arXiv subjects

O. Civitarese

Publications and source records attributed to O. Civitarese.

At least 37 records · Page 2Linked to original sources

arXiv2017

Decoherence effect in neutrinos produced in micro-quasar jets

We study the effect of decoherence upon the neutrino spectra produced in micro-quasar jets. In order to analyse the precession of the polarization vector of neutrinos we have calculated its time evolution by solving the corresponding equations of motion, and by assuming two different scenarios, namely: (i) the mixing between two active neutrinos, and (ii) the mixing between one active and one sterile neutrino. We have found that for the case with two active neutrinos and large values of the neutrino-neutrino interactions the onset of decoherence is not manifest. For the active-sterile scheme decoherence becomes manifest if the strength of the neutrino-neutrino interactions ($μ$) and the ratio between the square-mass difference and the energy ($ω=\frac{δm^2}{2 E}$) satisfy the relation $\fracμ{w_{E=E_{\rm min}}}>0.1$.

astro-ph.HE↗

arXiv2017

Low energy meson spectrum from a QCD approach based on many-body methods

The TDA and RPA many-body methods are applied to a QCD motivated Hamiltonian in the Coulomb gauge. The gluon effects in the low energy domain are accounted for by the Instantaneous color-Coulomb Interaction between color-charge densities, approximated by the sum of a Coulomb and a confining linear potentials. We use the eigenfunctions of the harmonic oscillator as a basis for the quantization of the quark fields, and discuss how suitable this basis is in various steps of the calculation. We show that the TDA results already reproduce the gross-structure of the light flavored meson states. The pion-like state in the RPA description, which is a highly collective state, is in a better agreement with the experimental value. The results are related to other nonperturbative treatments and compared to experimental data. We discuss the advantages of the present approach.

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arXiv2017

Study of the $β$-decay of $^{100}$Tc with Total Absorption $γ$-Ray Spectroscopy

The \b{eta}-decay of 100 Tc has been studied using the Total Absorption γ-Ray Spectroscopy technique at IGISOL. In this work the new DTAS spectrometer in coincidence with a cylindrical plastic \b{eta} detector has been employed. The \b{eta}-intensity to the ground state obtained from the analysis is in good agreement with previous high-resolution measurements. However, differences in the feeding to the first excited state as well as weak feeding to a new level at high excitation energy have been deduced from this experiment. Theoretical calculations performed in the quasiparticle random- phase approximation (QRPA) framework are also reported. Comparison of these calculations with our measurement serves as a benchmark for calculations of the double \b{eta}-decay of 100 Mo.

nucl-ex↗

arXiv2016

Calculated WIMP signals at the ANDES laboratory: comparison with northern and southern located dark matter detectors

Weakly Interacting Massive Particles (WIMP) are possible components of the Universe's Dark Matter. The detection of WIMP is signalled by the recoil of the atomic nuclei which form a detector. CoGeNT at the Soudan Underground Laboratory (SUL) and DAMA at the Laboratori Nazionali del Gran Sasso (LNGS) have reported data on annual modulation of signals attributed to WIMP. Both experiments are located in laboratories of the northern hemisphere. Dark matter detectors are planned to operate (or already operate) in laboratories of the southern hemisphere, like SABRE at Stawell Underground Physics Laboratory (SUPL) in Australia, and DM-ICE in the South Pole. In this work we have analysed the dependence of diurnal and annual modulation of signals, pertaining to the detection of WIMP, on the coordinates of the laboratory, for experiments which may be performed in the planned new underground facility ANDES (Agua Negra Deep Experimental Site), to be built in San Juan, Argentina. We made predictions for NaI and Ge-type detectors placed in ANDES, to compare with DAMA, CoGeNT, SABRE and DM-ICE arrays, and found that the diurnal modulation of the signals, at the site of ANDES, is amplified at its maximum value, both for NaI (Ge)-type detectors, while the annual modulation remains unaffected by the change in coordinates from north to south.

astro-ph.CO↗

arXiv2014

Calculation of primordial abundances of light nuclei including a heavy sterile neutrino

We include the coupling of a heavy sterile neutrino with active neutrinos in the calculation of primordial abundances of light-nuclei. We calculate neutrino distribution functions and primordial abundances, as functions depending on a renormalization of the sterile neutrino distribution function $(a)$, the sterile neutrino mass $(m_s)$ and the mixing angle $(ϕ)$. Using the observable data, we set constrains on these parameters, which have the values $a < 0.60$, $\sin^2 ϕ=0.15$ and $m_s \approx 4$ keV, for a fixed value of the baryon to photon ratio. When the baryon to photon ratio is allowed to vary, its extracted value is in agreement with the values constrained by Planck observations and by the Wilkinson Microwave Anisotropy Probe (WMAP). It is found that the anomaly in the abundance of $^7$Li persists, in spite of the inclusion of a heavy sterile neutrino.

