Modern Physics Letters A

We review the gauge symmetry breaking mechanism due to orbifold projections in orbifold model building. We explicitly show the existence of a scale of breaking if such a symmetry breaking is due to freely-acting orbifold operators only, i.e. in case the breaking is realized nonlocally in the internal space. We show that such a scale is related to the compactification moduli only, and that there are no extra continuous parameters, at least in semirealistic models with SUSY in four dimensions. In this sense, the mechanism is peculiarly different from the standard Higgs (or Hosotani) symmetry breaking mechanism. We show that the mechanism also differs from that present in standard orbifold models where, even in the presence of discrete Wilson lines, a scale of breaking is generically missing, since the breaking is localized in specific points in the internal space.

We review a set of background geometries where the described nonlocal breaking is realized, both in the case of two and six extra dimensions. In the latter case, relevant in string model building, we consider both heterotic and open string compactifications.

We analyze with the Bayesian method the solar and KamLAND neutrino data in terms of neutrino oscillations. We show that Bayesian credible regions with a flat prior in the plane strongly support the LMA solution, in agreement with the usual chi-square analysis. Other reasonable priors are considered in order to test the stability of the LMA solution. We show that priors which favor small or large values of the mixing angle lead to minor changes of the allowed LMA region, affecting mainly its large-tan²θ₁₂ part.

We discuss self-consistent geometries and behavior of dilaton in exactly solvable models of 2D dilaton gravity, with quantum fields in the Boulware state. If the coupling H(ϕ) between curvature and dilaton ϕ is non-monotonic, back-reaction can remove the classical singularity. As a result, an everywhere regular star-like configuration may appear, in which case the Boulware state, contrary to expectations, smooths out the system. For monotonic H(ϕ), exact solutions confirm the features found before with the help of numerical methods: the appearance of the bouncing point and the presence of isotropic singularity at the classically forbidden branch of the dilaton.

PPN-limit of alternative theories of gravity represents a still controversial matter of debate and no definitive answer has been provided, up to now, about this issue. By using the definition of the PPN-parameters γ and β in terms of f(R) theories of gravity, we show that a family of third-order polynomial theories, in the Ricci scalar R, turns out to be compatible with the PPN-limit and the deviation from General Relativity, theoretically predicted, can agree with experimental data.

Using the partition of the number p-1 into p-1 real parts which are not equal with each other necessarily, we develop the unitary parasupersymmetry algebra of arbitrary order p so that the well-known Rubakov–Spiridonov–Khare parasupersymmetry becomes a special case of the developed one. It is shown that the developed algebra is realized by simple harmonic oscillator and Landau problem on a flat surface with the symmetries of h₃ and h₄ Heisenberg–Lie algebras. For this new parasupersymmetry, the well-known unitary condition is violated, however, unitarity of the corresponding algebra is structurally conserved. Moreover, the components of the bosonic Hamiltonian operator are derived as functions from the mean value of the partition numbers with their label weight function.

Static quark potential is studied using a tadpole improved gauge lattice action. The scale is set using the potential for a wide range of bare parameters. The renormalized anisotropy of the lattice is also measured.

In this paper, a generalization and extension of some recent results is proposed. Namely, a four-dimensional quantum space is constructed and a differential calculus, obeying the usual criteria of covariance, is built upon this space. These results are then used to construct a gauge theory on this four-dimensional quantum space.