Particle theory

Abstract:

We investigate the low-lying spectrum of closed confining flux-tubes that wind around a spatial torus in D = 2+1 and carry flux in different representations of SU(N). We focus on our most recent calculations for N = 6 and β?= 171, where the calculated low-energy physics is very close to the continuum and large-N limits. We investigate the adjoint, 84, 120, k = 2A, 2S and k = 3A, 3M, 3S representations and show that the corresponding flux-tubes do exist. Similarly to the results for the fundamental representation, the ground state of a flux-tube with momentum along its axis appears to be well described by Nambu-Goto all the way down to very short tubes. In contrast, excited states have much larger deviations from Nambu-Goto. We discuss whether these states are non-string-like and associated with excitations of massive flux-tube modes.

Abstract:

Non-perturbative superpotential cross-couplings between visible sector matter and Kähler moduli can lead to significant flavour-changing neutral currents in compactifications of type IIB string theory. Here, we compute corrections to Yukawa couplings in orbifold models with chiral matter localised on D3-branes and non-perturbative effects on distant D7-branes. By evaluating a threshold correction to the D7-brane gauge coupling, we determine conditions under which the non-perturbative corrections to the Yukawa couplings appear. The flavour structure of the induced Yukawa coupling generically fails to be aligned with the tree-level flavour structure. We check our results by also evaluating a correlation function of two D7-brane gauginos and a D3-brane Yukawa coupling. Finally, by calculating a string amplitude between n hidden scalars and visible matter we show how non-vanishing vacuum expectation values of distant D7-brane scalars, if present, may correct visible Yukawa couplings with a flavour structure that differs from the tree-level flavour structure. © 2013 SISSA, Trieste, Italy.

Abstract:

We study the Galactic radio background produced by synchrotron emission of cosmic ray electrons in Galactic magnetic fields. The angular power spectrum of all-sky maps contains contributions from physical processes on a variety of scales, including the variation of emissivity on large (kpc) scales and the small-scale turbulence in the interstellar medium. Previous studies have focussed on one scale or the other but have not provided a self-consistent picture valid on all scales. We show that the angular power spectrum in the multipole range l ∼ 10 − 100 has a deficit with respect to observations at 408 MHz and suggest that the shells of old supernova remnants (like the well-known ‘radio loops’) can provide the missing power. We model the emissivity from individual shells and find that the Galactic population of O(1000) shells naturally provides the missing power and brings the modelled angular power spectrum into agreement with the observed one. This has important implications for studies of the cosmic microwave background, in particular B-mode polarisation.

Abstract:

We discuss the use of gauge fields to stabilize complex structure moduli in Calabi-Yau three-fold compactifications of heterotic string and M-theory. The requirement that the gauge fields in such models preserve supersymmetry leads to a complicated landscape of vacua in complex structure moduli space. We develop methods to systematically map out this multi-branched vacuum space, in a computable and explicit manner. In analysing the resulting vacua, it is found that the associated Calabi-Yau three-folds are sometimes stabilized at a value of complex structure resulting in a singular compactification manifold. We describe how it is possible to resolve these singularities, in some cases, while maintaining computational control over the moduli stabilization mechanism. The discussion is illustrated throughout the paper with explicit worked examples. © 2013 SISSA, Trieste, Italy.