both channels and a universal perturbative continuum; they quantitatively
describe hadronic tau-decay data and satisfy vacuum sum rules. These features
facilitate the implementation of chiral mixing without further assumptions, and
lead to in-medium spectral functions which exhibit a mutual tendency of
compensating resonance and dip structures, suggestive for an approach toward
structureless distributions. In the sum rule analysis, we account for pion mass
corrections, which turn out to be significant. While the Weinberg sum rules
remain satisfied even at high temperatures, the numerical evaluation of the QCD
sum rules for vector and axial-vector channels reveals significant deviations
setting in for temperatures beyond ~140 MeV, suggestive of additional physics
beyond low-energy chiral pion dynamics.Comment: 8 pages, 3 figure
Charmonium sum rules applied to a holographic model
The heavy-quark QCD sum rules are applied to a model of charmonium based upon
the gauge/gravity duality. We find that there is strong agreement between the
moments of the polarization function calculated from the holographic model and
the experimental data suggesting that the model is consistent with the
heavy-quark QCD sum rules at zero temperature.Comment: 9 pages, 2 figures, Corrected typos to bring inline with PRD versio
Doubly heavy hadrons and the domain of validity of doubly heavy diquark--anti-quark symmetry
In the limit of heavy quark masses going to infinity, a symmetry is known to
emerge in QCD relating properties of hadrons with two heavy quarks to analogous
states with one heavy anti-quark. A key question is whether the charm mass is
heavy enough so that this symmetry is manifest in at least an approximate
manner. The issue is crucial in attempting to understand the recent reports by
the SELEX Collaboration of doubly charmed baryons. We argue on very general
grounds that the charm quark mass is substantially too light for the symmetry
to emerge automatically via colour coulombic interactions. However, the
symmetry could emerge approximately depending on the dynamical details.Comment: 9 page
Thermal Dileptons from Coarse-Grained Transport as Fireball Probes at SIS Energies
Utilizing a coarse-graining method to convert hadronic transport simulations
of Au+Au collisions at SIS energies into local temperature, baryon and pion
densities, we compute the pertinent radiation of thermal dileptons based on an
in-medium spectral function that describes available spectra at
ultrarelativistic collision energies. In particular, we analyze how far the
resulting yields and slopes of the invariant-mass spectra can probe the
lifetime and temperatures of the fireball. We find that dilepton radiation sets
in after the initial overlap phase of the colliding nuclei of about 7 fm/c, and
lasts for about 13 fm/c. This duration closely coincides with the development
of the transverse collectivity of the baryons, thus establishing a direct
correlation between hadronic collective effects and thermal EM radiation, and
supporting a near local equilibration of the system. This fireball "lifetime"
is substantially smaller than the typical 20-30 fm/c that naive considerations
of the density evolution alone would suggest. We furthermore find that the
total dilepton yield radiated into the invariant-mass window of
GeV/, normalized to the number of charged pions, follows a relation to
the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion
collisions, and thus corroborates the versatility of this tool. The spectral
slopes of the invariant-mass spectra above the mass provide a
thermometer of the hottest phases of the collision, and agree well with the
maximal temperatures extracted from the coarse-grained hadron spectra.Comment: 9 pages, 6 figures; v2: extended discussion, matches the version
accepted for publicatio