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We study the implications of isospin symmetry in nuclear systems on two-body distributions and derive relations between different densities or momentum distributions of a given nucleus or of different nuclei in the same isospin multiplet. A connection between neutrinoless double beta decay transition of mirror nuclei and two-body densities is also obtained. The relations are numerically verified by means of quantum Monte Carlo calculations. We discuss the relevance of these results to the analysis of nuclear short-range correlations and also show that isospin symmetry allows us to extract information about the spectator nucleons when a short-range correlated pair is formed.

We present a variational Monte Carlo method that solves the nuclear many-body problem in the occupation number formalism exploiting an artificial neural network representation of the ground-state wave function. A memory-efficient version of the stochastic reconfiguration algorithm is developed to train the network by minimizing the expectation value of the Hamiltonian. We benchmark this approach against widely used nuclear many-body methods by solving a model used to describe pairing in nuclei for different types of interaction and different values of the interaction strength. Despite its polynomial computational cost, our method outperforms coupled-cluster and provides energies that are in excellent agreement with the numerically-exact full configuration interaction values., 9 pages, 3 figures

We introduce a family of neural quantum states for the simulation of strongly interacting systems in the presence of spatial periodicity. Our variational state is parameterized in terms of a permutationally-invariant part described by the Deep Sets neural-network architecture. The input coordinates to the Deep Sets are periodically transformed such that they are suitable to directly describe periodic bosonic systems. We show example applications to both one and two-dimensional interacting quantum gases with Gaussian interactions, as well as to $^4$He confined in a one-dimensional geometry. For the one-dimensional systems we find very precise estimations of the ground-state energies and the radial distribution functions of the particles. In two dimensions we obtain good estimations of the ground-state energies, comparable to results obtained from more conventional methods., 8 pages, 6 figures

The interpretation of the available and forthcoming data obtained from multimessenger astrophysical observations -- potentially providing unprecedented access to neutron star properties -- will require the development of novel, accurate theoretical models of dense matter. Of great importance, in this context, will be the capability to devise a description of thermal effects applicable to the study of quantities other than the equation of state, such as the transport coefficients and the neutrino mean free path in the nuclear medium. The formalism based on correlated basis states and the cluster expansion technique has been previously employed to derive a well-behaved effective interaction -- suitable for use in standard perturbation theory -- from a state-of-the-art nuclear Hamiltonian, including phenomenological two- and three-nucleon potentials. Here, we provide a comprehensive and self-contained account of the extension of this approach to the treatment of finite-temperature effects, and report the results of numerical calculations of a number of properties of nuclear matter with arbitrary neutron excess and temperature up to 50 MeV., Comment: 20 pages, 11 figures, typeset using aastex631

We discuss the construction of a nuclear Energy Density Functional (EDF) from ab initio calculations, and we advocate the need of a methodical approach that is free from ad hoc assumptions. The equations of state (EoS) of symmetric nuclear and pure neutron matter are computed using the chiral NNLO$_{\rm sat}$ and the phenomenological AV4$^\prime$+UIX$_{c}$ Hamiltonians as inputs in the Self-consistent Green's Function (SCGF) and Auxiliary Field Diffusion Monte Carlo (AFDMC) methods, respectively. We propose a convenient parametrization of the EoS as a function of the Fermi momentum and fit it on the SCGF and AFDMC calculations. We apply the ab initio-based EDF to carry out an analysis of the binding energies and charge radii of different nuclei in the local density approximation. The NNLO$_{\rm sat}$-based EDF produces encouraging results, whereas the AV4$^\prime$+UIX$_{c}$-based one is farther from experiment. Possible explanations of these different behaviors are suggested, and the importance of gradient and spin-orbit terms is analyzed. Our work paves the way for a practical and systematic way to merge ab initio nuclear theory and DFT, while at the same time it sheds light on some of the critical aspects of this procedure., Comment: 14 pages, 6 figures

We propose a novel semiclassical approach to intranuclear cascades, which takes as input quantum Monte Carlo nuclear configurations and uses a semiclassical, impact-parameter-based algorithm to model the propagation of protons and neutrons in the nuclear medium. We successfully compare our simulations to available proton-carbon scattering data and nuclear-transparency measurements. By analyzing the dependence of the simulated observables upon the ingredients entering our intranuclear cascade algorithm, we provide a quantitative understanding of their impact. Particular emphasis is devoted to the role played by nuclear correlations, the Pauli exclusion principle, and interaction probability distributions.

