arXiv subjects

Andrei Neacsu

Publications and source records attributed to Andrei Neacsu.

14 recordsLinked to original sources

arXiv2026

Exact Calculation of Two-neutrino Double Beta Decay Rate

The calculation of the two-neutrino double-beta decay (DBD) rates has relied so far on approximations that decouple the nuclear and atomic parts. To provide a more rigorous treatment, we propose an approach which incorporates the full interdependence between nuclear structure and lepton kinematics. Deviations of the decay rates and electron spectra from the traditional methods, such as closure, non-closure and Taylor expansion approximation, are presented and discussed for the isotopes $^{82}$Se and $^{136}$Xe. Our approach gives a more realistic description of the DBD process, and opens the avenue of additional, new theoretical and experimental investigations into nuclear and atomic effects in the process. Extensions of this framework to other isotopes and to neutrinoless double-beta decay are currently underway.

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arXiv2026

Uncertainty Quantification of the $^{76}$Ge Neutrinoless Double-Beta Decay Nuclear Matrix Element

The experimental pursuit of neutrinoless double-beta decay ($0\nu\beta\beta$) constitutes one of the most compelling avenues for probing lepton-number violation and exploring physics beyond the Standard Model. Within this landscape, $^{76}$Ge has consistently ranked among the most promising isotopes for current and next-generation bolometric and liquid-scintillator experiments, notably GERDA and LEGEND. In the present work, we adapt a rigorous statistical protocol previously established for $^{48}$Ca~\cite{Horoi-prc22} and $^{136}$Xe~\cite{Horoi-Xe-2023} to the $^{76}$Ge system, utilizing a valence configuration that aligns with our recent investigation of $^{82}$Se~\cite{Neacsu-Symmetry-2024}. Our methodology introduces systematic, bounded fluctuations to the two-body matrix elements of established effective interactions, subsequently monitoring how these perturbations propagate through a suite of low-energy nuclear observables. Special emphasis is placed on the $0\nu\beta\beta$ nuclear matrix element (NME), whose theoretical uncertainty currently dominates the interpretation of experimental half-life limits. By integrating these simulated variations into a Bayesian Model Averaging framework and benchmarking against empirical spectroscopic data, we derive a constrained probability distribution for the NME. The resulting analysis yields a central value of 2.46 with an associated standard deviation of 0.25, thereby quantifying the intrinsic theoretical spread within the interacting shell model approach. Furthermore, we perform a comprehensive correlation analysis across all computed observables to evaluate internal consistency, identify non-trivial structural dependencies, and establish benchmarks that may guide the refinement of future effective interactions.

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arXiv2023

Predicting the neutrinoless double-beta decay matrix element of $^{136}$Xe using a statistical approach

Calculation of the nuclear matrix elements (NMEs) for double-beta decay is of paramount importance for guiding experiments and for analyzing and interpreting the experimental data, especially for the search of the neutrinoless double beta decay mode ($0\nu\beta\beta$). However, there are currently still large differences between the NME values calculated by different methods, hence a quantification of their uncertainties is very much required. In this paper we propose a statistical analysis of $0\nu\beta\beta$ NME for the $^{136}Xe$ isotope, based on the interacting shell model, but using three independent effective Hamiltonians, emphasizing the range of the NMEs' most probable values and its correlations with observables that can be obtained from the existing nuclear data. Consequently, we propose a common probability distribution function for the $0\nu\beta\beta$ NME, which has a range of (1.55 - 2.65) at 90\% confidence level, with a mean value of 1.99 and a standard deviation of 0.37.

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arXiv2018

Shell model study on the possibility of using an effective field theory for disentangling several contributions to the neutrinoless double-beta decay

Neutrinoless double-beta decay represents the most promising approach for revealing some of the most important, yet-unknown, properties of neutrinos related to their absolute masses and their nature. This transition involves beyond standard model theories that predict the violation of the lepton number conservation by two units. There is no experimental confirmation yet for this decay, but new experiments have set lower-limits for the associated half-lives in the case of several isotopes. Using an effective field theory that describes this transition, we calculate half-life ratios for five experimentally interesting isotopes in the case of 12 lepton number violating couplings. These half-life ratios can be used to probe the sensitivity of the five isotopes in relation to their respective mechanisms, to predict the half-life limits needed to match the different experimental results, and in the case of experimental confirmation, these ratios could possibly indicate the dominant mechanism of the transition. We provide an analysis that could reveal valuable information regarding the dominant neutrinoless double-beta decay mechanism, if experimental half-life data becomes available for different isotopes.

