Modern Physics Letters A

Orbifolds in field theory are potentially singular objects for at their fixed points the curvature becomes infinite, therefore one may wonder whether field theory calculations near orbifold singularities can be trusted. String theory is perfectly well defined on orbifolds and can therefore be taken as a UV completion of field theory on orbifolds. We investigate the properties of field and string theory near orbifold singularities by reviewing the computation of a one-loop gauge field tadpole. We find that in string theory the twisted states give contributions that have a spread of a couple of string lengths around the singularity, but otherwise the field theory picture is confirmed. One additional surprise is that in some orbifold models one can identify local tachyons that give contributions near the orbifold fixed point.

In the present paper we present and analyze exact cylindrically symmetric diagonal and nondiagonal solutions in the so-called dilaton–axion gravity. It is shown that they are free of curvature singularities, (quasi)regular on the axis of symmetry, asymptotically flat and describe rotating or nonrotating cosmic strings interacting with gravitational and dilaton–axion waves.

The p-wave threshold effect is described in terms of the reduced scattering matrix. The relationship of this approach to previous theoretical threshold models is established. We prove that this phenomenon is related to the reaction-mechanism of quasiresonant scattering: a single particle neutron threshold state and direct interaction coupling to open observed channels. Spectroscopic aspects of the threshold effect, both with respect to the magnitude and microstructure, are discussed in terms of the neutron strength function.

We present a class of models of flavor that rely on the use of flavor symmetries and the Froggart–Nielsen mechanism in extra dimensions. The particle content is that of the standard model plus an additional flavon field; all the fields propagate in universal extra dimensions and the flavor scale is associated with the cutoff of the theory, which in 5D is ~ 10 TeV. The Yukawa matrices are generated by higher dimension operators involving flavon fields, whose vacuum expectation values break the flavor symmetry. We apply this framework and present a specific 5D model based on a discrete local symmetry that reproduces all fermion masses and mixing angles both in the quark and charged lepton sectors.

Recently Gambini and Pullin proposed a new consistent discrete approach to quantum gravity and applied it to cosmological models. One remarkable result of this approach is that the cosmological singularity can be avoided in a general fashion. However, whether the continuum limit of such discretized theories exists is model dependent. In the case of massless scalar field coupled to gravity with Λ=0, the continuum limit can only be achieved by fine tuning the recurrence constant. We regard this failure as the implication that cosmological constant should vary with time. For this reason we replace the massless scalar field by Chaplygin gas which may contribute an effective cosmological constant term with the evolution of the universe. It turns out that the continuum limit can indeed be reached in this case.