We perform first-principle calculations of electron-nucleus scattering on $^3$He and $^3$H using the Green's function Monte Carlo method and two approaches based on the factorization of the final hadronic state: the spectral-function formalism and the short-time approximation. These three methods are benchmarked among each other and compared to the experimental data for the longitudinal and transverse electromagnetic response functions of $^3$He, and the inclusive cross sections of both $^3$He and $^3$H. Since these three approaches are based on the same description of nuclear dynamics of the initial target state, comparing their results enables a precise quantification of the uncertainties inherent to factorization schemes. At sufficiently large values of the momentum transfer, we find an excellent agreement of the Green's function Monte Carlo calculation with experimental data and with both the spectral-function formalism and the short-time approximation. We also analyze the relevance of relativistic effects, whose inclusion becomes crucial to explain data at high momentum and energy transfer., Comment: 16 pages, 8 figures

Classes of two-nucleon ($2N$) contact interactions are developed in configuration space at leading order (LO), next-to-leading order (NLO), and next-to-next-to-next-to-leading order (N3LO) by fitting the experimental singlet $np$ scattering length and deuteron binding energy at LO, and $np$ and $pp$ scattering data in the laboratory-energy range 0--15 MeV at NLO and 0--25 MeV at N3LO. These interactions are regularized by including two Gaussian cutoffs, one for $T\,$=$\,0$ and the other for $T\,$=$\,1$ channels. The cutoffs are taken to vary in the ranges $R_0\,$=$(1.5$--2.3) fm and $R_1\,$=$(1.5$--3.0) fm. The 780 (1,100) data points up to 15 (25) MeV energy, primarily differential cross sections, are fitted by the NLO (N3LO) models with a $\chi^2$/datum about 1.7 or less (well below 1.5), when harder cutoff values are adopted. As a first application, we report results for the binding energies of nuclei with mass numbers $A\,$=$\,3$--6 and 16 obtained with selected LO and NLO $2N$ models both by themselves as well as in combination with a LO three-nucleon ($3N$) contact interaction. The latter is characterized by a single low-energy constant that is fixed to reproduce the experimental $^3$H binding energy. The inclusion of the $3N$ interaction largely removes the sensitivity to cutoff variations in the few-nucleon systems and leads to predictions for the $^3$He and $^4$He binding energies that cluster around 7.8 MeV and 30 MeV, respectively. However, in $^{16}$O this cutoff sensitivity remains rather strong. Finally, predictions at LO only are also reported for medium-mass nuclei with $A\,$=$\,40$, 48, and 90., Comment: 23 pages, 6 figures

We devise a framework based on the generalized contact formalism that combines the nuclear shell model and quantum Monte Carlo methods and compute the neutrinoless double-beta decay of experimentally relevant nuclei, including $^{76}$Ge, $^{130}$Te, and $^{136}$Xe. In light nuclei, we validate our nuclear matrix element calculations by comparing against accurate variational Monte Carlo results. Due to additional correlations captured by quantum Monte Carlo and introduced within the generalized contact formalism, in heavier systems, we obtain long-range nuclear matrix elements that are about 30% smaller than previous shell-model results. We also evaluate the recently recognized short-range nuclear matrix element estimating its coupling by the charge-independence-breaking term of the Argonne $v_{18}$ potential used in the Monte Carlo calculations. Our results indicate an enhancement of the total nuclear matrix element by around 30%., Comment: 14 pages, 8 figures

A new numerical study calculates the interaction cross section between neutrinos and the carbon nucleus and finds a good match with extant data, a key step toward characterizing neutrino flavor oscillations.

Traditional intranuclear cascades assume point like interactions which neglects the fact that the nuclear force has a finite interaction range. In this new cascade, we use Quantum Monte Carlo nuclear configurations, along with a model for nucleon wavepacket overlap to incorporate more quantum mechanical effects into the cascade. We demonstrate how this approach correctly reproduces the mean-free path, can be evolved using constant time steps, and reproduces the proton-Carbon reaction cross-section. Additionally, we vary the parameters of the new cascade model as a means to estimate the uncertainty associated with our calculation.

