Particle theory

Abstract:

We study the topological susceptibility, χ, in QCD with two quark flavours using lattice field configurations that have been produced with an O(a) improved quark action. We find clear evidence for the expected suppression at small quark mass, and examine the variation of χ with this mass. The resulting estimate of the pion decay constant, fπ = 105 ± 6-10⁺¹⁸ MeV, is consistent with the experimental value of ≃ 93 MeV. We compare χ to the large-Nc prediction and find consistency over a large range of quark masses. We discuss the benefits of the non-perturbative action improvement scheme and of the stategy of keeping the lattice spacing (nearly) fixed as the quark mass is varied. We compare our results with other studies and suggest why such a quark mass dependence has not always been seen. © 2001 Elsevier Science B.V. All rights reserved.

Abstract:

We construct cosmological solutions of four-dimensional effective heterotic M theory with a moving five-brane and evolving dilaton and T modulus. It is shown that the five-brane generates a transition between two asymptotic rolling-radii solutions. Moreover, the five-brane motion always drives the solutions towards strong coupling asymptotically. We present an explicit example of a negative-time branch solution which ends in a brane collision accompanied by a small-instanton transition. The five-dimensional origin of some of our solutions is also discussed.