Inclusive one- and two-jet cross sections in γγ reactions at e+e− colliders

We have calculated inclusive one- and two jet production in photon-photon collisions superimposing direct, single-resolved and double-resolved cross sections for center of mass energies of the LEP1, LEP2 and NLC range. The direct and single-resolved cross sections are calculated up to next-to-leading order. The double-resolved two-jet cross section is calculated only in LO with a k factor estimated from the NLO one jet cross section. Various differential cross sections as functions of transverse momenta and rapidities of the jets are evaluated.     

Optimal myopic policy for a stochastic inventory problem with fixed and proportional backorder costs

In this paper, we consider a single product, periodic review, stochastic demand inventory problem where backorders are allowed and penalized via fixed and proportional backorder costs simultaneously. Fixed backorder cost associates a one-shot penalty with stockout situations whereas proportional backorder cost corresponds to a penalty for each demanded but yet waiting to be satisfied item. We discuss the optimality of a myopic base-stock policy for the infinite horizon case. Critical number of the infinite horizon myopic policy, i.e., the base-stock level, is denoted by S. If the initial inventory is below S then the optimal policy is myopic in general, i.e., regardless of the values of model parameters and demand density. Otherwise, the sufficient condition for a myopic optimum requires some restrictions on demand density or parameter values. However, this sufficient condition is not very restrictive, in the sense that it holds immediately for Erlang demand density family. We also show that the value of S can be computed easily for the case of Erlang demand. This special case is important since most real-life demand densities with coefficient of variation not exceeding unity can well be represented by an Erlang density. Thus, the myopic policy may be considered as an approximate solution, if the exact policy is intractable. Finally, we comment on a generalization of this study for the case of phase-type demands, and identify some related research problems which utilize the results presented here.     

Mesure de la dispersion pour la solution éléments finis de l'équation de Helmholtz

The numerical solution of the Helmholtz equation suffers from a dispersion error. A general method to calculate a priori the dispersion error is formulated and applied to the methods suggested in the literature in order to minimize the dispersion.     

Search for antihelium in cosmic rays

The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle Discovery during flight STS-91 in a 51.7° orbit at altitudes between 320 and 390 km. A total of 2.86×10⁶ helium nuclei were observed in the rigidity range 1 to 140 GV. No antihelium nuclei were detected at any rigidity. An upper limit on the flux ratio of antihelium to helium of <1.1×10⁻⁶ is obtained.     

On the precision of calculations of initial state radiation in the LEP Z line-shape fits

Calculation of initial state radiation (ISR) in the fitting program MIZA and in the Standard Model program BHM is improved within the analysis of the Z line-shape measurements at LEP. The uncertainty on the determination of the Z line-shape parameters coming from the precision of ISR calculations is 2×10⁻⁴ on σ₀ and 0.1 MeV on m_Z and Γ_Z.     

On the quark-antiquark potential at short distances

We investigate the static quark-antiquark potential up to distances of 8 lattice units for pure SU(2) gauge theory on lattices with anisotropic couplings. The action is the Wilson action with a coupling for time-like plaquettes which differs from those for space-like ones. Numerical simulations are performed in a large range of β The potential is obtained by fitting “cooled” Wilson loops with up to four exponential terms. An interpolation of the potentials by a sum of a perturbative term, a linear term and by lattice artifacts shows poor scaling in comparison with he isotropic case. If the coupling in the time-like region is reduced, the linear term is much smaller than in the isotropic case, and vice versa. Consequences for the bag picture for hadrons are discussed.     

Charge/color breaking minima and A-parameter bounds in supersymmetric models

In supersymmetric models, the requirement that the global potential minimum not violate color and/or electric charge invariance implies restrictions on the parameters of the model. We reassess and improve upon constraints on the A parameter that have been obtained previously in the literature. In particular, we demonstrate that no universally applicable bound which is either necessary or sufficient can be given.