Cosmology of the Next-to-Minimal Supersymmetric Standard Model

We discuss the domain wall problem in the Next-to-Minimal Supersymmetric Standard Model, with particular attention to the usual solution of explicit breaking of the discrete symmetry by non-renormalisable operators. This ``solution'' leads to a contradiction between the requirements of cosmology and those of avoiding the destabilisation of the hierarchy.Comment: 6 pages LaTeX, needs sprocl.sty (included at end) Talk presented by P.L. White at Valencia 9

Familial and peer influences on sport participation among adolescents in rural South African secondary schools

This study was designed to investigate the influences of family and peers on sport participation amongst adolescents in secondary schools at Hlanganani rural area of Limpopo Province, South Africa. A total of 172 learners (108 females and 64 males) attending three public secondary schools in Hlanganani rural area volunteered to participate in the study. Results indicated that adolescents preferred peers to family for support in sport participation. In terms of gender, no significant differences (p>0.05) were noted between mean values for school boys and girls with regard to family influence. Boys reported more tangible support, whereas girls indicated a preference for emotional support. Sport participation among learners is likely to increase when they receive informational, tangible, emotional and appraisal support from their parents and peers

Cosmic Reionisation by Stellar Sources: Population II Stars

We study the reionisation of the Universe by stellar sources using a numerical approach that combines fast 3D radiative transfer calculations with high resolution hydrodynamical simulations. Ionising fluxes for the sources are derived from intrinsic star formation rates computed in the underlying hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x 10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in previous studies of this kind. Our calculations reveal that the reionisation process is sensitive to the inclusion of dim sources with masses below ~10^9 h^-1 M_sol. We present the results of our reionisation simulation assuming a range of escape fractions for ionising photons and make statistical comparisons with observational constraints on the neutral fraction of hydrogen at z~6 derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting model has an escape fraction of ~20% and causes reionisation to occur by z~8, although the IGM remains fairly opaque until z~6. In order to simultaneously match the observations from the z=6.28 SDSS quasar and the optical depth measurement from WMAP with the sources modeled here, we require an evolving escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure

String Thermodynamics in D-Brane Backgrounds

We discuss the thermal properties of string gases propagating in various D-brane backgrounds in the weak-coupling limit, and at temperatures close to the Hagedorn temperature. We determine, in the canonical ensemble, whether the Hagedorn temperature is limiting or non-limiting. This depends on the dimensionality of the D-brane, and the size of the compact dimensions. We find that in many cases the non-limiting behaviour manifest in the canonical ensemble is modified to a limiting behaviour in the microcanonical ensemble and show that, when there are different systems in thermal contact, the energy flows into open strings on the `limiting' D-branes of largest dimensionality. Such energy densities may eventually exceed the D-brane intrinsic tension. We discuss possible implications of this for the survival of Dp-branes with large values of p in an early cosmological Hagedorn regime. We also discuss the general phase diagram of the interacting theory, as implied by the holographic and black-hole/string correspondence principles.Comment: 50 pages, LaTeX, 4 eps figures. Added discussion of random walk picture. Corrected technical error in the treatment of ND strings (notice some formulas are rewritten). Conclusions unchange

Some Thermodynamical Aspects of String Theory

Thermodynamical aspects of string theory are reviewed and discussed.Comment: 22 Pages plain latex; based on contributions to Golfand Memorial Volume and Englertfest by E.Rabinovic

Fermion Masses and Mixing in Four and More Dimensions

We give an overview of recent progress in the study of fermion mass and flavor mixing phenomena. Mass matrix ansatze are considered within the SM and SUSY GUTs where some predictive frameworks based on SU(5) and SO(10) are reviewed. We describe a variety of schemes to construct quark mass matrices in extra dimensions focusing on four major classes: models with the SM residing on 3-brane, models with universal extra dimensions, models with split fermions and models with warped extra dimensions. We outline how realistic patterns of quark mass matrices could be derived from orbifold models in heterotic superstring theory. Finally, we address the fermion mass problem in intersecting D-branes scenarios, and present models with D6-branes able to give a good quantitatively description of quark masses and mixing. The role of flavor/CP violation problem as a probe of new physics is emphasized.Comment: a review based on seminars presented by S.K. in different places, 34 pages, late

Meta-stable SUSY Breaking Model in Supergravity

We analyze a supersymmetry (SUSY) breaking model proposed by Intriligator, Seiberg and Shih in a supergravity (SUGRA) framework. This is a simple and natural setup which demands neither extra superpotential interactions nor an additional gauge symmetry. In the SUGRA setup, the U(1)R symmetry is explicitly broken by the constant term in the superpotential, and pseudo-moduli field naturally takes non-zero vacuum expectation value through a vanishing cosmological constant condition. Sfermions tend to be heavier than gauginos, and the strong-coupling scale is determined once a ratio of sfermion to gaugino masses is fixed.Comment: 13 page

Noncommutativity from the string perspective: modification of gravity at a mm without mm sized extra dimensions

We explore how the IR pathologies of noncommutative field theory are resolved when the theory is realized as open strings in background B-fields: essentially, since the IR singularities are induced by UV/IR mixing, string theory brings them under control in much the same way as it does the UV singularities. We show that at intermediate scales (where the Seiberg-Witten limit is a good approximation) the theory reproduces the noncommutative field theory with all the (un)usual features such as UV/IR mixing, but that outside this regime, in the deep infra-red, the theory flows continuously to the commutative theory and normal Wilsonian behaviour is restored. The resulting low energy physics resembles normal commutative physics, but with additional suppressed Lorentz violating operators. We also show that the phenomenon of UV/IR mixing occurs for the graviton as well, with the result that, in configurations where Planck's constant receives a significant one-loop correction (for example brane-induced gravity), the distance scale below which gravity becomes non-Newtonian can be much greater than any compact dimensions.Comment: 30 pages. Slight revision: clarified some points and added a referenc

Instanton induced charged fermion and neutrino masses in a minimal Standard Model scenario from intersecting D-branes

String instanton Yukawa corrections from Euclidean D-branes are investigated in an effective Standard Model theory obtained from the minimal U(3)xU(2)xU(1) D-brane configuration. In the case of the minimal chiral and Higgs spectrum, it is found that superpotential contributions are induced by string instantons for the perturbatively forbidden entries of the up and down quark mass matrices. Analogous non-perturbative effects generate heavy Majorana neutrino masses and a Dirac neutrino texture with factorizable Yukawa couplings. For this latter case, a specific example is worked out where it is shown how this texture can reconcile the neutrino data.Comment: 17 pages, 3 figure

WKB formalism and a lower limit for the energy eigenstates of bound states for some potentials

In the present work the conditions appearing in the WKB approximation formalism of quantum mechanics are analyzed. It is shown that, in general, a careful definition of an approximation method requires the introduction of two length parameters, one of them always considered in the text books on quantum mechanics, whereas the second one is usually neglected. Afterwards we define a particular family of potentials and prove, resorting to the aforementioned length parameters, that we may find an energy which is a lower bound to the ground energy of the system. The idea is applied to the case of a harmonic oscillator and also to a particle freely falling in a homogeneous gravitational field, and in both cases the consistency of our method is corroborated. This approach, together with the Rayleigh--Ritz formalism, allows us to define an energy interval in which the ground energy of any potential, belonging to our family, must lie.Comment: Accepted in Modern Physics Letters