3,122 research outputs found
Dynamical Formation of Disoriented Chiral Condensates
We study the dynamical formation of disoriented chiral condensates in very
high energy nucleus-nucleus collisions using Bjorken hydrodynamics and
relativistic nucleation theory. It is the dynamics of the first order
confinement phase transition which controls the evolution of the system. Every
bubble or fluctuation of the new, hadronic, phase obtains its own chiral
condensate with a probability determined by the Boltzmann weight of the finite
temperature effective potential of the linear sigma model. We evaluate domain
size and chiral angle distributions, which can be used as initial conditions
for the solution of semiclassical field equations.Comment: 17 pages, latex and 10 ps figures available at
http://www.nbi.dk/~vischer/dcc.htm
Three-Field Potential for Soft-Wall AdS/QCD
The AdS/CFT correspondence may offer new and useful insights into the
non-perturbative regime of strongly coupled gauge theories such as Quantum
Chromodynamics. Soft-wall AdS/QCD models have reproduced the linear
trajectories of meson spectra by including background dilaton and chiral
condensate fields. Efforts to derive these background fields from a scalar
potential have so far been unsuccessful in satisfying the UV boundary
conditions set by the AdS/CFT dictionary while reproducing the IR behavior
needed to obtain the correct chiral symmetry breaking and meson spectra.
We present a three-field scalar parametrization that includes the dilaton
field and the chiral and glueball condensates. This model is consistent with
linear trajectories for the meson spectra and the correct mass-splitting
between the vector and axial-vector mesons. We also present the resulting meson
trajectories.Comment: 6 pages, 2 figures, Presented at The 7th International Workshop on
Chiral Dynamics, August 6 -10, 2012, Jefferson Lab, Newport News, Virginia,
US
Dileptons and Photons from Coarse-Grained Microscopic Dynamics and Hydrodynamics Compared to Experimental Data
Radiation of dileptons and photons from high energy nuclear collisions
provides information on the space-time evolution of the hot dense matter
produced therein. We compute this radiation using relativistic hydrodynamics
and a coarse-grained version of the microscopic event generator UrQMD, both of
which provide a good description of the hadron spectra. The currently most
accurate dilepton and photon emission rates from perturbative QCD and from
experimentally-based hadronic calculations are used. Comparisons are made to
data on central Pb-Pb and Pb-Au collisions taken at the CERN SPS at a beam
energy of 158 A GeV. Both hydrodynamics and UrQMD provide very good
descriptions of the photon transverse momentum spectrum measured between 1 and
4 GeV, but slightly underestimate the low mass spectrum of e+e- pairs, even
with greatly broadened rho and omega vector mesons. Predictions are given for
the transverse momentum distribution of dileptons.Comment: 35 pages, 17 figure
Vacuum Structure and Dark Energy
We consider that the universe is trapped in an excited vacuum state and the
resulting excitation energy provides the observed dark energy. We explore the
conditions under which this situation can arise from physics already known.
Considering the example of how macroscopic QED fields alter the vacuum
structure, we find that the energy scale 1 meV --- 1 eV is particularly
interesting. We discuss how dark energy of this form is accessible to
laboratory experiments.Comment: 5 pages, 2 figures; recognized for Honorable Mention in 2010 Gravity
Research Foundation Awards for Essays on Gravitation, in press with Int. J.
Mod. Phys.
The Kohn-Luttinger Effect in Gauge Theories
Kohn and Luttinger showed that a many body system of fermions interacting via
short range forces becomes superfluid even if the interaction is repulsive in
all partial waves. In gauge theories such as QCD the interaction between
fermions is long range and the assumptions of Kohn and Luttinger are not
satisfied. We show that in a U(1) gauge theory the Kohn-Luttinger phenomenon
does not take place. In QCD attractive channels always exist, but there are
cases in which the primary pairing channel leaves some fermions ungapped. As an
example we consider the unpaired fermion in the 2SC phase of QCD with two
flavors. We show that it acquires a very small gap via a mechanism analogous to
the Kohn-Luttinger effect. The gap is too small to be phenomenologically
relevant.Comment: 5 pages, 2 figure, minor revisions, to appear in PR
High momentum lepton pairs from jet-plasma interactions
We discuss the emission of high momentum lepton pairs (p_T>4 GeV) with low
invariant masses (M << p_T) in central Au+Au collisions at RHIC
(\sqrt{s_{NN}}=200 GeV). The spectra of dileptons produced through interactions
of quark and antiquark jets with the quark-gluon plasma (QGP) have been
calculated. Annihilation and Compton scattering processes, as well as processes
benefitting from collinear enhancement, including Landau-Pomeranchuk-Migdal
(LPM) effects, are calculated and convolved with a one dimensional hydrodynamic
expansion. The jet-induced contributions are compared to thermal dilepton
emission and Drell-Yan processes, and are found to dominate around p_T=4 GeV.Comment: Parallel talk given at QM2006, Shanghai November 2006. 4 pages, 3
figure
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