Quantum Monte Carlo methods are powerful numerical tools to accurately solve the Schr\"odinger equation for nuclear systems, a necessary step to describe the structure and reactions of nuclei and nucleonic matter starting from realistic interactions and currents. These ab-initio methods have been used to accurately compute properties of light nuclei -- including their spectra, moments, and transitions -- and the equation of state of neutron and nuclear matter. In this work we review selected results obtained by combining quantum Monte Carlo methods and recent Hamiltonians constructed within chiral effective field theory., Comment: 36 pages, 9 figures

An ab initio Green's function Monte Carlo (GFMC) method is introduced for calculating total rates of muon weak capture in light nuclei with mass number $A\ensuremath{\le}12$. As a first application of the method, we perform a calculation of the rate in $^{3}\mathrm{H}$ and $^{4}\mathrm{He}$ in a dynamical framework based on realistic two- and three-nucleon interactions and realistic nuclear charge-changing weak currents. The currents include one- and two-body terms induced by $\ensuremath{\pi}$- and $\ensuremath{\rho}$-meson exchange, and $N$-to-$\mathrm{\ensuremath{\Delta}}$ excitation, and are constrained to reproduce the empirical value of the Gamow-Teller matrix element in tritium. We investigate the sensitivity of theoretical predictions to current parametrizations of the nucleon axial and induced pseudoscalar form factors as well as to two-body contributions in the weak currents. The large uncertainties in the measured $^{4}\mathrm{He}$ rates obtained from bubble-chamber experiments (carried out over 50 years ago) prevent us from drawing any definite conclusions. No data exist for $^{3}\mathrm{H}$, but results are compared to those of a recent Faddeev calculation as a validation of the present GFMC method.

We report benchmark calculations of the energy per particle of pure neutron matter as a function of the baryon density using three independent many-body methods: Brueckner-Bethe-Goldstone, Fermi hypernetted chain/single-operator chain, and auxiliary-field diffusion Monte Carlo. Significant technical improvements are implemented in the latter two methods. The calculations are made for two distinct families of realistic coordinate-space nucleon-nucleon potentials fit to scattering data, including the standard Argonne $v_{18}$ interaction and two of its simplified versions, and four of the new Norfolk $\Delta$-full chiral effective field theory potentials. The results up to twice nuclear matter saturation density show some divergence among the methods, but improved agreement compared to earlier work. We find that the potentials fit to higher-energy nucleon-nucleon scattering data exhibit a much smaller spread of energies., Comment: 16 pages, 7 figures

We present results for the quasi-elastic weak production of $\Lambda$ and $\Sigma$ hyperons induced by $\bar{\nu}$ scattering off nuclei, in the kinematical region of interest for accelerator neutrino experiments. We employ realistic hole spectral functions and we describe the propagation of the hyperons in the nuclear medium by means of a Monte Carlo cascade. The latter strongly modifies the kinematics and the relative production rates of the hyperons, leading to a non-vanishing $\Sigma^+$ cross section, to a sizable enhancement of the $\Lambda$ production and to a drastic reduction of the $\Sigma^0$ and $\Sigma^-$ distributions. We also compute the quasi-elastic weak $\Lambda_c$ production cross section, paying special attention to estimate the uncertainties induced by the model dependence of the vacuum $n\to \Lambda_c$ weak matrix element. In this regard, the recent BESIII measurements of the branching ratios of $\Lambda_c\rightarrow \Lambda l^+\nu_l$ ($l=e,\mu$) are used to benchmark the available theoretical predictions., Comment: 18 pages, 10 figures

In recent years, the combination of precise quantum Monte Carlo (QMC) methods with realistic nuclear interactions and consistent electroweak currents, in particular those constructed within effective field theories (EFTs), has lead to new insights in light and medium-mass nuclei, neutron matter, and electroweak reactions. This compelling new body of work has been made possible both by advances in QMC methods for nuclear physics, which push the bounds of applicability to heavier nuclei and to asymmetric nuclear matter and by the development of local chiral EFT interactions up to next-to-next-to-leading order and minimally nonlocal interactions including $\Delta$ degrees of freedom. In this review, we discuss these recent developments and give an overview of the exciting results for nuclei, neutron matter and neutron stars, and electroweak reactions., Comment: 27 pages, 8 figures; Invited review prepared for the Annual Review of Nuclear and Particle Science, Volume 69 (2019)