astro-ph.CO↗

arXiv2008

Effects due to Resonant and Continuum States on the Neutrino-Nucleus Cross Section

Estimates of the neutrino-nucleus cross section, for the charged-current process nu+208Pb-> e+208Bi, are presented. The nuclear structure calculations have been performed by considering bound, resonant, and continuum states in the single-particle basis used to construct correlated proton-particle neutron-hole configurations. The observed features of the spectrum of 208Bi have been reproduced, as accurately as possible, by diagonalizing a phenomenological multipole-multipole interaction. Calculations of the cross section, for values of q 200 $ MeV, were performed, and the dependence of the results upon the choice of the residual proton-neutron interaction was investigated. It is found that the inclusion of resonant states in the calculation of the nuclear wave functions increases the neutrino-nucleus cross section, and that the contribution of the continuum is negligible.

nucl-th↗

arXiv2008

Testing Primordial Abundances With Sterile Neutrinos

The mixing between sterile and active neutrinos is taken into account in the calculation of Big Bang Nucleosynthesis. The abundances of primordial elements, like D, 3He, 4He and 7Li, are calculated by including sterile neutrinos, and by using finite chemical potentials. It is found that the resulting theoretical abundances are consistent with WMAP data on baryonic densities, and with limits of LSND on mixing angles, only if 7Li is excluded from the statistical analysis of theoretical and experimental results.

astro-ph↗

arXiv2008

Effects of orbital occupancies on the neutrinoless beta-beta matrix element of 76Ge

In this work we use the recently measured neutron occupancies in the 76Ge and 76Se nuclei as a guideline to define the neutron quasiparticle states in the 1p0f0g shell. We define the proton quasiparticles by inspecting the odd-mass nuclei adjacent to 76Ge and 76Se. We insert the resulting quasiparticles in a proton-neutron quasiparticle random-phase approximation (pnQRPA) calculation of the nuclear matrix element of the neutrinoless double beta (0-nu-beta-beta) decay of 76Ge. A realistic model space and effective microscopic two-nucleon interactions are used. We include the nucleon-nucleon short-range correlations and other relevant corrections at the nucleon level. It is found that the resulting 0-nu-beta-beta matrix element is smaller than in the previous pnQRPA calculations, and closer to the recently reported shell-model results.

nucl-th↗

arXiv2007

Generalized rotational hamiltonians from nonlinear angular momentum algebras

Higgs algebras are used to construct rotational Hamiltonians. The correspondence between the spectrum of a triaxial rotor and the spectrum of a cubic Higgs algebra is demonstrated. It is shown that a suitable choice of the parameters of the polynomial algebra allows for a precise identification of rotational properties. The harmonic limit is obtained by a contraction of the algebra, leading to a linear symmetry.

nucl-th↗

arXiv2007

The Friedrichs-Model with fermion-boson couplings

In this work we present an extended version of the Friedrichs Model, which includes fermion-boson couplings. The set of fermion bound states is coupled to a boson field with discrete and continuous components. As a result of the coupling some of the fermion states may become resonant states. This feature suggests the existence of a formal link between the occurrence of Gamow Resonant States in the boson sector, as predicted by the standard Friedrichs Model, with similar effects in the set of solutions of the fermion central potential (Gamow fermion resonances). The structure of the solutions of the model is discussed by using different approximations to the model space. Realistic couplings constants are used to calculate fermion resonances in a heavy mass nucleus.

nucl-th↗

arXiv2007

The Friedrichs-Model with fermion-boson couplings II

In this work we present a formal solution of the extended version of the Friedrichs Model. The Hamiltonian consists of discrete and continuum bosonic states, which are coupled to fermions. The simultaneous treatment of the couplings of the fermions with the discrete and continuous sectors of the bosonic degrees of freedom leads to a system of coupled equations, whose solutions are found by applying standard methods of representation of bound and resonant states.

