arXiv subjects

O. Civitarese

Publications and source records attributed to O. Civitarese.

At least 19 recordsLinked to original sources

arXiv2026

Solution to the uncertainty problem of nuclear matrix element for neutrinoless double-$β$ decay

The neutrinoless double-$β$ decay ($0νββ$) of nuclei is one of the major research subjects of neutrino physics nowadays because of its influence on particle physics and astrophysics. The predicted nuclear matrix elements (NMEs) for the $0νββ$ decay exhibit large uncertainties depending on the models employed. This problem has affected the development of neutrino physics for many years. We have recently performed the calculation of the NMEs for the $0νββ$ and two-neutrino double-$β$ decay ($2νββ$) modes with a perturbed transition operator and found that the effective axial-vector current coupling $g_A^\mathrm{eff}$ is similar for the two decay modes. Based on this finding, we calculate the $0νββ$ NMEs using the phenomenological $g_A^\mathrm{eff}$ that reproduces the measured half-life of the $2νββ$ decay. We apply this method to the NMEs for $^{136}$Xe, $^{130}$Te, and $^{76}$Ge obtained by several groups and show that the uncertainty of the $0νββ$ NME is dramatically reduced. Based on this result, we calculate the effective neutrino mass, consistent with the current experimental lower limit of the half-life for the $0νββ$ decay. The results indicate that the extracted value of the effective neutrino mass does not yet reach the inverted-mass-hierarchy region allowed by neutrino oscillation data when the lightest neutrino mass is assumed to be below 10 meV. We also calculate the perturbed $0νββ$ and $2νββ$ NMEs of $^{110}$Pd.

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arXiv2025

Half-life of $^{136}$Xe for neutrinoless double-$β$ decay calculated with effective axial-vector current coupling unified for two-neurtino and neutrinoless double-$β$ decay modes

The upper limit on the mass of the Majorana neutrino, extracted from the limits on the nonobservation of the neutrinoless double-$β$ ($0νββ$) decay, is hampered by uncertainties in the matrix elements of the transition operators. Recently, we have shown that the values of the effective axial-vector current coupling constants ($g_A^\textrm{eff}$) for the $0νββ$ and the two-neutrino double-$β$ decays are close. This striking result was obtained for the first time by including vertex corrections and two-body currents in these matrix elements. In this letter, we calculate the half-life for the $0νββ$ decay ($T_{1/2}^{0ν}$) of $^{136}$Xe using this closeness and show the convergence of the half-life with respect to the variation of the method to determine $g_A^\textrm{eff}$. The closeness of the $g_A^\textrm{eff}$ of the two decay modes plays a decisive role in predicting $T_{1/2}^{0ν}$. The appropriate value of $g_A^\textrm{eff}$ depends on the assumptions made for the sectors of the nuclear structure and transition operators of the calculations within the perturbation scheme. The value $g_A^\textrm{eff}\approx 1$ is obtained when the SkM$^\ast$ is used to describe the nuclear structure component, while a smaller value of $g_A^\textrm{eff}$ is obtained by applying a less realistic interaction like the SGII one.

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arXiv2025

Effects of perturbation for transition operator of double-$β$ decay on nuclear matrix element, effective axial-vector current coupling, and half-life

We calculate the nuclear matrix element (NME), effective axial-vector current coupling $g_A^\mathrm{eff}$, and half-life of the double-$β$ ($ββ$) decay using the transition operator perturbed by the nuclear interaction. The correction terms for the NME are obtained by extending the hadron sector to a higher order in terms of the Rayleigh-Schrödinger perturbation theory. The NME calculations are performed for the neutrinoless $ββ$ ($0νββ$) and the two-neutrino $ββ$ ($2νββ$) decays of $^{136}$Xe. The nuclear wave functions are calculated by the quasiparticle random-phase approximation (QRPA) with the Skyrme, the Coulomb, and the contact pairing interactions. Sufficiently large single-particle valence spaces are used. The correction terms for the NME are comparable with the leading term in absolute value, and the sum of the corrections has the opposite sign to that of the leading term. The $g_A^\mathrm{eff}$'s for the $ββ$ NME are calculated by a few methods depending on the truncation of the NME and the half-life referred to. Similarities are found between some of these $g_A^\mathrm{eff}$'s including those of the $0νββ$ NME. This leads to the conclusion that the value of $g_A^\mathrm{eff}$ can indeed be determined by the perturbed transition operator. It is in a comparable range of the $g_A$ for the $2νββ$ NME. The perturbation effect on the $2νββ$ half-life is discussed by comparing the calculated half-lives with the different $g_A$'s and the NME components.

