W.M. Alberico

Weak decay of Λ-hypernuclei

Abstract In this review we discuss the present status of strange nuclear physics, with special attention to the weak decay of Λ-hypernuclei. The models proposed for the evaluation of the Λ decay widths are summarized and their results are compared with the data. The rates Γ NM =Γ n +Γ p (+Γ 2 ) , Γπ0 and Γπ− are well explained by several calculations. Despite the intensive investigations of the last years, the main open problem remains a sound theoretical interpretation of the large experimental values of the ratio Γn/Γp. However, the large uncertainties involved in the experimental determination of the ratio do not allow to reach any definitive conclusion. The Γn/Γp puzzle is strongly related to the so-called ΔI=1/2 rule on the isospin change in the non-mesonic decay, whose possible violation cannot be established at present, again due to the insufficient precision of the data. Although recent works offer a step forward in the solution of the puzzle, further efforts (especially on the experimental side) must be invested in order to understand the detailed dynamics of the non-mesonic decay. Even if, by means of single nucleon spectra measurements, the error bars on Γn/Γp have been considerably reduced very recently at KEK (however, with central data compatible with older experiments), a clean extraction of Γn/Γp is needed. What is missing at present, but planned for the next future, are measurements of (1) nucleon energy spectra in double coincidence and (2) nucleon angular correlations: such observations allow to disentangle the nucleons produced in one- and two-body induced decays and lead to a direct determination of Γn/Γp. Notably, the two-body component of the non-mesonic decay rates has not been measured yet, due to the too low counting rates expected for a coincidence experiment. For the asymmetric non-mesonic decay of polarized hypernuclei the situation is even more puzzling. Indeed, strong inconsistencies appear already among data. A recent experiment obtained a positive intrinsic Λ asymmetry parameter, aΛ, for Λ 5 H → e . This is in complete disagreement with a previous measurement, which obtained a large and negative aΛ for p-shell hypernuclei, and with theory, which predicts a negative value moderately dependent on nuclear structure effects. Also in this case, improved experiment establishing with certainty the sign and magnitude of aΛ for s- and p-shell hypernuclei will provide a guidance for a deeper understanding of hypernuclear dynamics and decay mechanisms.

Quenching and hardening in the transverse quasi-elastic peak

Abstract We study in the RPA framework the collective response of symmetric, infinite nuclear matter to a spin-isospin sensitive probe with both σ · q and σ × q couplings. The two responses, similar in the low- q region, differ markedly for moderate momenta (≳1 fm −1 ). Indeed the collective effect manifests itself quite differently in the two responses; whereas the longitudinal one displays a softening and an enhancement (due to the attractive character of the associated particle-hole force), the transverse response is quenched and hardened with respect to the free Fermi gas. The existing experimental data, which we analyze, are compatible with our results. We also explore the total strengths and find that for repulsive forces they are appreciably reduced by the RPA correlations. A large part of this quenching comes from the Δ-excitation (LLEE effect), but some reduction is still present even when the nucleonic degrees of freedom are neglected. This illustrates a violation of strength conservation brought about by the RPA correlations in the spin-isospin channel.

Heavy-flavour spectra in high-energy nucleus-nucleus collisions

The propagation of the heavy quarks produced in relativistic nucleus–nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors RAA and the elliptic-flow coefficients v2 of electrons from heavy-flavor decay.