nucl-th↗

arXiv2007

Short-range correlations and neutrinoless double beta decay

In this work we report on the effects of short-range correlations upon the matrix elements of neutrinoless double beta decay. We focus on the calculation of the matrix elements of the neutrino-mass mode of neutrinoless double beta decays of 48Ca and 76Ge. The nuclear-structure components of the calculation, that is the participant nuclear wave functions, have been calculated in the shell-model scheme for 48Ca and in the proton-neutron quasiparticle random-phase approximation (pnQRPA) scheme for 76Ge. We compare the traditional approach of using the Jastrow correlation function with the more complete scheme of the unitary correlation operator method (UCOM). Our results indicate that the Jastrow method vastly exaggerates the effects of short-range correlations on the neutrinoless double beta decay nuclear matrix elements.

nucl-th↗

arXiv2006

The role of the continuum and the spurious 1- transitions in incoherent mu - e conversion rate calculations

By using the Continuum RPA (CRPA) method, the incoherent transition strength of the exotic mu - e conversion in the 208Pb and 40Ca nuclei is investigated. The question whether excited nuclear states lying high in the continuum give an important contribution to the incoherent rate is addressed. The admixture of spurious components in the rate coming from 1- excitations is investigated in detail by using the self-consistent CRPA with Skyrme interactions as well as a less consistent version and by employing two ways to remove the spurious strength: the use of effective operators or simply the exclusion of the spurious state appearing close to zero energy. In all cases, the correction achieved is quite large.

nucl-th↗

arXiv2004

Modelling Pentaquark and Heptaquark States

A schematic model for hadronic states, based on constituent quarks and antiquarks and gluon pairs, is discussed. The phenomenological interaction between quarks and gluons is QCD motivated. The obtained hadronic spectrum leads to the identification of nucleon and $Δ$ resonances and to pentaquark and heptaquark states. The predicted lowest pentaquark state ($J^π = {1/2}^-$) lies at the energy of 1.5 GeV and it is associated to the observed $Θ^+(1540)$ state. For heptaquarks ($J^π = {1/2}^+, {3/2}^+$) the model predicts the lowest state at 2.5 GeV.

nucl-th↗

arXiv2004

A schematic model for QCD. III : Hadronic states

The hadronic spectrum obtained in the framework of a QCD-inspired schematic model, is presented. The model is the extension of a previous version, whose basic degrees of freedom are constituent quarks and antiquarks, and gluons. The interaction between quarks and gluons is a phenomenological interaction and its parameters are fixed from data. The classification of the states, in terms of quark and antiquark and gluon configurations is based on symmetry considerations, and it is independent of the chosen interaction. Following this procedure, nucleon and $Δ$ resonances are identified, as well as various penta- and hepta-quarks states. The lowest pentaquarks state is predicted at 1.5 GeV and it has negative parity, while the lowest hepta-quarks state has positive parity and its energy is of the order of 2.5 GeV.

nucl-th↗

arXiv2003

Light-neutrino mass hierarchies, nuclear matrix elements, and the observability of neutrinoless double-beta decay

Results for neutrino flavor oscillations and neutrino mixing mechanisms, obtained from the analysis of the Sudbury Neutrino Observatory (SNO), the SuperKamiokande (SK), CHOOZ, KamLAND and WMAP data, are used to calculate the effective neutrino mass relevant for the neutrinoless double-beta decay. The observability of the decay of 76Ge is discussed within different light-neutrino mass hierarchies and by presenting a systematics on the available nuclear matrix elements.

nucl-th↗

arXiv2003

A schematic model for QCD I: Low energy meson states

A simple model for QCD is presented, which is able to reproduce the meson spectrum at low energy. The model is a Lipkin type model for quarks coupled to gluons. The basic building blocks are pairs of quark-antiquarks coupled to a definite flavor and spin. These pairs are coupled to pairs of gluons with spin zero. The multiplicity problem, which dictates that a given experimental state can be described in various manners, is removed when a particle-mixing interaction is turned on. In this first paper of a series we concentrates on the discussion of meson states at low energy, the so-called zero temperature limit of the theory. The treatment of baryonic states is indicated, also.

nucl-th↗

arXiv2003

A schematic model for QCD II: finite temperature regime

A schematic model for QCD, developed in a previous paper, is applied to calculate meson properties in the high temperature (up to 0.5 GeV) regime. It is a Lipkin model for quark-antiquark pairs coupled to gluon pairs of spin zero. The partition function is constructed with the obtained meson spectrum and several thermodynamical observables are calculated, like: the energy density, heat capacity, as well as relative production rates of mesons and absolute production rates for pions and kaons. The model predictions show a qualitative agreement with data. Based on these results we advocate the use of the model as a toy model for QCD.

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