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arXiv2023

Nuclear response to dark matter signals in Ge and Xe odd-mass targets

Abstract: The interaction of dark matter particles (WIMPs) with the odd-mass $^{73}$Ge and $^{131}$Xe target nuclei ,{ {that is the recoil rates corresponding to the elastic scattering of WIMPs by these nuclei}}, is analysed in the context of the minimal extensions of the SUSY model. The BCS+QRPA technique plus the quasiparticle-phonon coupling scheme is used to describe the nuclear structure part of the calculations. The resulting values for the nuclear spin content of both nuclei are compared to values previously reported in the literature.

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arXiv2022

Dark matter, supernova neutrinos and other backgrounds in direct dark matter searches. The ANDES laboratory prospects

Dark Matter particles can be detected directly via their elastic scattering with nuclei. Next generation experiments can eventually find physical evidences about dark matter candidates. With this motivation in mind, we have calculated the expected signals of dark matter particles in xenon detectors. The calculations were performed by considering different masses and parameters within the minimal supersymmetric standard model. Since the detectors can also detect neutrinos, we have analyzed the supernova neutrino signal including a sterile neutrino in the formalism. Using this 3+1 scheme, we make predictions for both the normal and inverse mass hierarchy. In order to perform a study of the response of planned direct-detection experiments, to be located in ANDES (Agua Negra Deep Experimental Site), we have calculated the neutrino contributions to the background by taken into account reactor's neutrinos and geoneutrinos at the site of the lab. As a test detector, we take a Xenon1T-like array.

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arXiv2022

Neutrino interactions in liquid scintillators including active-sterile neutrino mixing

Neutrinos play an important role in core-collapse supernova events since they are a key piece to understand the explosion mechanisms. The analysis of the neutrino fluxes can bring answers to neutrino's related problems e.g.: mass hierarchy, spectral splitting, sterile neutrinos, etc. In this work we study the impact of neutrino oscillations and the possible existence of eV sterile neutrinos upon the supernova neutrino flux $F_ν(E)$. We have calculated the energy distribution of the neutrino flux from a supernova and the total number of events which would be detected in a liquid scintillator. We also present an analysis for the conversion probabilities as a function of the active-sterile neutrino mixing parameters. Finally, we have carried out a statistical analysis to extract values for the mixing parameters of the model.

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arXiv2021

MSSM WIMPs-nucleon cross section for E$_χ<$ 500 GeV

Among dark-matter candidates are the WIMPs (Weekly Interacting Massive Particles). Low-threshold detectors could directly detect dark-matter by measuring the energy deposited by the particles. In this work we examine the cross section for the elastic scattering of WIMPs on nucleons, in the spin-dependent and spin-independent channels. WIMPs are taken as neutralinos in the context of the minimal super-symmetric extension of the standard model (MSSM). The dependence of the results with the adopted MSSM parameters is discussed.

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arXiv2021

X-ray spectra, light-curves and SEDs of blazars frequently observed by Swift

Blazars research is one of the hot topics of contemporary extra-galactic astrophysics. That is because these sources are the most abundant type of extra-galactic gamma-ray sources and are suspected to play a central role in multi-messenger astrophysics. We have used swift_xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count-rates and flux estimations in several energy bands of over 31,000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multi-frequency archival data to assemble the broad-band Spectral Energy Distributions (SEDs) and the long-term light-curves of all sources in the sample. Our study shows that large X-ray luminosity variability on different timescales is present in all objects. Spectral changes are also frequently observed with a "harder-when-brighter" or "softer-when-brighter" behavior depending on the SED type of the blazars. The peak energy of the synchrotron component nu_peak in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.

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arXiv2021

Limits on active-sterile neutrino mixing parameters using heavy nuclei abundances

The production of heavy-mass elements due to the rapid neutron-capture mechanism (r-process) is associated with astrophysical scenarios, such as supernovae and neutron-star mergers. In the r-process the capture of neutrons is followed by $β$-decays until nuclear stability is reached. A key element in the chain of nuclear weak-decays leading to the production of isotopes may be the change of the parameters controlling the neutrino sector, due to the mixing of active and sterile species. In this work we have addressed this question and calculated $β$-decay rates for the nuclei involved in the r-process chains as a function of the neutrino mixing parameters. These rates were then used in the calculation of the abundance of the heavy elements produced in core-collapse supernova and in neutron-star mergers, starting from different initial mass-fraction distributions. The analysis shows that the core-collapse supernova environment contributes with approximately $30\%$ of the total heavy nuclei abundance while the neutron-star merger contributes with about $70\%$ of it. Using available experimental data we have performed a statistical analysis to set limits on the active-sterile neutrino mixing angle and found a best-fit value $\sin^2 2θ_{14}=0.22$, a value comparable with those found in other studies reported in the literature.

