Holographic Baryons from Oblate Instantons

We investigate properties of baryons in a family of holographic field theories related to the Sakai-Sugimoto model of holographic QCD. Starting with the $N_f=2$ Sakai-Sugimoto model, we truncate to a 5D Yang-Mills action for the gauge fields associated with the noncompact directions of the flavour D8-branes. We define a free parameter $\gamma$ that controls the strength of this Yang-Mills term relative to the Chern-Simons term that couples the abelian gauge field to the SU(2) instanton density. Moving away from $\gamma = 0$ should incorporate some of the effects of taking the Sakai-Sugimoto model away from large 't Hooft coupling $\lambda$. In this case, the baryon ground state corresponds to an oblate SU(2) instanton on the bulk flavour branes: the usual SO(4) symmetric instanton is deformed to spread more along the field theory directions than the radial direction. We numerically construct these anisotropic instanton solutions for various values of $\gamma$ and calculate the mass and baryon charge profile of the corresponding baryons. Using the value $\gamma = 2.55$ that has been found to best fit the mesonic spectrum of QCD, we find a value for the baryon mass of 1.19 GeV, significantly more realistic than the value 1.60 GeV computed previously using an SO(4) symmetric ansatz for the instanton.Comment: 22 pages, 9 figures. v2: Minor corrections, version accepted to JHEP. v3: A minor correctio

Quantum spin glass and the dipolar interaction

Systems in which the dipolar energy dominates the magnetic interaction, and the crystal field generates strong anisotropy favoring the longitudinal interaction terms, are considered. Such systems in external magnetic field are expected to be a good experimental realization of the transverse field Ising model. With random interactions this model yields a spin glass to paramagnet phase transition as function of the transverse field. Here we show that the off-diagonal dipolar interaction, although effectively reduced, destroys the spin glass order at any finite transverse field. Moreover, the resulting correlation length is shown to be small near the crossover to the paramagnetic phase, in agreement with the behavior of the nonlinear susceptibility in the experiments on \LHx. Thus, we argue that the in these experiments a cross-over to the paramagnetic phase, and not quantum criticality, was observed.Comment: To appear in Phys. Rev. Let

Simulating disease transmission dynamics at a multi-scale level

We present a model of the global spread of a generic human infectious disease using a Monte Carlo micro-simulation with large-scale parallel-processing. This prototype has been constructed and tested on a model of the entire population of the British Isles. Typical results are presented. A microsimulation of this order of magnitude of population simulation has not been previously attained. Further, an efficiency assessment of processor usage indicates that extension to the global scale is feasible. We conclude that the flexible approach outlined provides the framework for a virtual laboratory capable of supporting public health policy making at a variety of spatial scales.high-performance computing; global modelling; disease transmission

RANDOM MATRIX THEORY APPROACH TO THE INTENSITY DISTRIBUTIONS OF WAVES PROPAGATING IN A RANDOM MEDIUM

Statistical properties of coherent radiation propagating in a quasi - 1D random media is studied in the framework of random matrix theory. Distribution functions for the total transmission coefficient and the angular transmission coefficient are obtained.Comment: 8 pages, latex, no figures. Submitted to Phys.Rev.

Modeling SiO Maser Emission from Late-Type Stars

We have performed a thorough study of both radiative and collisional pumping of the SiO masers around late-type stars, carefully considering the combined and separate actions of each type of pump in order to gauge its effectiveness. We find that collisional pumping is severely underestimated when the model calculations use a small number (less than about 18) of rotational levels in each vibrational state. We have developed a procedure that corrects this problem and gives results that are nearly independent of the number of levels utilized in the calculations. We recognize, but do not solve, an important problem that afflicts all escape probability treatments which include maser saturation effects on the level populations. Maser radiation is strongly beamed and the functional form of the beaming angle must be known to properly calculate the maser escape probability. However, the beam pattern for saturated masers in the presence of large velocity gradients has yet to be studied in the literature. Our model is based on observations and theoretical arguments that place the SiO masers in high-density clumps rather than in the smooth stellar wind. Significantly, general conclusions can be reached which are independent of the pumping mechanism. Most importantly, the overall molecular density is restricted to lie between ~109-1010 cm-3, regardless of the type of pumping. In addition, both collisional and radiative pumps result in the production of a maser chain within each vibrational state, as observed. There are some important differences, however, between the pumping mechanisms. All pumps based on stellar radiation become less efficient with distance from the star because of the rapid decline in pump rate. This prevents any radiative pump from being able to produce the observed maser emission over most of the observed maser region. The only feasible radiative pumps require fine tuning of physical conditions and produce inversion only over a narrow range of optical depths that depends sensitively on the size of the velocity gradient and the form of the escape probability expression. In addition, these radiative pumps have difficulty in explaining the simultaneous production of masers in the same rotational transitions of adjacent vibrational states as is observed. We find that collisional pumping produces the strongest maser emission and, in contrast to radiative pumping, generates maser radiation over the entire observed region and does not require fine tuning of the physical parameters for its operation. Furthermore, there is a significant range of overlapping column densities where collisional pumping produces maser emission in the same rotational transitions of adjacent vibrational states, as observed. Collisional pumping thus appears to be the primary pumping mechanism responsible for the SiO maser phenomenon

A weather-driven model of malaria transmission

BACKGROUND: Climate is a major driving force behind malaria transmission and climate data are often used to account for the spatial, seasonal and interannual variation in malaria transmission. METHODS: This paper describes a mathematical-biological model of the parasite dynamics, comprising both the weather-dependent within-vector stages and the weather-independent within-host stages. RESULTS: Numerical evaluations of the model in both time and space show that it qualitatively reconstructs the prevalence of infection. CONCLUSION: A process-based modelling structure has been developed that may be suitable for the simulation of malaria forecasts based on seasonal weather forecasts

Statistics of fluctuations for two types of crossover: from ballistic to diffusive regime and from orthogonal to unitary ensemble

In our previous publication [Kogan et al, Phys. Rev. {\bf 48}, 9404 (1993)] we considered the issue of statistics of radiation diffusively propagating in a disordered medium. The consideration was in the framework of diagrammatic techniques and a new representation for the intensity distribution function in terms of connected diagrams only was proposed. Here we use similar approach to treat the issue of statistics in the regime of the crossover between ballistic and diffusive transport. We find that even small contribution from coherent component decreases by one half the intensity distribution function for small values of intensity and also produces oscillations of the distribution function. We also apply this method to study statistics of fluctuations of wave functions of chaotic electrons in a quantum dot in an arbitrary magnetic field, by calculating the single state local density in the regime of the crossover between the orthogonal and unitary ensemble.Comment: Revtex, 3 pages + 2 ps.figures in uuencoded file, a version which clarifies and unites the results of two previous submission