Particle theory

Abstract:

We investigate the topological structure of the vacuum in SU(3) lattice gauge theory. We use under-relaxed cooling to remove the high-frequency fluctuations and a variety of "filters" to identify the topological charges in the resulting smoothened field configurations. We find a densely packed vacuum with an average instanton size, in the continuum limit, of ρ̄∼0.5 fm. The density at large ρ decreases paidly as 1/ρ∼11. At small sizes we see some signs of a trend towards the asymptotic perturbative behavior of D(∝)ρ6. We find that an interesting polarization phenomenon occurs: the large topological charges tend to have, on the average, the same sign and are over-screened by the smaller charges which tend to have, again on the average, the opposite sign to the larger instantons. We also calculate the topological susceptibility, χt, for which we obtain a continuum value of χ1/4t∼187 MeV. We perform the calculations for various volumes, lattice spacings and numbers of cooling sweeps, so as to obtain some control over the associated systematic errors. The coupling range is 6.0≤β≤6.4 and the lattice volumes range from 163×48 to 323×64.

Abstract:

Massive neutrinos were the first proposed, and remain the most natural, particle candidate for the dark matter. In the absence of firm laboratory evidence for neutrino mass, considerations of the formation of large scale structure in the universe provide a sensitive, albeit indirect, probe of this possibility. Observations of galaxy clustering and large angle anisotropy in the cosmic microwave background have been interpreted as requiring that neutrinos provide similar to 20% of the critical density. However the need for such 'hot' dark matter is removed if the primordial spectrum of density fluctuations is tilted below scale-invariance, as is often the case in physically realistic inflationary models. This question will be resolved by forthcoming precision measurements of microwave background anisotropy on small angular scales. This data will also improve the nucleosynthesis bound on the number of neutrino species and test whether decays of relic neutrinos could have ionized the intergalactic medium.