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arXiv2020

The Open Universe survey of Swift-XRT GRB fields: a complete sample of HBL blazars

We have analysed all the X-ray images centred on Gamma Ray Bursts generated by Swift over the last 15 years using automatic tools that do not require any expertise in X-ray astronomy, producing results in excellent agreement with previous findings. This work, besides presenting the largest medium-deep survey of the X-ray sky and a complete sample of blazars, wishes to be a step in the direction of achieving the ultimate goal of the Open Universe Initiative, that is to enable non expert people to fully benefit of space science data, possibly extending the potential for scientific discovery, currently confined within a small number of highly specialised teams, to a much larger population. We have used the Swift_deepsky Docker container encapsulated pipeline to build the largest existing flux-limited and unbiased sample of serendipitous X-ray sources. Swift_deepsky runs on any laptop or desktop computer with a modern operating system. The tool automatically downloads the data and the calibration files from the archives, runs the official Swift analysis software and produces a number of results including images, the list of detected sources, X-ray fluxes, SED data, and spectral slope estimations. We used our source list to build the LogN-LogS of extra-galactic sources, which perfectly matches that estimated by other satellites. Combining our survey with multi-frequency data we selected a complete radio flux-density limited sample of High Energy Peaked (HBL) blazars.

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arXiv2020

Neutrino mixing in nuclear rapid neutron-capture processes

A possible mechanism for the formation of heavy-mass elements in supernovae is the rapid neutron-capture-mechanism (r-process). It depends upon the electron-fraction $Y_e$, a quantity which is determined by beta-decay-rates. In this paper, we focus on the calculation of electroweak decay-rates in presence of massive neutrinos. The resulting expressions are then used to calculate nuclear reactions entering the rapid-neutron capture. We fix the astrophysical parameters to the case of a core-collapse supernova. The neutrino sector includes a mass scheme and mixing angles for active neutrinos, and also by including the mixing between active and sterile neutrinos. The results of the calculations show that the predicted abundances of heavy-mass nuclei are indeed affected by the neutrino mixing.

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arXiv2019

Open Universe for Blazars: a new generation of astronomical products based on 14 years of Swift-XRT data

Open Universe for blazars is a set of high-transparency data products for blazar science, and the tools designed to generate them. Blazar astrophysics is becoming increasingly data driven, depending on the integration and combined analysis of large quantities of data from the entire span of observational astrophysics techniques. The project was therefore chosen as one of the pilot activities within the United Nations Open Universe Initiative. In this work we developed a data analysis pipeline called Swift_deepsky, based on the Swift XRTDAS software and the XIMAGE package, encapsulated into a Docker container. Swift_deepsky, downloads and reads low-level data, generates higher-level products, detects X-ray sources and estimates several intensity and spectral parameters for each detection, thus facilitating the generation of complete and up-to-date science-ready catalogues from an entire space-mission dataset. The Docker version of the pipeline and its derived products is publicly available from the Open Universe Website at openuniverse.asi.it. We present the results of a detailed X-ray image analysis based on Swift_deepsky on all Swift XRT observations including a known blazar, carried out during the first 14 years of operations of the Swift Observatory. The resulting database includes over 27,000 images integrated in different X-ray bands, and a catalogue, called 1OUSXB, that provides intensity and spectral information for 33,396 X-ray sources, 8,896 of which are single or multiple detections of 2,308 distinct blazars. All the results can be accessed on-line in a variety of ways: e.g., from the Open Universe portal at openuniverse.asi.it, through Virtual Observatory services, via the VOU-Blazar tool and the SSDC SED builder. One of the most innovative aspects of this work is that the results can be safely reproduced and extended by anyone.

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arXiv2019

Neutrino oscillations and decoherence in short-GRB progenitors

Neutrinos are produced in cosmic accelerators, like active galactic nuclei (AGNs), blazars, supernova (SN) remnants and gamma-ray bursts (GRBs). On their way to the Earth they experience flavor-oscillations. The interactions of the neutrinos coming from the source with other particles, e.g. intergalactic primordial neutrinos or heavy-mass right-handed neutrinos, in their way to the detector may transform the original wave packet in pointer states. This phenomenon, known as decoherence, becomes important in the reconstruction of processes at the source. In this work we study neutrino emission in short GRBs by adopting the Fireshell Model. We consider $e^{-}e^{+}$-pair annihilation as the main channel for neutrino production. We compare the properties of the neutrino-flux with the characteristic photon-signal produced once the transparency condition is reached. We study the effects of flavor-oscillations and decoherence as neutrinos travel from the region near the black-hole (BH) event-horizon outwards. We consider the source to be in thermal equilibrium, and calculate energy distribution functions for electrons and neutrinos. To compute the effects of decoherence we use a Gaussian model. In this scenario the emitted electron-neutrinos transform into pointer states consisting of $67.8\%$ electron-neutrinos and $32.2\%$ as a combination of mu and tau neutrinos. We found that decoherence plays an important role in the evolution of the neutrino wave packet, leading to the detected pointer states on Earth.

