How good is the quenched approximation of QCD?

The quenched approximation for QCD is, at present and in the foreseeable future, unavoidable in lattice calculations with realistic choices of the lattice spacing, volume and quark masses. In this talk, I review an analytic study of the effects of quenching based on chiral perturbation theory. Quenched chiral perturbation theory leads to quantitative insight on the difference between quenched and unquenched QCD, and reveals clearly some of the diseases which are expected to plague quenched QCD. Uses jnl.tex and epsf.tex for figure 3. Figures 1 and 2 not included, sorry. Available as hardcopy on request.Comment: 22 pages, Wash. U. HEP/94-62 (Forgotten set of macros now included, sorry.

String breaking in Lattice QCD

The separation of a heavy quark and antiquark pair leads to the formation of a tube of flux, or string, which should break in the presence of light quark-antiquark pairs. This expected zero temperature phenomenon has proven elusive in simulations of lattice QCD. We present simulation results that show that the string does break in the confining phase at nonzero temperature.Comment: LATTICE98(hightemp), 3 pages, 4 figures, LaTe

Rates and Equilibria for a Photoisomerizable Antagonist at the Acetylcholine Receptor of Electrophorus Electroplaques

Voltage-jump and light-flash experiments have been performed on isolated Electrophorus electroplaques exposed simultaneously to nicotinic agonists and to the photoisomerizable compound 2,2'-bis-[α-(trimethylammonium)methyl]-azobenzene (2BQ). Dose-response curves are shifted to the right in a nearly parallel fashion by 2BQ, which suggests competitive antagonism; dose-ratio analyses show apparent dissociation constants of 0.3 and 1 µM for the cis and trans isomers, respectively. Flash-induced trans → cis concentration jumps produce the expected decrease in agonist-induced conductance; the time constant is several tens of milliseconds. From the concentration dependence of these rates, we conclude that the association and dissociation rate constants for the cis-2BQ-receptor binding are approximately ~ 10^8 M^(-1) s^(-1) and 60 s^(-1) at 20ºC; the Q_(10) is 3. Flash-induced cis → trans photoisomerizations produce molecular rearrangements of the ligand-receptor complex, but the resulting relaxations probably reflect the kinetics of buffered diffusion rather than of the interaction between trans-2BQ and the receptor. Antagonists seem to bind about an order of magnitude more slowly than agonists at nicotinic receptors

Threshold Photo/Electro Pion Production - Working Group Summary

We summarize the pertinent experimental and theoretical developments in the field of pion photo- and electroproduction in the threshold region. We discuss which experiments and which calculations should be done/performed in the future.Comment: plain TeX (macro included), 6pp, summary talk presented at the workshop on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199

The isentropic equation of state of 2-flavor QCD

Using Taylor expansions of the pressure obtained previously in studies of 2-flavor QCD at non-zero chemical potential we calculate expansion coefficients for the energy and entropy densities up to ${\cal O}(\mu_q^6)$ in the quark chemical potential. We use these series in $\mu_q/T$ to determine lines of constant entropy per baryon number ($S/N_B$) that characterize the expansion of dense matter created in heavy ion collisions. In the high temperature regime these lines are found to be well approximated by lines of constant $\mu_q/T$. In the low temperature phase, however, the quark chemical potential is found to increase with decreasing temperature. This is in accordance with resonance gas model calculations. Along the lines of constant $S/N_B$ we calculate the energy density and pressure. Within the accuracy of our present analysis we find that the ratio $p/\epsilon$ for $T>T_0$ as well as the softest point of the equation of state, $(p/\epsilon)_{min}\simeq 0.075$, show no significant dependence on $S/N_B$.Comment: 7 pages, 10 figure

Topics in Chiral Perturbation Theory

I consider some selected topics in chiral perturbation theory (CHPT). For the meson sector, emphasis is put on processes involving pions in the isospin zero S-wave which require multi-loop calculations. The advantages and shortcomings of heavy baryon CHPT are discussed. Some recent results on the structure of the baryons are also presented.Comment: 30 pp, TeX, Review talk, Third Workshop on High Energy Particle Physics (WHEPP III), Madras, India, January 1994. 7 figures available upon request. CRN--94/0

Staggered Fermion Actions with Improved Rotational Invariance

We introduce a class of improved actions for staggered fermions which to O(p^4) and O(p^6), respectively, lead to rotationally invariant propagators. We discuss the resulting reduction of flavour symmetry breaking in the meson spectrum and comment on the improvement in the calculation of thermodynamic observables.Comment: 3 pages and 4 figures, Contribution to Lattice 97 (Poster Session), late

Scaling functions for O(4) in three dimensions

Monte Carlo simulation using a cluster algorithm is used to compute the scaling part of the free energy for a three dimensional O(4) spin model. The results are relevant for analysis of lattice studies of high temperature QCD.Comment: 12 pages, 6 figures, uses epsf.st

Virtual Compton Scattering off the Nucleon in Chiral Perturbation Theory

We investigate the spin-independent part of the virtual Compton scattering (VCS) amplitude off the nucleon within the framework of chiral perturbation theory. We perform a consistent calculation to third order in external momenta according to Weinberg's power counting. With this calculation we can determine the second- and fourth-order structure-dependent coefficients of the general low-energy expansion of the spin-averaged VCS amplitude based on gauge invariance, crossing symmetry and the discrete symmetries. We discuss the kinematical regime to which our calculation can be applied and compare our expansion with the multipole expansion by Guichon, Liu and Thomas. We establish the connection of our calculation with the generalized polarizabilities of the nucleon where it is possible.Comment: 26 pages, 2 Postscript figures, RevTex using epsfi