arXiv subjects

A. Lavagno

Publications and source records attributed to A. Lavagno.

At least 55 records · Page 3Linked to original sources

arXiv2003

Non-extensive resonant reaction rates in astrophysical plasmas

We study two different physical scenarios of thermonuclear reactions in stellar plasmas proceeding through a narrow resonance at low energy or through the low energy wing of a wide resonance at high energy. Correspondingly, we derive two approximate analytical formulae in order to calculate thermonuclear resonant reaction rates inside very coupled and non ideal astrophysical plasmas in which non-extensive effects are likely to arise. Our results are presented as simple first order corrective factors that generalize the well known classical rates obtained in the framework of Maxwell-Boltzmann statistical mechanics. As a possible application of our results, we calculate the dependence of the total corrective factor with respect to the energy at which the resonance is located, in an extremely dense and non ideal carbon plasma.

nucl-th↗

arXiv2003

Color Superconductivity in Compact Stars and Gamma Ray Bursts

We study the effects of color superconductivity on the structure and formation of compact stars. We show that it is possible to satisfy most of recent observational boundaries on masses and radii if a diquark condensate forms in a hybrid or a quark star. Moreover, we find that a huge amount of energy, of the order of $10^{53}$ erg, can be released in the conversion from a (metastable) hadronic star into a (stable) hybrid or quark star, if the presence of a color superconducting phase is taken into account. Accordingly to the scenario proposed in Astrophys.J.586(2003)1250, the energy released in this conversion can power a Gamma Ray Burst. This mechanism can explain the recent observations indicating a delay, of the order of days or years, between a few Supernova explosions and the subsequent Gamma Ray Burst.

nucl-th↗

arXiv2003

Gamma Ray Bursts from delayed collapse of neutron stars to quark matter stars

We propose a model to explain how a Gamma Rays Burst can take place days or years after a supernova explosion. Our model is based on the conversion of a pure hadronic star (neutron star) into a star made at least in part of deconfined quark matter. The conversion process can be delayed if the surface tension at the interface between hadronic and deconfined-quark-matter phases is taken into account. The nucleation time (i.e. the time to form a critical-size drop of quark matter) can be extremely long if the mass of the star is small. Via mass accretion the nucleation time can be dramaticaly reduced and the star is finally converted into the stable configuration. A huge amount of energy, of the order of 10$^{52}$--10$^{53}$ erg, is released during the conversion process and can produce a powerful Gamma Ray Burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay proposed in GRB990705 and in GRB011211.

astro-ph↗

arXiv2002

Isospin dependence of the critical quark-deconfinement densities

We explore the dependence of the critical density, separating hadronic matter from a mixed phase of quarks and hadrons, on the ratio $Z/A$. We use both the MIT bag model and the Color Dielectric Model to describe the quark dynamics, while for the hadronic phase we employ various relativistic equations of state. We find that, if the parameters of quark models are fixed so that the existence of quark stars is allowed, then the critical density drops dramatically in the range $Z/A \sim $ 0.3--0.4. Moreover, for $Z/A \sim $ 0.3 the critical density is only slightly larger than the saturation density of symmetric nuclear-matter. This opens the possibility to verify the Witten-Bodmer hypothesis on absolute stability of quark matter using ground-based experiments in which neutron-rich nuclei are tested.

nucl-th↗

arXiv2002

Relativistic Nonextensive Thermodynamics

Starting from the basic prescriptions of the Tsallis' nonextensive thermostatistics, i.e. generalized entropy and normalized q-expectation values, we study the relativistic nonextensive thermodynamics and derive a Boltzmann transport equation that implies the validity of the H-theorem where a local nonextensive four-entropy density is considered. Macroscopic thermodynamic functions and the equation of state for a perfect gas are derived at the equilibrium.

cond-mat.stat-mech↗

arXiv2002

Non-conventional statistical effects in relativistic heavy-ions collisions

We show that non-conventional statistical effects (due to the presence of long range forces, memory effects, correlations and fluctuations) can be very relevant in the interpretation of the experimental observables in relativistic heavy-ions collisions. Transverse mass spectrum, transverse momentum fluctuations and rapidity spectra are analysed in the framework of the non-extensive statistical mechanics.

nucl-th↗

arXiv2001

Anomalous diffusion in non-equilibrium relativistic heavy-ion rapidity spectra

There is experimental and theoretical evidence that the broad rapidity distribution of net proton yield in central heavy-ion collisions at SPS energies could be a signal of non-equilibrium properties of the system. We show that the broad rapidity shape can be well reproduced in the framework of a non-linear relativistic Fokker-Planck dynamics which incorporates non-extensive statistics and anomalous diffusion.

hep-ph↗

arXiv2001

Generalized thermodynamics of q-deformed bosons and fermions

We study the thermostatistics of q-deformed bosons and fermions obeying the symmetric algebra and show that it can be built on the formalism of q-calculus. The entire structure of thermodynamics is preserved if ordinary derivatives are replaced by an appropriate Jackson derivative. In this framework, we derive the most important thermodynamic functions describing the q-boson and q-fermion ideal gases in the thermodynamic limit. We also investigate the semi-classical limit and the low temperature regime and demonstrate that the nature of the q-deformation gives rise to pure quantum statistical effects stronger than undeformed boson and fermion particles.

cond-mat.stat-mech↗

arXiv2001

Solar reaction rates, non-extensivity and quantum uncertainty

We show that in weakly non-ideal plasmas, like the solar interior, both non-extensivity and quantum uncertainty (a' la Galitskii and Yakimets) should be taken into account to derive equilibrium ion distribution functions and to estimate nuclear reaction rates and solar neutrino fluxes.