The magnetic form factors of $^2$H, $^3$H, and $^3$He, deuteron photodisintegration cross sections at low energies, and deuteron threshold electrodisintegration cross sections at backward angles in a wide range of momentum transfers, are calculated with the chiral two-nucleon (and three-nucleon) interactions including $\Delta$ intermediate states that have recently been constructed in configuration space. The $A\,$=$\,$3 wave functions are obtained from hyperspherical-harmonics solutions of the Schr\"odinger equation. The electromagnetic current includes one- and two-body terms, the latter induced by one- and two-pion exchange (OPE and TPE, respectively) mechanisms and contact interactions. The contributions associated with $\Delta$ intermediate states are only retained at the OPE level, and are neglected in TPE loop (tree-level) corrections to two-body (three-body) current operators. Expressions for these currents are derived and regularized in configuration space for consistency with the interactions. The low-energy constants that enter the contact few-nucleon systems. The predicted form factors and deuteron electrodisintegration cross section are in excellent agreement with experiment for momentum transfers up to 2--3 fm$^{-1}$. However, the experimental values for the deuteron photodisintegration cross section are consistently underestimated by theory, unless use is made of the Siegert form of the electric dipole transition operator. A complete analysis of the results is provided, including the clarification of the origin of the aforementioned discrepancy., Comment: 24 pages, 13 figures

The electromagnetic responses obtained from Green's function Monte Carlo (GFMC) calculations are based on realistic treatments of nuclear interactions and currents. The main limitations of this method comes from its nonrelativistic nature and its computational cost, the latter hampering the direct evaluation of the inclusive cross sections as measured by experiments. We extend the applicability of GFMC in the quasielastic region to intermediate momentum transfers by performing the calculations in a reference frame that minimizes nucleon momenta. Additional relativistic effects in the kinematics are accounted for employing the two-fragment model. In addition, we developed a novel algorithm, based on the concept of first-kind scaling, to compute the inclusive electromagnetic cross section of $^4$He through an accurate and reliable interpolation of the response functions. A very good agreement is obtained between theoretical and experimental cross sections for a variety of kinematical setups. This offers a promising prospect for the data analysis of neutrino-oscillation experiments that requires an accurate description of nuclear dynamics in which relativistic effects are fully accounted for., 10 pages, 7 figures

Scaling features of the nuclear electromagnetic response functions unveil aspects of nuclear dynamics that are crucial for interpretating neutrino- and electron-scattering data. In the large momentum-transfer regime, the nucleon-density response function defines a universal scaling function, which is independent of the nature of the probe. In this work, we analyze the nucleon-density response function of $^{12}$C, neglecting collective excitations. We employ particle and hole spectral functions obtained within two distinct many-body methods, both widely used to describe electroweak reactions in nuclei. We show that the two approaches provide compatible nucleon-density scaling functions that for large momentum transfers satisfy first-kind scaling. Both methods yield scaling functions characterized by an asymmetric shape, although less pronounced than that of experimental scaling functions. This asymmetry, only mildly affected by final state interactions, is mostly due to nucleon-nucleon correlations, encoded in the continuum component of the hole SF., 15 pages, 11 figures

Galilean invariance is usually violated in self-consistent mean-field calculations that employ effective density-dependent nuclear forces. We present a novel approach, based on variational quantum Monte Carlo techniques, suitable to preserve this symmetry and assess the effect of its violation, seldom attempted in the past. To this aim, we generalize the linear optimization method to encompass the density-dependence of effective Hamiltonians, and study $^4$He, $^{16}$O, and $^{40}$Ca ground-state properties employing the Gogny interaction., Comment: 24 pages, 6 figures

The factorization scheme, based on the impulse approximation and the spectral function formalism, has been recently generalized to allow the description of electromagnetic nuclear interactions driven by two-nucleon currents. We have extended this framework to the case of weak charged and neutral currents and carried out calculations of the double-differential neutrino-carbon and neutrino-oxygen cross sections using two different models of the target spectral functions. The results, showing a moderate dependence on the input spectral function, confirm that our approach provides a consistent treatment of all reaction mechanisms contributing to the signals detected by accelerator-based neutrino experiments., Comment: 18 pages, 7 figures