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arXiv2017

Towards an effective field theory approach to the neutrinoless double-beta decay

Weak interaction in nuclei represents a well-known venue for testing many of the fundamental symmetries of the Standard Model. In particular, neutrinoless double-beta decay offers the possibility to test Beyond Standard Model theories predicting that neutrinos are Majorana fermions and the lepton number conservation is violated. This paper focuses on an effective field theory approach to neutrinoless double-beta decay for extracting information regarding the properties of the Beyond Standard Model Lagrangian responsible for this process. We use shell model nuclear matrix elements and the latest experimental lower limits for the half-lives to extract the lepton number violating parameters of five nuclei of experimental interest, and lower limits for the energy scales of the new physics.

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arXiv2017

Interference Between Light and Heavy Neutrinos for $0\nu\beta\beta$ Decay in the Left-Right Symmetric Model

Neutrinoless double-beta decay is proposed as an important low energy phenomenon that could test beyond the Standard Model physics. There are several potentially competing beyond the Standard Model mechanisms that can induce the process. It thus becomes important to disentangle the different processes. In the present study we consider the interference effect between the light left-handed and heavy right-handed Majorana neutrino exchange mechanisms. The decay rate, and consequently, the phase-space factors for the interference term are derived, based on the left-right symmetric model. The numerical values for the interference phase-space factors for several nuclides are calculated, taking into consideration the relativistic Coulomb distortion of the electron wave function and finite-size of the nucleus. The variation of the interference effect with the Q-value of the process is studied.

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arXiv2016

Shell model studies of competing mechanisms to the neutrinoless double-beta decay in $^{124}$Sn, $^{130}$Te, and $^{136}$Xe

Neutrinoless double-beta decay is a predicted beyond Standard Model process that could clarify some of the not yet known neutrino properties, such as the mass scale, the mass hierarchy, and its nature as a Dirac or Majorana fermion. Should this transition be observed, there are still challenges in understanding the underlying contributing mechanisms. We perform a detailed shell model investigation of several beyond Standard Model mechanisms that consider the existence of right-handed currents. Our analysis presents different venues that can be used to identify the dominant mechanisms for nuclei of experimental interest in the mass A$\sim$130 region ($^{124}$Sn, $^{130}$Te, and $^{136}$Xe). It requires an accurate knowledge of nine nuclear matrix elements that we calculate, in addition to the associated energy dependent phase-space factors.

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arXiv2016

Analysis of mechanisms that could contribute to the neutrinoless double-beta decay

Neutrinoless double-beta decay, if observed, would signal physics beyond the Standard Model that could be discovered at energies significantly lower than those at which the relevant degrees of freedom could be excited. Therefore, it could be challenging to further use the neutrinoless double-beta decay observations to distinguish between many beyond Standard Model mechanisms contributing to this process. Accurate nuclear structure calculations of the nuclear matrix elements necessary to analyze the decay rates could be helpful to narrow down the list of contributing mechanisms, and to better identify the more exotic properties of the neutrinos. We investigate the information one can get from the angular and energy distribution of the emitted electron assuming that the right-handed currents exist. For the analysis of these distributions we calculate the necessary nuclear matrix elements using shell model techniques.

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arXiv2015

Shell model predictions for $^{124}$Sn double-beta decay

Neutrinoless double-beta ($0 νββ$) decay is a promising beyond Standard Model process. Two-neutrino double-beta ($2 νββ$) decay is an associated process that is allowed by the Standard Model, and it was observed in about 10 isotopes, including decays to the excited states of the daughter. $^{124}$Sn was the first isotope whose double-beta decay modes were investigated experimentally, and despite few other recent efforts, no signal has been seen so far. Shell model calculations were able to make reliable predictions for $2 νββ$ decay half-lives. Here we use shell model calculations to predict the $2 νββ$ decay half-life of $^{124}$Sn. Our results are quite different from the existing quasiparticle random-phase approximation (QRPA) results, and we envision that they will be useful for guiding future experiments. We also present shell model nuclear matrix elements for two potentially competing mechanisms to the $0 νββ$ decay of $^{124}$Sn.

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arXiv2015

An effective method to accurately calculate the phase space factors for $\beta^- \beta^-$ decay

Accurate calculations of the electron phase space factors are necessary for reliable predictions of double-beta decay rates, and for the analysis of the associated electron angular and energy distributions. We present an effective method to calculate these phase space factors that takes into account the distorted Coulomb field of the daughter nucleus, yet allows one to easily calculate the phase space factors with good accuracy relative to the most exact methods available in the recent literature.

