Two-Loop O(αsGFmt2){\cal O}(\alpha_sG_Fm_t^2) Corrections to the Fermionic Decay Rates of the Standard-Model Higgs Boson

Low- and intermediate mass Higgs bosons decay preferably into fermion pairs. The one-loop electroweak corrections to the respective decay rates are dominated by a flavour-independent term of ${\cal O}(G_Fm_t^2)$. We calculate the two-loop gluon correction to this term. It turns out that this correction screens the leading high-$m_t$ behaviour of the one-loop result by roughly 10\%. We also present the two-loop QCD correction to the contribution induced by a pair of fourth-generation quarks with arbitrary masses. As expected, the inclusion of the QCD correction considerably reduces the renormalization-scheme dependence of the prediction.Comment: 14 pages, latex, figures 2-5 appended, DESY 94-08

Formation of superdense hadronic matter in high energy heavy-ion collisions

We present the detail of a newly developed relativistic transport model (ART 1.0) for high energy heavy-ion collisions. Using this model, we first study the general collision dynamics between heavy ions at the AGS energies. We then show that in central collisions there exists a large volume of sufficiently long-lived superdense hadronic matter whose local baryon and energy densities exceed the critical densities for the hadronic matter to quark-gluon plasma transition. The size and lifetime of this matter are found to depend strongly on the equation of state. We also investigate the degree and time scale of thermalization as well as the radial flow during the expansion of the superdense hadronic matter. The flow velocity profile and the temperature of the hadronic matter at freeze-out are extracted. The transverse momentum and rapidity distributions of protons, pions and kaons calculated with and without the mean field are compared with each other and also with the preliminary data from the E866/E802 collaboration to search for experimental observables that are sensitive to the equation of state. It is found that these inclusive, single particle observables depend weakly on the equation of state. The difference between results obtained with and without the nuclear mean field is only about 20\%. The baryon transverse collective flow in the reaction plane is also analyzed. It is shown that both the flow parameter and the strength of the ``bounce-off'' effect are very sensitive to the equation of state. In particular, a soft equation of state with a compressibility of 200 MeV results in an increase of the flow parameter by a factor of 2.5 compared to the cascade case without the mean field. This large effect makes it possible to distinguish the predictions from different theoretical models and to detect the signaturesComment: 55 pages, latex, + 39 figures available upon reques

Conformal Field Theory Approach to the 2-Impurity Kondo Problem: Comparison with Numerical Renormalization Group Results

Numerical renormalization group and conformal field theory work indicate that the two impurity Kondo Hamiltonian has a non-Fermi liquid critical point separating the Kondo-screening phase from the inter-impurity singlet phase when particle-hole (P-H) symmetry is maintained. We clarify the circumstances under which this critical point occurs, pointing out that there are two types of P-H symmetry. Only one of them guarantees the occurance of the critical point. Much of the previous numerical work was done on models with the other type of P-H symmetry. We analyse this critical point using the boundary conformal field theory technique. The finite-size spectrum is presented in detail and compared with about 50 energy levels obtained using the numerical renormalization group. Various Green's functions, general renormalization group behaviour, and a hidden $SO(7)$ are analysed.Comment: 38 pages, RevTex. 2 new sections clarify the circumstances under which a model will exhibit the non-trivial critical point (hence potentially resolving disagreements with other Authors) and explain the hidden SO(7) symmetry of the model, relating it to an alternative approach of Sire et al. and Ga

Heavy-Higgs Lifetime at Two Loops

The Standard-Model Higgs boson with mass $M_H >> 2M_Z$ decays almost exclusively to pairs of $W$ and $Z$ bosons. We calculate the dominant two-loop corrections of $O( G_F^2 M_H^4 )$ to the partial widths of these decays. In the on-mass-shell renormalization scheme, the correction factor is found to be $1 + 14.6$, where the second term is the one-loop correction. We give full analytic results for all divergent two-loop Feynman diagrams. A subset of finite two-loop vertex diagrams is computed to high precision using numerical techniques. We find agreement with a previous numerical analysis. The above correction factor is also in line with a recent lattice calculation.Comment: 26 pages, 6 postscript figures. The complete paper including figures is also available via WWW at http://www.physik.tu-muenchen.de/tumphy/d/T30d/PAPERS/TUM-HEP-247-96.ps.g

Heavy quark mass determination from the quarkonium ground state energy: a pole mass approach

The heavy quark pole mass in perturbation theory suffers from a renormalon caused, inherent uncertainty of $O(\Lambda_{\rm QCD})$. This fundamental difficulty of determining the pole mass to an accuracy better than the inherent uncertainty can be overcome by direct resummation of the first infrared renormalon. We show how a properly defined pole mass as well as the $\bar {\rm MS}$ mass for the top and bottom quarks can be determined accurately from the $O(m\alpha_s^5)$ quarkonium ground state energy.Comment: 16 pages; published versio