We have studied the scaling properties of the electromagnetic response functions of $^4$He and $^{12}$C nuclei computed by the Green's Function Monte Carlo approach, retaining only the one-body current contribution. Longitudinal and transverse scaling functions have been obtained in the relativistic and non relativistic cases and compared to experiment for various kinematics. The characteristic asymmetric shape of the scaling function exhibited by data emerges in the calculations in spite of the non relativistic nature of the model. The results are consistent with scaling of zeroth, first and second kinds. Our analysis reveals a direct correspondence between the scaling and the nucleon-density response functions., 11 pages, 15 figures

An updated and improved version of the effective interaction based on the Argonne\textendash Urbana nuclear Hamiltonian\textemdash derived using the formalism of Correlated Basis Functions (CBF) and the cluster expansion technique\textemdash is employed to obtain a number of properties of cold nuclear matter at arbitrary neutron excess within the formalism of many-body perturbation theory. The numerical results\textemdash including the ground-state energy per nucleon, the symmetry energy, the pressure, the compressibility, and the single-particle spectrum\textemdash are discussed in the context of the available empirical information, obtained from measured nuclear properties and heavy-ion collisions.

Variational calculations of ground-state properties of $^4$He, $^{16}$O, and $^{40}$Ca are carried out employing realistic phenomenological two- and three-nucleon potentials. The trial wave function includes two- and three-body correlations acting on a product of single-particle determinants. Expectation values are evaluated with a cluster expansion for the spin-isospin dependent correlations considering up to five-body cluster terms. The optimal wave function is obtained by minimizing the energy expectation value over a set of up to 20 parameters by means of a nonlinear optimization library. We present results for the binding energy, charge radius, one- and two-body densities, single-nucleon momentum distribution, charge form factor, and Coulomb sum rule. We find that the employed three-nucleon interaction becomes repulsive for $A\geq16$. In $^{16}$O the inclusion of such a force provides a better description of the properties of the nucleus. In $^{40}$Ca instead, the repulsive behavior of the three-body interaction fails to reproduce experimental data for the charge radius and the charge form factor. We find that the high-momentum region of the momentum distributions, determined by the short-range terms of nuclear correlations, exhibit a universal behavior independent of the particular nucleus. The comparison of the Coulomb sum rules for $^4$He, $^{16}$O, and $^{40}$Ca reported in this work will help elucidate in-medium modifications of the nucleon form factors., 24 pages, 25 figures

We extend the prediction range of Pionless Effective Field Theory with an analysis of the ground state of $^{16}$O in leading order. To renormalize the theory, we use as input both experimental data and lattice QCD predictions of nuclear observables, which probe the sensitivity of nuclei to increased quark masses. The nuclear many-body Schr\"odinger equation is solved with the Auxiliary Field Diffusion Monte Carlo method. For the first time in a nuclear quantum Monte Carlo calculation, a linear optimization procedure, which allows us to devise an accurate trial wave function with a large number of variational parameters, is adopted. The method yields a binding energy of $^{4}$He which is in good agreement with experiment at physical pion mass and with lattice calculations at larger pion masses. At leading order we do not find any evidence of a $^{16}$O state which is stable against breakup into four $^4$He, although higher-order terms could bind $^{16}$O., Comment: 11 pages, 4 figures

The presence of exotic states of matter in neutron stars (NSs) is currently an open issue in physics. The appearance of muons, kaons, hyperons, and other exotic particles in the inner regions of the NS, favored by energetic considerations, is considered to be an effective mechanism to soften the equation of state (EoS). In the so-called two-families scenario, the softening of the EoS allows for NSs characterized by very small radii, which become unstable and convert into a quark stars (QSs). In the process of conversion of a NS into a QS material can be ablated by neutrinos from the surface of the star. Not only neutron-rich nuclei, but also more exotic material, such as hypernuclei or deconfined quarks, could be ejected into the atmosphere. In the NS atmosphere, atoms like H, He, and C should exist, and attempts to model the NS thermal emission taking into account their presence, with spectra modified by the extreme magnetic fields, have been done. However, exotic atoms, like muonic hydrogen $(p\,\mu^-)$ or the so-called Sigmium $(\Sigma^+\,e^-)$, could also be present during the conversion process or in its immediate aftermath. At present, analytical expressions of the wave functions and eigenvalues for these atoms have been calculated only for H. In this work, we extend the existing solutions and parametrizations to the exotic atoms $(p\,\mu^-)$ and $(\Sigma^+\,e^-)$, making some predictions on possible transitions. Their detection in the spectra of NS would provide experimental evidence for the existence of hyperons in the interior of these stars., Comment: 10 pages, 6 figures, proceedings of the "International Conference on Exotic Atoms and Related Topics - EXA2017", Austrian Academy of Sciences, Austria, September 11-15, 2017

