Particle theory

Abstract:

In grand unified theories based on orbifold constructions in higher dimensions, Higgsino-mediated proton decay is absent. However, proton decay mediated by $X$ and $Y$ gauge bosons is typically enhanced to levels detectable by current and future experiments. We analyse the phenomenology of proton decay induced by the minimal coupling of $X,Y$ gauge bosons. In particular, we show that the novel realization of matter in orbifold GUTs can lead to unusual final state flavour structure, for example, the dominance of the $p\to K^0\mu^+$ mode. Furthermore, we discuss proton decay induced by higher-derivative brane operators, finding potentially observable rates for natural values of the operator coefficients.

Abstract:

In the context of brane-world theories, the production cross-section for black holes may be greatly enhanced. Such black holes can in principle lead to detectable signals via their Hawking evaporation to brane-localized modes. We calculate, in the semiclassical approximation, the leading corrections to the energy spectrum (the greybody factors) for decay into scalar fields, as a function of the number of toroidally compactified extra dimensions, and partial wave number.

Abstract:

At a formal level, there appears to be no difficulty involved in introducing a chemical potential for a globally conserved quantum number into the partition function for space-time including a black hole. Were this possible, however, it would provide a form of black hole hair, and contradict the idea that global quantum numbers are violated in black hole evaporation. We demonstrate dynamical mechanisms that negate the formal procedure, both for topological charge (Skyrmions) and complex scalar-field charge. Skyrmions collapse to the horizon; scalar-field charge fluctuates uncontrollably.

Abstract:

The large N limit of SU(N) gauge theories in 3+1 dimensions is investigated on the lattice by extrapolating results obtained for 2 ≤ N ≤ 5. A numerical determination of the masses of the lowest-lying glueball states and of the topological susceptibility in the limit N → ∞ is provided. Ratios of the tensions of stable κ-strings over the tension of the fundamental string are investigated in various regimes and the results are compared with expectations based on several scenarios - in particular MQCD and Casimir scaling. While not conclusive at zero temperature in D=3+1, in the other cases investigated our data seem to favour the latter.