Weak decay of Lambda hypernuclei

Abstract In this review we discuss the present status of strange nuclear physics, with special attention to the weak decay of Λ-hypernuclei. The models proposed for the evaluation of the Λ decay widths are summarized and their results are compared with the data. The rates Γ NM =Γ n +Γ p (+Γ 2 ) , Γπ0 and Γπ− are well explained by several calculations. Despite the intensive investigations of the last years, the main open problem remains a sound theoretical interpretation of the large experimental values of the ratio Γn/Γp. However, the large uncertainties involved in the experimental determination of the ratio do not allow to reach any definitive conclusion. The Γn/Γp puzzle is strongly related to the so-called ΔI=1/2 rule on the isospin change in the non-mesonic decay, whose possible violation cannot be established at present, again due to the insufficient precision of the data. Although recent works offer a step forward in the solution of the puzzle, further efforts (especially on the experimental side) must be invested in order to understand the detailed dynamics of the non-mesonic decay. Even if, by means of single nucleon spectra measurements, the error bars on Γn/Γp have been considerably reduced very recently at KEK (however, with central data compatible with older experiments), a clean extraction of Γn/Γp is needed. What is missing at present, but planned for the next future, are measurements of (1) nucleon energy spectra in double coincidence and (2) nucleon angular correlations: such observations allow to disentangle the nucleons produced in one- and two-body induced decays and lead to a direct determination of Γn/Γp. Notably, the two-body component of the non-mesonic decay rates has not been measured yet, due to the too low counting rates expected for a coincidence experiment. For the asymmetric non-mesonic decay of polarized hypernuclei the situation is even more puzzling. Indeed, strong inconsistencies appear already among data. A recent experiment obtained a positive intrinsic Λ asymmetry parameter, aΛ, for Λ 5 H → e . This is in complete disagreement with a previous measurement, which obtained a large and negative aΛ for p-shell hypernuclei, and with theory, which predicts a negative value moderately dependent on nuclear structure effects. Also in this case, improved experiment establishing with certainty the sign and magnitude of aΛ for s- and p-shell hypernuclei will provide a guidance for a deeper understanding of hypernuclear dynamics and decay mechanisms.

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

Abstract. 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 non-extensive 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 non-extensive 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 GeV.

Mesonic correlation functions at finite temperature and density in the Nambu-Jona-Lasinio model with a Polyakov loop

We investigate the properties of scalar and pseudoscalar mesons at finite temperature and quark chemical potential in the framework of the Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (PNJL model) with the aim of taking into account features of both chiral symmetry breaking and deconfinement. The mesonic correlators are obtained by solving the Schwinger-Dyson equation in the RPA approximation with the Hartree (mean field) quark propagator at finite temperature and density. In the phase of broken chiral symmetry, a narrower width for the {sigma} meson is obtained with respect to the NJL case; on the other hand, the pion still behaves as a Goldstone boson. When chiral symmetry is restored, the pion and {sigma} spectral functions tend to merge. The Mott temperature for the pion is also computed.

Strangeness in the nucleon: neutrino-nucleon and polarized electron-nucleon scattering

Abstract After the EMC and subsequent experiments at CERN, SLAC and DESY on the deep inelastic scattering of polarized leptons on polarized nucleons, it is now established that the Q 2 =0 value of the axial strange form factor of the nucleon, a quantity which is connected with the spin of the proton and is quite relevant from the theoretical point of view, is relatively large. In this review, we consider different methods and observables that allow one to obtain information on the strange axial and vector form factors of the nucleon at different values of Q 2 . These methods are based on the investigation of the neutral current induced effects such as the P-odd asymmetry in the scattering of polarized electrons on protons and nuclei, the elastic neutrino (antineutrino) scattering on protons and the quasi-elastic neutrino (antineutrino) scattering on nuclei. We discuss in detail the phenomenology of these processes and the existing experimental data.

Non-extensive statistical effects in high-energy collisions

Following the basic prescriptions of the relativistic Tsallis’ non-extensive thermostatistics, we investigate from a phenomenological point of view the relevance of non-extensive statistical effects on relativistic heavy-ion collisions observables, such as rapidity spectra of the net proton production, transverse momentum distributions and transverse momentum fluctuations. Moreover, we study the nuclear and the subnuclear equation of state, investigating the critical densities of a phase transition to a hadron-quark-gluon mixed phase by requiring the Gibbs conditions on the global conservation of the electric and the baryon charges. The relevance of small deviations from the standard extensive statistics is studied in the context of intermediate- and high-energy heavy-ion collisions.

Freeze-out conditions from net-proton and net-charge fluctuations at RHIC

Abstract We calculate ratios of higher-order susceptibilities quantifying fluctuations in the number of net-protons and in the net-electric charge using the Hadron Resonance Gas (HRG) model. We take into account the effect of resonance decays, the kinematic acceptance cuts in rapidity, pseudo-rapidity and transverse momentum used in the experimental analysis, as well as a randomization of the isospin of nucleons in the hadronic phase. By comparing these results to the latest experimental data from the STAR Collaboration, we determine the freeze-out conditions from net-electric charge and net-proton distributions and discuss their consistency.

Heavy flavors in AA collisions: production, transport and final spectra

A multi-step setup for heavy-flavor studies in high-energy nucleus-nucleus (AA) collisions—addressing within a comprehensive framework the initial