In a core-collapse supernova, a huge amount of energy is released in the Kelvin-Helmholtz phase subsequent to the explosion, when the proto-neutron star cools and deleptonizes as it loses neutrinos. Most of this energy is emitted through neutrinos, but a fraction of it can be released through gravitational waves. We model the evolution of a proto-neutron star in the Kelvin-Helmholtz phase using a general relativistic numerical code, and a recently proposed finite temperature, many-body equation of state; from this we consistently compute the diffusion coefficients driving the evolution. To include the many-body equation of state, we develop a new fitting formula for the high density baryon free energy at finite temperature and intermediate proton fraction. We estimate the emitted neutrino signal, assessing its detectability by present terrestrial detectors, and we determine the frequencies and damping times of the quasi-normal modes which would characterize the gravitational wave signal emitted in this stage., Comment: 25 pages, 14 figures, accepted by PRD

A major goal of nuclear theory is to explain the spectra and stability of nuclei in terms of effective many-body interactions amongst the nucleus' constituents-the nucleons, i.e., protons and neutrons. Such an approach, referred to below as the basic model of nuclear theory, is formulated in terms of point-like nucleons, which emerge as effective degrees of freedom, at sufficiently low energy, as a result of a decimation process, starting from the fundamental quarks and gluons, described by Quantum Chromodynamics (QCD). A systematic way to account for the constraints imposed by the symmetries of QCD, in particular chiral symmetry, is provided by chiral effective field theory, in the framework of a low-energy expansion. Here we show, in quantum Monte Carlo calculations accurate to $\leq\!2\%$ of the binding energy, that two- and three-body chiral interactions fitted {\sl only} to bound- and scattering-state observables in, respectively, the two- and three-nucleon sectors, lead to predictions for the energy levels and level ordering of nuclei in the mass range $A\,$=$\,$4-12 in very satisfactory agreement with experimental data. Our findings provide strong support for the fundamental assumptions of the basic model, and pave the way to its systematic application to the electroweak structure and response of these systems as well as to more complex nuclei., Comment: 21 pages, 3 figures

We studied the Fermi and Gamow-Teller responses of cold symmetric nuclear matter within a unified dynamical model, suitable to account for both short- and long-range correlation effects. The formalism of correlated basis functions has been used to construct two-body effective interactions and one-body effective weak operators. The inclusion of the three-body cluster term allowed for incorporating in the effective interaction a realistic model of three- nucleon forces, namely the UIX potential. Moreover, the sizable unphysical dependence of the effective weak operator is removed once the three-body cluster term is taken into account., 26 pages, 15 figures

The longitudinal and transverse electromagnetic response functions of ^{12}C are computed in a "first-principles" Green's function Monte Carlo calculation, based on realistic two- and three-nucleon interactions and associated one- and two-body currents. We find excellent agreement between theory and experiment and, in particular, no evidence for the quenching of the measured versus calculated longitudinal response. This is further corroborated by a reanalysis of the Coulomb sum rule, in which the contributions from the low-lying J^{π}=2^{+}, 0_{2}^{+} (Hoyle), and 4^{+} states in ^{12}C are accounted for explicitly in evaluating the total inelastic strength.

The formalism based on factorization and nuclear spectral functions has been generalized to treat transition matrix elements involving two-nucleon currents, whose contribution to the nuclear electromagnetic response in the transverse channel is known to be significant. We report the results of calculations of the inclusive electron-carbon cross section, showing that the inclusion of processes involving two-nucleon currents appreciably improves the agreement between theory and data in the dip region, between the quasielastic and Δ-production peaks. The relation to approaches based on the independent particle of the nucleus and the implications for the analysis of flux-integrated neutrino-nucleus cross sections are discussed.

