arXiv subjects
Publications and source records attributed to Robert D. Pisarski.
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.
Using constituent quarks coupled to a linear sigma model at nonzero temperature, I show that many anomalous mesonic amplitudes, such as $π^0 \rightarrow γγ$, vanish in a chirally symmetric phase. Processes which are allowed, such as $π^0 σ\rightarrow γγ$, are computed to leading order in a loop expansion.
In a constituent quark model at nonzero temperature, the amplitude for $π^0 \rightarrow 2 γ$ vanishes in a chirally symmetric phase, while that for $π^0 σ\rightarrow 2 γ$ does not.
I review results on the shift of the $ρ$ meson mass at nonzero temperature in a gauged linear sigma model, under the assumption of strict vector meson dominance. (Based upon a talk presented at the International Workshop on "Chiral Dynamics in Hadrons and Nuclei", Seoul, Korea, Feb., 1995).
I discuss several topics in the applications of chiral symmetry at nonzero temperature, including: where the rho goes, disoriented chiral condensates, and the phase diagram for $QCD$ with $2+1$ flavors. (Based upon talks presented at the "Workshop on Finite Temperature $QCD$", Wuhan, P.R.C., April, 1994.)
I discuss different models which predict changes in the mass of the thermal $ρ$ field. I emphasize that while the predictions are strongly model dependent, nevertheless substantial shifts in the thermal $ρ$ mass are expected to occur at the point of phase transition. As long as the thermal $ρ$ peak does not become too broad, this should provide a striking signature of the existence of a phase transition. (Based upon a talk presented at Quark Matter '95, Monterey, CA, Jan., 1995.)
If the phase transition of $QCD$ at nonzero temperature is dominated by the (approximate) restoration of chiral symmetry, then the transition might be characterized using a gauged linear sigma model. Assuming that vector meson dominance holds, such sigma models predict that at the temperature of chiral restoration, the pole mass of the thermal $ρ$ meson is greater than that at zero temperature; in the chiral limit and in weak coupling this mass is $\sim 962 \, MeV$. The width of the thermal $ρ-a_1$ peak is estimated to be about $200 - 250 \, MeV$.
We compute the effective potential for the phase of the Wilson line at high temperature in the standard model to one loop order. Besides the trivial vacua, there are metastable states in the direction of $U(1)$ hypercharge. Assuming that the universe starts out in such a metastable state at the Planck scale, it easily persists to the time of the electroweak phase transition, which then proceeds by an unusual mechanism. All remnants of the metastable state evaporate about the time of the $QCD$ phase transition.
Rajagopal and Wilczek have proposed that relativistic nuclear collisions can generate domains in which the chiral condensate is disoriented. If sufficiently large ({\it i.e.} nucleus sized), such domains can yield measurable fluctuations in the number of neutral and charged pions. However, by numerical simulation of the zero-temperature two-flavor linear sigma model, we find that domains are essentially {\it pion} sized. Nevertheless, we show that large domains can occur if the effective mesons masses are much lighter.