I. Roditi

A family of nonextensive entropies

A generalized nonextensive two-parameter entropy is developed, along lines which unify current nonextensive frameworks. It recovers, as particular cases, the Tsallis and symmetric entropies, as well as the Boltzmann-Gibbs entropy. The properties of the new (q, q′)-entropy are analysed.

λ-point transition in quantum q-gases

Abstract We show that Bose-Einstein condensation is present in highly deformed q -gases, the critical temperature being higher than for the usual ideal boson gas. The specific heat C V has a λ-point transition behaviour not exhibited by non-deformed ideal gases.

HIGHLY DEFORMED q-OSCILLATOR SYSTEMS

We consider the large q limit of systems made of deformed Heisenberg operators. When the deformation parameter is infinite the Fock space and the statistical properties have a fermionic behaviour. We also investigate the ideal q-gas and find the virial expansion of its equation of state.

ν-DIMENSIONAL IDEAL QUANTUM q-GAS: BOSE-EINSTEIN CONDENSATION AND λ-POINT TRANSITION

We consider an ideal quantum q-gas in ν spatial dimensions and energy spectrum ωiα pα Departing from the Hamiltonian H=ω[N], we study the effect of the deformation on thermodynamic functions and equation of state of that system. The virial expansion is obtained for the high temperature (or low density) regime. The critical temperature is higher than in non-deformed ideal gases. We show that Bose-Einstein condensation always exists (unless when ν/α=1) for finite q but not for q=∞. Employing numerical calculations and selecting for v/α the values 3/2, 2 and 3, we show the critical temperature as a function of q, the specific heat CV and the chemical potential µ as functions of for q=1.05 and q=4.5. CV exhibits a λ-point discontinuity in all cases, instead of the cusp singularity found in the usual ideal gas. Our results indicate that physical systems which have quantum symmetries can exhibit Bose-Einstein condensation phenomenon, the critical temperature being favored by the deformation parameter.

A simpler version of the PMS application to perturbation theory

Abstract We present a slightly modified approach to the application of the principle of minimal sensitivity (PMS) used to improved results in perturbation theory. Calculations are shown to be substantially easier and essentially give the same results.

VIRIAL EXPANSION FOR AN ε-DEFORMED SYSTEM

We compute the virial expansion of an ideal quantum q gas equation of state for small values of the deformation parameter.

Vacuum polarization in d + 1/2 dimensions

Abstract We study the main properties of the one-loop vacuum polarization function ( Π α β ) for spinor QED in ‘ d + 1 2 dimensions’, i.e., with fields defined on M ⊂ R d + 1 such that M = { ( x 0 , … , x d ) | x d ⩾ 0 } , with bag-like boundary conditions on the boundary ∂ M = { ( x 0 , … , x d ) | x d = 0 } . We obtain an exact expression for the induced current due to an external constant electric field normal to the boundary. We show that, for the particular case of 2 + 1 dimensions, there is a transverse component for the induced current, which is localized on a region close to ∂ M . This current is a parity breaking effect purely due to the boundary.

The DELPHI microvertex detector

The main characteristics of the DELPHI Microvertex Detector are presented. The performance in terms of impact parameter resolution, association efficiency, and ambiguity is evaluated after two years of data taking at LEP.

The role of the central element in the quantum algebra underlying the twisted XXZ chain

Abstract We study the relationship among the XXZ Heisenberg model and three models obtained from it by various transformations. In particular, we emphasize the role of a non-trivial central element t Z in the underlying algebra and its relationship with the twisted boundary conditions, S N +1 ± = t ± N S 1 ± .We study the relationship among the XXZ Heisenberg model and three models obtained from it by various transformations. In particular, we emphasize the role of a non trivial central element

A simple interpolation for high and low momentum transfers in QCD

t^Z