The transport properties of neutron star matter play an important role in many astrophysical processes. We report the results of a calculation of the shear viscosity and thermal conductivity coefficients of the hard-sphere fermion system of degeneracy $\ensuremath{\nu}=2$, that can be regarded as a model of pure neutron matter. Our approach is based on the effective interaction obtained from the formalism of correlated basis functions and the cluster expansion technique. The resulting transport coefficients show a strong sensitivity to the quasiparticle effective mass, reflecting the effect of second-order contributions to the self-energy that are not taken into account in nuclear matter studies available in the literature.

A quantitative understanding of neutrino-nucleus interactions is demanded to achieve precise measurement of neutrino oscillations, and hence the determination of their masses. In addition, next generation detectors will be able to detect supernovae neutrinos, which are likely to shed some light on the open questions on the dynamics of core collapse. In this context, it is crucial to account for two-body meson-exchange currents along within realistic models of nuclear dynamics. We summarize our progresses towards the construction of a consistent framework, based on the Green's function Monte Carlo method, that can be exploited to accurately describe neutrino interactions with atomic nuclei in the quasi-elastic sector., Comment: 8 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:1509.00451

The excitation of two particle-two hole final states in neutrino-nucleus scattering has been advocated by many authors as the source of the excess cross section observed by the MiniBooNE Collaboration in the quasi elastic sector. We analyse the mechanisms leading to the appearance of these final states, and illustrate their significance through the results of accurate calculations of the nuclear electromagnetic response in the transverse channel. A novel approach, allowing for a consistent treatment of the amplitudes involving one- and two-nucleon currents in the kinematical region in which the non relativistic approximation breaks down, is outlined., Comment: Physical Review C, in press. arXiv admin note: text overlap with arXiv:1312.1210

We briefly review the growing efforts to set up a unified framework for the description of neutrino interactions with atomic nuclei and nuclear matter, applicable in the broad kinematical region corresponding to neutrino energies ranging between few MeV and few GeV. The emerging picture suggests that the formalism of nuclear many-body theory can be exploited to obtain the neutrino-nucleus cross sections needed for both the interpretation of oscillation signals and simulation of neutrino transport in compact stars., To appear in International Journal of Modern Physics E

The authors calculate $a\phantom{\rule{0}{0ex}}b$-$i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ quasi-elastic electromagnetic and neutral-weak response functions for $A=4$ and $A=12$ nuclei by using realistic nuclear two- and three-body forces and one- and two-body electroweak currents, in combination with quantum Monte Carlo methods and maximum-entropy techniques. In the electromagnetic case, their novel approach allows a direct comparison with experimental data. The results challenge the conventional picture that quasi-elastic inclusive scattering is dominated by single-nucleon knockout.

The formalism based on correlated basis functions and the cluster expansion technique has been recently employed to derive an effective interaction from a realistic nuclear hamiltonian. To gauge the reliability of this scheme, we perform a systematic comparison between the results of its application to the Fermi hard-sphere system and the predictions obtained from low-density expansions, as well as from other many-body techniques. The analysis of a variety of properties, including the ground state energy, the effective mass and the momentum distribution, shows that the effective interaction approach is remarkably accurate, thus suggesting that it may be employed to achieve a consistent description of the structure and dynamics of nuclear matter in the density region relevant to astrophysical applications., Comment: 10 pages, 13 figures

The onset of hyperons in the core of neutron stars and the consequent softening of the equation of state have been questioned for a long time. Controversial theoretical predictions and recent astrophysical observations of neutron stars are the grounds for the so-called hyperon puzzle. We calculate the equation of state and the neutron star mass-radius relation of an infinite systems of neutrons and $\Lambda$ particles by using the auxiliary field diffusion Monte Carlo algorithm. We find that the three-body hyperon-nucleon interaction plays a fundamental role in the softening of the equation of state and for the consequent reduction of the predicted maximum mass. We have considered two different models of three-body force that successfully describe the binding energy of medium mass hypernuclei. Our results indicate that they give dramatically different results on the maximum mass of neutron stars, not necessarily incompatible with the recent observation of very massive neutron stars. We conclude that stronger constraints on the hyperon-neutron force are necessary in order to properly assess the role of hyperons in neutron stars., Comment: 6 pages, 2 figures

