We report on determinations of the low-energy constants α 5 and α 8 in the effective chiral Lagrangian at O(p 4), using lattice simulations with N f = 2 flavours of dynamical quarks. Precise knowledge of these constants is required to test the hypothesis whether or not the up-quark is massless. Our results are obtained by studying the quark mass dependence of suitably defined ratios of pseudosc...

We obtain a limit when mass tends to zero of the relativistic diffusion of Schay and Dudley. The diffusion process has the log-normal distribution. We discuss Langevin stochastic differential equations leading to an equilibrium distribution. We show that for the Jüttner equilibrium distribution the relativistic diffusion is a linear approximation to the Kompaneetz equation describing a photon d...

Preheating can convert superhorizon fluctuations of light scalar fields present at the end of inflation into observable density perturbations. We show in detail how lattice field theory simulations and the separate universes approximation can be used to calculate these perturbations and make predictions for the nonlinearity parameter fNL. We also present a simple approximation scheme that can r...

is almost coeval with Maxwell’s equations themselves, although a proof of its validity awaited over a century [1,2]. In addition to countless generalizations of ‘‘duality’’ in field and string theory, it has led to an enormous variety of more precise analogs, in particular to spin 2 [2,3], then to all free massless (integer or half-integer) spin s > 0 systems in flat space [4,5]. In de Sitter (...

Due to their weak interactions, neutrinos can polarize a medium and acquire an induced charge. We consider the Cherenkov radiation emitted by neutrinos due to their effective electromagnetic interactions as they pass through a polarizable medium. The effect exists even for massless, chiral neutrinos, where no physics beyond the standard model needs to be assumed. The study of the electromagneti...

In cosmological backgrounds, there can be ’partially massless’ higher spin fields which have fewer degrees of freedom than their massive partners. The equations for the partially massless spin-2 fields are usually taken to be the linearized Einstein equations augmented with a ’tuned’ Pauli-Fierz mass. Here, we add more powers of curvatures and show that for the string-generated Einstein-Gauss-B...

The regularized expectation value of the stress-energy tensor for a massless bosonic or fermionic field in 1+1 dimensions is calculated explicitly for the instantaneous vacuum relative to any Cauchy surface (here a spacelike curve) in terms of the length L of the curve (if closed), the local extrinsic curvature K of the curve, its derivative K ′ with respect to proper distance x along the curve...

The next-to-leading order cross section for the inclusive production of charm quarks in γγ collisions is calculated as a function of the transverse momentum pT and the rapidity y in approaches using massive or massless charm quarks. For the direct cross section we derive the massless limit from the massive theory with the result that this limit differs from the massless version with MS factoriz...

We present a path integral representation for massless spin one-half particles. It is shown that this gives us a super–symmetric, P − and T −non–invariant pseudoclassical model for relativistic massless spinning particles. Dirac quantization of this model is considered.