Flat histogram simulation of lattice polymer systems

We demonstrate the use of a new algorithm called the Flat Histogram sampling algorithm for the simulation of lattice polymer systems. Thermodynamics properties, such as average energy or entropy and other physical quantities such as end-to-end distance or radius of gyration can be easily calculated using this method. Ground-state energy can also be determined. We also explore the accuracy and limitations of this method. Key words: Monte Carlo algorithms, flat histogram sampling, HP model, lattice polymer systemsComment: 7 RevTeX two-column page

A role for dual viral hits in causation of subacute sclerosing panencephalitis

Subacute sclerosing panencephalitis (SSPE) is a progressive fatal neurodegenerative disease associated with persistent infection of the central nervous system (CNS) by measles virus (MV), biased hypermutations of the viral genome affecting primarily the matrix (M) gene with the conversion of U to C and A to G bases, high titers of antibodies to MV, and infiltration of B cells and T cells into the CNS. Neither the precipitating event nor biology underlying the MV infection is understood, nor is their any satisfactory treatment. We report the creation of a transgenic mouse model that mimics the cardinal features of SSPE. This was achieved by initially infecting mice expressing the MV receptor with lymphocytic choriomeningitis virus Cl 13, a virus that transiently suppressed their immune system. Infection by MV 10 days later resulted in persistent MV infection of neurons. Analysis of brains from infected mice showed the biased U to C hypermutations in the MV M gene and T and B lymphocyte infiltration. These sera contained high titers of antibodies to MV. Thus, a small animal model is now available to both molecularly probe the pathogenesis of SSPE and to test a variety of therapies to treat the disease

Feasibility, Compliance, and Efficacy of a Randomized Controlled Trial Using Vibration in Pre-pubertal Children

Objective: Interventions utilizing vibration may increase bone mass and size which may reduce forearm fractures in children. This randomized controlled pilot trial tested the feasibility, compliance and efficacy of forearm loading regimes in an after-school program in pre-pubertal children aged 6-10 years. Methods: A 12-week randomized controlled trial incorporated high (HMMS; N=10) and low (LMMS; N=10) magnitude mechanical stimulation vibration, floor exercises (N=9), and controls (N=10). Radial bone measures by DXA and pQCT were compared at the end of intervention (12-weeks) and 4-months post-intervention (4- months post). Results: Percent changes were significantly greater in floor vs. control for ultra-distal areal BMD by DXA at 12- weeks (1%[-2,5] vs.-5%[-8,-2] respectively, p=0.02) and 4-months post (5%[1,8] vs -2%[-5,2], p=0.03) and in HMMS vs. controls for trabecular vBMD by pQCT at 12-weeks (4%[0, 8], vs. -8% [-14, -2], p=0.02). Children exposed to HMMS showed positive changes in cortical BMC, area, and cortical vBMD after 12 weeks that remained 4 months post-intervention. Children exposed to floor exercise showed positive changes in cortical BMC, area, and periosteal circumference 4-months post-intervention. Controls had decreased trabecular BMD, but increased bone area and periosteal circumference. Conclusions: Exposure to floor exercise and HMMS increased trabecular aBMD and vBMD in the radius

Determining the density of states for classical statistical models: A random walk algorithm to produce a flat histogram

We describe an efficient Monte Carlo algorithm using a random walk in energy space to obtain a very accurate estimate of the density of states for classical statistical models. The density of states is modified at each step when the energy level is visited to produce a flat histogram. By carefully controlling the modification factor, we allow the density of states to converge to the true value very quickly, even for large systems. This algorithm is especially useful for complex systems with a rough landscape since all possible energy levels are visited with the same probability. In this paper, we apply our algorithm to both 1st and 2nd order phase transitions to demonstrate its efficiency and accuracy. We obtained direct simulational estimates for the density of states for two-dimensional ten-state Potts models on lattices up to $200 \times 200$ and Ising models on lattices up to $256 \times 256$. Applying this approach to a 3D $\pm J$ spin glass model we estimate the internal energy and entropy at zero temperature; and, using a two-dimensional random walk in energy and order-parameter space, we obtain the (rough) canonical distribution and energy landscape in order-parameter space. Preliminary data suggest that the glass transition temperature is about 1.2 and that better estimates can be obtained with more extensive application of the method.Comment: 22 pages (figures included

Reexamination of the long-range Potts model: a multicanonical approach

We investigate the critical behavior of the one-dimensional q-state Potts model with long-range (LR) interaction $1/r^{d+\sigma}$, using a multicanonical algorithm. The recursion scheme initially proposed by Berg is improved so as to make it suitable for a large class of LR models with unequally spaced energy levels. The choice of an efficient predictor and a reliable convergence criterion is discussed. We obtain transition temperatures in the first-order regime which are in far better agreement with mean-field predictions than in previous Monte Carlo studies. By relying on the location of spinodal points and resorting to scaling arguments, we determine the threshold value $\sigma_c(q)$ separating the first- and second-order regimes to two-digit precision within the range $3 \leq q \leq 9$. We offer convincing numerical evidence supporting $\sigma_c(q)Comment: 18 pages, 18 figure