Particle theory

Abstract:

We present a supersymmetric model of electroweak symmetry-breaking exhibiting improved naturalness, wherein the stop mass can be pushed beyond the reach of the Large Hadron Collider without unnatural fine tuning. This implies that supersymmetry may still solve the hierarchy problem, even if it eludes detection at the LHC.

Abstract:

It is known that the cosmological baryon density (Omega(b)) and dark matter density (Q(dm)) have strikingly similar values. However, in most theories of the early Universe, each density is explained by separate dynamics and consequently there is no compelling reason for this observation. In this note, I briefly review a model in which the dark matter species possesses a particle-antiparticle asymmetry. This asymmetry determines both the baryon asymmetry and strongly affects the dark matter density, thus naturally linking Omega(b) and Omega(dm). In these models it is shown that sneutrinos can play the role of such dark matter

Abstract:

We find a strong-to-weak coupling crossover in D=2+1 SU(N) lattice gauge theories that appears to become a third-order phase transition at N=∞, in a similar way to the Gross-Witten transition in the D=1+1 SU(N→∞) lattice gauge theory. There is, in addition, a peak in the specific heat at approximately the same coupling that increases with N, which is connected to ZN monopoles (instantons), reminiscent of the first-order bulk transition that occurs in D=3+1 lattice gauge theories for N≥5. Our calculations are not precise enough to determine whether this peak is due to a second-order phase transition at N=∞ or to the third-order phase transition having a critical behavior different to that of the Gross-Witten transition. We show that as the lattice spacing is reduced, the N=∞ gauge theory on a finite 3-torus appears to undergo a sequence of first-order ZN symmetry breaking transitions associated with each of the tori (ordered by size). We discuss how these transitions can be understood in terms of a sequence of deconfining transitions on ever-more dimensionally reduced gauge theories. We investigate whether the trace of the Wilson loop has a nonanalyticity in the coupling at some critical area, but find no evidence for this. However we do find that, just as one can prove occurs in D=1+1, the eigenvalue density of a Wilson loop forms a gap at N=∞ at a critical value of its trace. We show that this gap formation is in fact a corollary of a remarkable similarity between the eigenvalue spectra of Wilson loops in D=1+1 and D=2+1 (and indeed D=3+1): for the same value of the trace, the eigenvalue spectra are nearly identical. This holds for finite as well as infinite N; irrespective of the Wilson loop size in lattice units; and for Polyakov as well as Wilson loops. © 2006 The American Physical Society.

Abstract:

We study the predictions of holographic QCD for various observable four-point quark flavour current-current correlators. The dual 5-dimensional bulk theory we consider is a $SU(3)_L \times SU(3)_R$ Yang Mills theory in a slice of $AdS_5$ spacetime with boundaries. Particular UV and IR boundary conditions encode the spontaneous breaking of the dual 4D global chiral symmetry down to the $SU(3)_V$ subgroup. We explain in detail how to calculate the 4D four-point quark flavour current-current correlators using the 5D holographic theory, including interactions. We use these results to investigate predictions of holographic QCD for the $\Delta I = 1/2$ rule for kaon decays and the $B_K$ parameter. The results agree well in comparison with experimental data, with an accuracy of 25% or better. The holographic theory automatically includes the contributions of the meson resonances to the four-point correlators. The correlators agree well in the low-momentum and high-momentum limit, in comparison with chiral perturbation theory and perturbative QCD results, respectively.