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arXiv2014

Shell model studies of the $^{130}Te$ neutrinoless double-beta decay

Most uncertainties regarding the theoretical study of the neutrinoless double-beta decay are related to the accuracy of the nuclear matrix elements that appear in the expressions of the lifetimes. We calculate the nuclear matrix elements for the $0νββ$ decay of $^{130}Te$ in a shell model approach, using a recently proposed effective Hamiltonian. To ensure the reliability of the results, we investigate this Hamiltonian by performing calculations of spectroscopic quantities and comparing them to the latest experimental data available, and we analyze the $2νββ$ and the $0νββ$ decay nuclear matrix elements of $^{136}$Xe. Finally, we report new nuclear matrix for the $^{130}$Te considering the light neutrino exchange and heavy neutrino exchange mechanisms, alongside with an overview of some recent values reported in the literature.

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arXiv2014

Constraints on light neutrino parameters derived from the study of neutrinoless double beta decay

The study of the neutrinoless double beta ($0 ββ$) decay mode can provide us with important information on the neutrino properties, particularly on the electron neutrino absolute mass. In this work we revise the present constraints on the neutrino mass parameters derived from the $0 ββ$ decay analysis of the experimentally interesting nuclei. We use the latest results for the phase space factors (PSFs) and nuclear matrix elements (NMEs), as well as for the experimental lifetimes limits. For the PSFs we use values computed with an improved method reported very recently. For the NMEs we use values chosen from literature on a case-by-case basis, taking advantage of the consensus reached by the community on several nuclear ingredients used in their calculation. Thus, we try to restrict the range of spread of the NME values calculated with different methods and, hence, to reduce the uncertainty in deriving limits for the Majorana neutrino mass parameter. Our results may be useful to have an up-date image on the present neutrino mass sensitivities associated with $0 ββ$ measurements for different isotopes and to better estimate the range of values of the neutrino masses that can be explored in the future double beta decay (DBD) experiments.

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arXiv2013

Study of Nuclear Effects in the Computation of the 0νββ Decay Matrix Elements

We analyse the effects that different nuclear structure approximations associated with the short range correlations (SRC), finite nucleon size (FNS), higher order terms in the nucleon currents (HOC) and with some nuclear input parameters, have on the values of the nuclear matrix elements (NMEs) for the neutrinoless double beta (0νββ) decay. The calculations are performed with a new Shell Model(ShM) code which allows a fast computation of the two-body matrix elements of the transition operators. The treatment of SRC, FNS and HOC and include the use of quenched or unquenched values for the axial vector coupling constant produce the most important effects on the NMEs values. Equivalent effects of some of these approximations are also possible, which may lead (accidentally) to close final results. We found that the cummulative effect of all these nuclear ingredients on the calculated nuclear matrix elements NMEs is significant. Since the NMEs values are often obtained with different approximations and/or with different input parameters, a convergent view point on their inclusion/neglection and an uniformization of the calculations are needed, in order to enter in an era of precision concerning the computation of the NMEs for double beta deacay.

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arXiv2012

Fast, Efficient Calculations of the Two-Body Matrix Elements of the Transition Operators for Neutrinoless Double Beta Decay

To extract information about the neutrino properties from the study of neutrinoless double-beta (0νββ) decay one needs a precise computation of the nuclear matrix elements (NMEs) associated with this process. Approaches based on the Shell Model (ShM) are among the nuclear structure methods used for their computation. ShM better incorporates the nucleon correlations, but have to face the problem of the large model spaces and computational resources. The goal is to develop a new, fast algorithm and the associated computing code for efficient calculation of the two-body matrix elements (TBMEs) of the 0νββ decay transition operator, which are necessary to calculate the NMEs. This would allow us to extend the ShM calculations for double-beta decays to larger model spaces, of about 9-10 major harmonic oscillator shells. The improvement of our code consists in a faster calculation of the radial matrix elements. Their computation normally requires the numerical evaluation of two-dimensional integrals: one over the coordinate space and the other over the momentum space. By rearranging the expressions of the radial matrix elements, the integration over the coordinate space can be performed analytically, thus the computation reduces to sum up a small number of integrals over momentum. Our results for the NMEs are in a good agreement with similar results from literature, while we find a significant reduction of the computation time for TBMEs, by a factor of about 30, as compared with our previous code that uses two-dimensional integrals.

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