Particle theory

Abstract:

We discuss the implications of the recent measurement of a 17 keV mass component in the electron neutrino sector. Such a heavy state must decay in order to be compatible with cosmology; this requires states additional to those of the standard model. The most likely candidates are either majorons, allowing the decay into massless weakly interacting scalars, or single neutrinos, allowing the decay via the Z into light neutrinos. We show that in the latter case there is a lower bound on the lifetime in conflict with the cosmological constraints on this decay channel and on the photon plus neutrino decay channel which is also present. The extensions of the standard model needed to reduce the lifetime are considered. We analyze the mass matrix, playing particular regard to the singlet neutrino case, and discuss how it might be extended to explain the solar neutrino deficit. © 1991.

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.