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