Soluble theory with massive ghosts
To investigate the unitarity of asymptotically free, higher-derivative theories, like certain models of quantum gravity, I study a prototype in two space-time dimensions. The prototype is a kind of higher-derivative nonlinear sigma model; it is asymptotically free, exhibits dimensional transmutation, and is soluble in a large-N expansion. The S-matrix elements, constructed from the analytic continuation of the Euclidean Green's functions, conserve probability to approx.O(N/sup -1/), but violate unitarity at approx.O(N/sup -2/). The model demonstrates that in higher-derivative theories unitarity, or the lack thereof, cannot be decided without explicit control over the infrared limit. Even so, the results suggest that there may exist some (rather special) asymptotically free, higher-derivative theories which are unitary.

title: Theory at quark matter '02
abstract: I summarize theory at Quark Matter '02, stressing the continuing inability model to describe all notable features of the data from √ s /A: 55 → 200GeV

title: Damping rates for moving particles in hot QCD.
abstract: Using a program of perturbative resummation I compute the damping rates for fields at nonzero spatial momentum to leading order in weak coupling in hot QCD. Sum rules for spectral densities are used to simplify the calculations. For massless fields the damping rate has an apparent logarithmic divergence in the infrared limit, which is cut off by the screening of static magnetic fields ("magnetic mass"). This demonstrates how at high temperature even perturbative quantities are sensitive to nonperturbative phenomena.

title: Nonabelian Debye screening, tsunami waves, and worldline fermions
abstract: I give a pedagogical review of the derivation for the effective lagrangian for nonabelian Debye screening, or hard thermal loops. Following Kelly, Liu, Lucchesi, and Manuel, I give the simplest derivation possible, using classical kinetic theory. 
The result is valid not just for a thermal, but for an arbitrary initial distributions. I use this to study the evolution, at short times, of a gluonic ``tsunami wave''. 
I also suggest how classical kinetic theory may arise at one loop order. Using the wordline representation of the one loop effective action, I follow D'Hoker and Gagn\'{e} to replace the Wilson line by an integral over worldline fermions. A bilinear of these worldline fermions naturally defines a nonabelian charge, whose equation of motion is Wong's equation.

title: Cool pions move at less than the speed of light
abstract: At nonzero temperature, pions propagate through a thermal medium at less than the speed of light. About low temperature, this effect begins not at $\sim T^2$, but at next to leading order, $\sim T^4$. We also derive the generalization of the relation of Gell-Mann, Oakes, and Renner to nonzero temperature.

title: Soft Amplitudes in Hot Gauge Theories: A General Analysis
abstract: A systematic method for the calculation of amplitudes in hot gauge theories is developed. It is necessary to distinguish between hard momenta (of order T ) and soft momenta (of order gT ). Ordinary perturbation theory applies at hard momenta, but over soft momenta, effective propagators and vertices are required. These effective quantities resum the leading contributions from thermal fluctuations with hard virtual momenta. These “hard thermal loops” arise solely from subdiagrams at one-loop order: they are ultraviolet finite, gauge independent, and satisfy simple Ward identities. To illustrate the method we apply it to the quark and gluon self-energies. Ward identities are used to show that to one-loop order in this effective perturbation expansion, the two-point T -matrix elements constructed from the self-energies are gauge invariant. This proves that to leading order in g , the quark and gluon damping rates are gauge invariant.

title: The ROSAT HRI X-Ray Survey of the Cygnus Loop
abstract: We describe and report progress on the joint U.S. and German campaign to map the X-ray emission from the entire Cygnus Loop with the ROSAT High Resolution Imager. The Cygnus Loop is the prototype for a supernova remnant that is dominated by interactions with the interstellar medium and supplies fundamental physical information on this basic mechanism for shaping the interstellar medium. The global view that these high-resolution (FWHM ~ 10'') observations provide emphasizes the inhomogeneity of the interstellar medium and the pivotal nature of cloud-blast-wave interactions in determining the X-ray morphology of the supernova remnant. While investigating the details of the evolution of the blast wave, we also describe the interstellar medium in the vicinity of the Cygnus Loop, which the progenitor star has processed. Although we do not expect the X-ray observations to be complete until 1997 September, the incomplete data combined with deep Hα images provide definitive evidence that the Cygnus Loop was formed by an explosion within a preexisting cavity.