The prediction of neutron stars properties is strictly connected to the employed nuclear interactions. The appearance of hyperons in the inner core of the star is strongly dependent on the details of the underlying hypernuclear force. We summarize our recent quantum Monte Carlo results on the development of realistic two- and three-body hyperon-nucleon interactions based on the available experimental data for light- and medium-heavy hypernuclei., Comment: 4 pages, 1 figure, Proceedings of the 21st International Conference on Few-Body Problems in Physics, Chicago, Illinois, USA

We report the results of a calculation of the response of cold neutron matter to neutral-current interactions with low energy neutrinos, carried out using an effective interaction and effective operators consistently derived within the formalism of Correlated Basis Functions. The neutrino mean free path obtained from the calculated responses turns out to be strongly affected by both short and long range correlations, leading to a sizable increase with respect to the prediction of the Fermi gas model. The consistency between the proposed approach and Landau theory of normal Fermi liquids also has been investigated, using a set of Landau parameters obtained from the matrix elements of the effective interaction., 9 pages, 8 figures

Precise measurement of neutrino oscillations, and hence the determination of their masses demands a quantitative understanding of neutrino-nucleus interactions. To this aim, two-body meson-exchange currents have to be accounted for along within realistic models of nuclear dynamics. We summarize our progresses towards the construction of a consistent framework, based on quantum Monte Carlo methods and on the spectral function approach, that can be exploited to accurately describe neutrino interactions with atomic nuclei over the broad kinematical region covered by neutrino experiments., 8 pages, 4 figure, Proceedings of the 21st International Conference on Few-Body Problems in Physics, Chicago, Illinois, USA

A new generation of local three-body potentials providing an excellent description of the properties of light nuclei, as well as of the neutron-deuteron doublet scattering length, has been recently derived. We have performed a comparative analysis of the equations of state of both pure neutron matter and symmetric nuclear matter obtained using these models of three-nucleon forces. None of the considered potentials simultaneously explains the empirical equilibrium density and binding energy of symmetric nuclear matter. However, two of them provide reasonable values of the saturation density. The ambiguity concerning the treatment of the contact term of the chiral inspired potentials is discussed., Comment: 14 pages, 8 figures

A density-dependent two-nucleon potential has been derived within the formalism of correlated basis function. The effects of three-particle interactions has been included by integrating out the degrees of freedom of the third nucleon. The potential can be easily employed in nuclear matter calculations. It yields results in agreement with those obtained from the underlying three-body potential. Using the density dependent potential, we have carried out a study of the effects of three-nucleon interactions in symmetric nuclear matter within the Auxiliary Field Diffusion Monte Carlo (AFDMC) computational scheme.

Microscopic calculations based on realistic nuclear hamiltonians, while yielding accurate results for the energies of the ground and low-lying excited states of nuclei with $A \leq 12$, fail to reproduce the empirical equilibrium properties of nuclear matter, that are known to be significantly affected by three-nucleon forces. We discuss a scheme suitable to construct a density-dependent two-nucleon potential, in which the effects of $n$-particle interactions can be included by integrating out the degrees of freedom of $(n-2)$-nucleons. Our approach, based on the formalism of correlated basis function and state-of-the-art models of the two- and three-nucleon potentials, leads to an effective interactionthat can be easily employed in nuclear matter calculations, yielding results in good agreement with those obtained from the underlying three-body potential.

A microscopic description of the interaction of atomic nuclei with external electroweak probes is required for elucidating aspects of short-range nuclear dynamics and for the correct interpretation of neutrino oscillation experiments. Nuclear quantum Monte Carlo methods infer the nuclear electroweak response functions from their Laplace transforms. Inverting the Laplace transform is a notoriously ill-posed problem; and Bayesian techniques, such as maximum entropy, are typically used to reconstruct the original response functions in the quasielastic region. In this work, we present a physics-informed artificial neural network architecture suitable for approximating the inverse of the Laplace transform. Utilizing simulated, albeit realistic, electromagnetic response functions, we show that this physics-informed artificial neural network outperforms maximum entropy in both the low-energy transfer and the quasielastic regions, thereby allowing for robust calculations of electron scattering and neutrino scattering on nuclei and inclusive muon capture rates., 12 pages, 10 figures