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Abstract:

We present our preliminary results of SO(2N) gauge theories, approaching the large-N limit. SO(2N) theories may help us to understand QCD at finite chemical potential since there is an orbifold equivalence between SO(2N) and SU(N) gauge theories at large-N and SO(2N) theories do not have the sign problem present in QCD. We consider the string tensions, mass spectra, and deconfinement temperatures in the SO(2N) pure gauge theories in 2+1 dimensions, comparing them to their corresponding SU(N) theories.

Abstract:

We perform an exploratory investigation of how rapidly the physics of SO(2N) gauge theories approaches its N = ∞ limit. This question has recently become topical because SO(2N) gauge theories are orbifold equivalent to SU(N) gauge theories, but do not have a finite chemical potential sign problem. It is therefore interesting to know how close is the physics of SO(N) to that of SU(3) for the modest values of N where one might be able to perform chemical potential calculations. We consider only the pure gauge theory and, because of the inconvenient location of the lattice strong-to-weak coupling 'bulk' transition in 3 + 1 dimensions, we largely confine our numerical calculations to 2 + 1 dimensions in this paper. We provide some analytic estimates of the SO(2N) spectrum in both D = 2 + 1 and D = 3 + 1, and show, numerically, that the D = 2 + 1 SO(6) and SU(4) low-lying spectra do indeed appear to be the same. Our numerical calculations of a number of mass ratios show that the leading O(1/N) correction already dominates for N ≥ 6, and in some cases down to N = 4, and that, as expected, these ratios become consistent with those of SU(N) as N → ∞. In particular we see that SO(6) and SU(3) gauge theories are quite similar except for the values of the string tension and coupling, both of which differences can be readily understood. © 2013 SISSA, Trieste, Italy.

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.