nucl-th↗

arXiv2000

On the use of a non-extensive distribution in the solar plasma

We describe the physical motivations why we are urged to look for an ionic equilibrium distribution function slightly different from the Maxwellian to calculate nuclear reaction rates in the solar core plasma answering, at the same time, to a recent paper by Bahcall et al. (astro-ph/0010055).

astro-ph↗

arXiv2000

Non-extensive statistics effects in quark-gluon plasma and in relativistic heavy-ion collisions

The presence of memory effects and color long-range forces among the many-parton system in the early stage of heavy-ion collisions can affect the particle statistical behavior at the freeze-out temperature. In this context, we calculate, in the framework of the equilibrium generalized non-extensive thermostatistics, the shape of pion transverse mass spectrum and the value of the transverse momentum correlation function of the pions emitted during the central Pb+Pb collisions and we show that the experimental results is well reproduced assuming very small deviations from the standard statistics.

nucl-th↗

arXiv2000

Generalized symmetric nonextensive thermostatistics and q-modified structures

We formulate a convenient generalization of the q-expectation value, based on the analogy of the symmetric quantum groups and q-calculus, and show that the q->q^{-1} symmetric nonextensive entropy preserves all of the mathematical structure of thermodynamics just as in the case of non-symmetric Tsallis statistics. Basic properties and analogies with quantum groups are discussed.

cond-mat.stat-mech↗

arXiv1999

Nonextensive statistics in stellar plasma and solar neutrinos

Nonextensive and quantum uncertainty effects (related to the quasiparticles composing the stellar core) have strong influence on the nuclear rates and, of course, affect solar neutrino fluxes. Both effects do coexist and are due to the frequent collisions among the ions. The weakly nonextensive nature of the solar core is confirmed. The range of predictions for the neutrino fluxes is enlarged and the solar neutrino problem becomes less dramatic.

nucl-th↗

arXiv1999

Thermostatistics of q-deformed boson gas

We show that a natural realization of the thermostatistics of q-bosons can be built on the formalism of q-calculus and that the entire structure of thermodynamics is preserved if we use an appropriate Jackson derivative in place of the ordinary thermodynamics derivative. This framework allows us to obtain a generalized q-boson entropy which depends on the q-basic number. We study the ideal q-boson gas in the thermodynamic limit which is shown to exhibit Bose-Einstein condensation with higher critical temperature and discontinuous specific heat.

quant-ph↗

arXiv1999

Non-extensive statistics, fluctuations and correlations in high energy nuclear collisions

Starting from the experimental evidence that high-energy nucleus-nucleus collisions cannot be described in terms of superpositions of elementary nucleon-nucleon interactions, we analyze the possibility that memory effects and long-range forces imply a nonextensive statistical regime during high energy heavy ion collisions. The relevance of these statistical effects and their compatibility with the available experimental data are discussed. In particular we show that theoretical estimates, obtained in the framework of the generalized nonextensive thermostatistics, can reproduce the shape of the pion transverse mass spectrum and explain the different physical origin of the transverse momentum correlation function of the pions emitted during the central Pb+Pb and during the p+p collisions at 158 A GeV.

nucl-th↗

arXiv1998

Nonextensive statistical effects in nuclear physics problems

Recent progresses in statistical mechanics indicate the Tsallis nonextensive thermostatistics as the natural generalization of the standard classical and quantum statistics, when memory effects and long-range forces are not negligible. In this framework, weakly nonextensive statistical deviations can strongly reduce the puzzling discrepancies between experimental data and theoretical previsions for solar neutrinos and for pion transverse-momentum correlations in Pb-Pb high-energy nuclear collisions.

nucl-th↗

arXiv1998

Thermal distributions in stellar plasmas, nuclear reactions and solar neutrinos

The physics of nuclear reactions in stellar plasma is reviewed with special emphasis on the importance of the velocity distribution of ions. Then the properties (density and temperature) of the weak-coupled solar plasma are analysed, showing that the ion velocities should deviate from the Maxwellian distribution and could be better described by a weakly-nonexstensive (|q-1|<0.02) Tsallis' distribution. We discuss concrete physical frameworks for calculating this deviation: the introduction of higher-order corrections to the diffusion and friction coefficients in the Fokker-Plank equation, the influence of the electric-microfield stochastic distribution on the particle dynamics, a velocity correlation function with long-time memory arising from the coupling of the collective and individual degrees of freedom. Finally, we study the effects of such deviations on stellar nuclear rates, on the solar neutrino fluxes, and on the pp neutrino energy spectrum, and analyse the consequences for the solar neutrino problem.

nucl-th↗

arXiv1998

Non-Markovian effects in the solar neutrino problem

The solar core, because of its density and temperature, is not a weakly-interacting or a high-temperature plasma. Collective effects have time scales comparable to the average time between collisions, and the microfield distribution influences the particle dynamics. In this conditions ion and electron diffusion is a non-Markovian process, memory effects are present and the equilibrium statistical distribution function differs from the Maxwellian one. We show that, even if the deviations from the standard velocity distribution that are compatible with our present knowledge of the solar interior are small, they are sufficient to sensibly modify the sub-barrier nuclear reaction rates. The consequent changes of the neutrino fluxes are comparable to the flux deficits that constitute the solar neutrino problem.

physics.plasm-ph↗