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arXiv2019

Time-delay between neutrinos and gamma-rays in short GRBs

In this work we make an estimate of the time-delay between signals, recorded at detectors on Earth, of neutrinos and photons originated in a short gamma-ray burst. We describe the geometry and dynamics of the system according to the Fireshell model. The delay in the photon's arrival time is produced because the system is originally opaque to radiation; thus, the photons remain trapped and thermalize until the transparency condition is reached. We calculate the time interval between neutrino- and photon- emission in the black hole frame and transform it to the observer-frame using Lorentz transformations. We obtain a difference in the arrival time at Earth of $Δt_{Earth} \approx 854.57$ s.

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arXiv2018

Muon-electron lepton-flavor violating transitions: shell-model calculations of transitions in 27Al

In this paper we present the results of large-scale shell-model calculations of muon-to-electron lepton-flavor violating transitions for the case of the target nucleus 27Al. We extend the previous shell-model calculations, done in the sd model space, by including also the p orbitals in order to see whether the negative-parity states produce any significant effect in the conversion rate. The analysis of the results shows the dominance of coherent transitions mediated by isovector operators and going by the ground state of the target, with practically null influence of excited positive- or negative-parity states.

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arXiv2018

Effects of neutrino mixing upon electron fraction in core collapse supernovae

The inclusion of massive neutrinos affects the cross sections involved in the formation of heavy nuclei, modifying their abundances. Rapid neutron capture processes (r-process) are often associated with explosive events such as core-collapse supernovae. In this work we study the effects of active and sterile neutrino oscillations and interactions, upon the calculation of neutrino fluxes, the baryonic density and the electron fraction of the material. We have considered two different initial distribution functions of the neutrinos and different combinations of mixing parameters (including θ 34 = 0). We use the formalism of density matrices for the calculations and included the effects of neutrino oscillations, interactions with matter and self-neutrino interactions. We found that the interactions of the neutrinos with matter and with themselves change the electron fraction, affecting the onset of the r-process.

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arXiv2018

Neutrino induced reactions in core-collapse supernovae: effects on the electron fraction

Neutrino induced reactions are a basic ingredient in astrophysical processes like star evolution. The existence of neutrino oscillations affects the rate of nuclear electroweak decays which participates in the chain of events that determines the fate of the star. Among the processes of interest, the production of heavy elements in core-collapse supernovae is strongly dependent upon neutrino properties, like the mixing between different species of neutrinos. In this work we study the effects of neutrino oscillations upon the electron fraction as a function of the neutrino mixing parameters, for two schemes: the $1+1$ scheme (one active neutrino and one sterile neutrino) and the $2+1$ scheme (two active neutrinos and one sterile neutrino). We have performed this analysis considering a core-collapse supernovae and determined the physical conditions needed to activate the nuclear reaction chains involved in the r-process. We found that the interactions of the neutrinos with matter and among themselves and the initial amount of sterile neutrinos in the neutrino-sphere might change the electron fraction, therefore affecting the onset of the r-process. We have set constrains on the active-sterile neutrino mixing parameters. They are the square-mass-difference $Δm^2_{14} $, the mixing angle $\sin^2 2θ_{14}$, and the hindrance factor $ξ_s $ for the occupation of sterile neutrinos. The calculations have been performed for different values of $X_α$, which is the fraction of $α$-particles. For $X_α=0$ the r-process is taking place if $Δm^2_{14} \geq 2 \, {\rm eV}^2 $, $\sin^2 2θ_{14} < 0.8 $ and $ξ_s < 0.5$. For larger values of $X_α$ the region of parameters is strongly reduced. The present results are compared to results available in the literature.

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arXiv2018

Decoherence-effects in the neutrino-mixing mechanism: active and sterile neutrinos in the three flavor scheme

In this work we analyze the effects of quantum decoherence upon the precession of the neutrino polarization vector in a supernovae-like environment. In order to perform the study we have determined the time-dependence of the polarization vector by solving the equation of motion for different neutrino-mixing schemes. The results of the calculations show that the onset of decoherence depends strongly on the parameters of the adopted mixing scheme. As examples we have considered : a) the mixing between active neutrinos and b) the mixing between active end sterile neutrinos.

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