Particle theory

Abstract:

We investigate some basic aspects of topological fluctuations in lattice QCD, in the version with two colours and four light flavours; and we do so in both the confining, chiral symmetry broken phase and in the non-confining, chirally symmetric phase. This latter phase is found to ccut not only high temperatures, just as in the pure gauge system, but also in small spatial volumes, which is unlike the pure gauge case. We derive the way the topological susceptibility should vary with quark mass at small quark masses. We find that the calculated topological susceptibility decerases to zero with the quark mass, with the theretically expected powers except - in the symmetric phase - at the very smallest values of the quark mass. We demonstrate that this anomalous behaviour can be understood as arising from the fact that the lattice topological "zero modes" are in fact sufficiently far from being zero. We also show, in the chirally symmetric phase, that, just as expected, the average distance between instantons and anti-instantons decreases with decreasing quark mass. We finish with a new and more precise estimate of the location of the finite-temperature transition in SU(2) with four light flavours. © 1991.

Abstract:

We obtain estimates of the lightest glueball masses, the string tension, and the topological susceptibility in an exploratory study of QCD with two light flavors of quarks. Our calculations are performed at =5.6 with staggered quark masses mq=0.010 and 0.025 and on lattices ranging from 124 to 164. Our estimates suggest that, just as in the pure gauge theory, the 0++ is the lightest glueball with the 2++ about 50% heavier. Our mq=0.01 results predict a 0++ glueball mass of about 1.6 times the mass and the square root of the string tension of about 0.48 times the mass, which is surprisingly close to the usual phenomenologically motivated estimates of around 0.55. Our value of the topological susceptibility at mq=0.01 is consistent with the prediction, to O(mq) of the standard anomalous Ward identity. However, the variation of this susceptibility between mq=0.01 and mq=0.025 is weaker than the linear dependence one expects at small mq in the broken-chiral-symmetry phase of QCD. © 1991 The American Physical Society.

Abstract:

We calculate various meson and glueball masses in the version of lattice QCD with two colors and four (light) flavors. We do so both in the chirally symmetric and in the spontaneously broken phases. In the spontaneously broken phase (performed on 10320 lattices at =1.975) we find the following mass ratios: mG(0+)m=1.1(2), mG(0+)=2.8(5), m=0.39(3), mG(2+)mG(0+)=1.7(5). In the small-spatial-volume, chirally symmetric phase we explicitly observe parity doubling among the mesons but not among the glueballs. All these calculations are exploratory for a variety of reasons which we discuss. © 1991 The American Physical Society.

Abstract:

We study hadron thermodynamics with Wilson quarks. The crossover curve between the high-and low-temperature phases is determined as a function of gauge coupling and hopping parameter on 83×4 lattices. Screening lengths are calculated in the vicinity of the crossover region, and meson masses are calculated along the crossover curve on 82×16×4 and 83×16 lattices, respectively. © 1991 The American Physical Society.

Abstract:

Under the DOE Grand Challenge program, we have calculated the hadron spectrum of QCD with two flavors of dynamical quarks. We set 6 g2 = 5.6 and use bare quark masses amq = 0.025 and 0.01 to generate gauge configurations. The spectrum is calculated using both staggered and Wilson valence quarks. There are several positive features when these results are compared to those with larger lattice spacing; however, the nucleon to rho mass ratio is still too large. © 1991.