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 Λ 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.

We present fully local versions of the minimally non-local nucleon-nucleon potentials constructed in a previous paper [M. Piarulli et al., Phys. Rev. C 91, 024003 (2015)], and use them in hypersperical-harmonics and quantum Monte Carlo calculations of ground and excited states of 3 H, 3 He, 4 He, 6 He, and 6 Li nuclei. The long-range part of these local potentials includes oneand two-pion exchange contributions without and with ∆-isobars in the intermediate states up to order Q 3 (Q denotes generically the low momentum scale) in the chiral expansion, while the short-range part consists of contact interactions up to order Q 4 . The low-energy constants multiplying these contact interactions are fitted to the 2013 Granada database in two different ranges of laboratory energies, either 0-125 MeV or 0-200 MeV, and to the deuteron binding energy and nn singlet scattering length. Fits to these data are performed for three models characterized by long-and short-range cutoffs, R L and R S respectively, ranging from (R L , R S ) = (1.2, 0.8) fm down to (0.8, 0.6) fm. The long-range (short-range) cutoff regularizes the one-and two-pion exchange (contact) part of the potential.

No abstract

An ab initio calculation of the 12 C elastic form factor, and sum rules of longitudinal and transverse response functions measured in inclusive (e, e ) scattering, is reported, based on realistic nuclear potentials and electromagnetic currents. The longitudinal elastic form factor and sum rule are found to be in satisfactory agreement with available experimental data. A direct comparison between theory and experiment is difficult for the transverse sum rule. However, it is shown that the calculated one has large contributions from two-body currents, indicating that these mechanisms lead to a significant enhancement of the quasi-elastic transverse response. This fact may have implications for the anomaly observed in recent neutrino quasi-elastic charge-changing scattering data off 12 C. The current picture of the nucleus as a system of protons and neutrons interacting among themselves via two-and three-body forces and with external electroweak probes via one-and two-body currents-a dynamical framework we will refer to below as the standard nuclear physics approach (SNPA)-has been shown to reproduce satisfactorily a variety of empirical properties of light nuclei with mass number A ≤ 12, including energy spectra [1][2][3][4][5][6][7], static properties [1,3,4,8,9] of low-lying states, such as charge radii, and magnetic and quadrupole moments, and longitudinal electron scattering [10,11]. However, it has yet to be established conclusively whether such a picture quantitatively and successfully accounts for the observed electroweak structure and response of these systems, at least those with A > 4, in a wide range of energy and momentum transfers. This issue has acquired new and pressing relevance in view of the anomaly seen in recent neutrino quasielastic charge-changing scattering data on 12 C [12], i.e., the excess, at relatively low energy, of measured cross section relative to theoretical calculations. Analyses based on these calculations have led to speculations that our present understanding of the nuclear response to chargechanging weak probes may be incomplete [13], and, in particular, that the momentum-transfer dependence of the axial form factor of the nucleon may be quite different from that obtained from analyses of pion electroproduction data [14] and measurements of neutrino and anti-neutrino reactions on protons and deuterons [15][16][17][18]. However, it should be emphasized that the calculations on which these analyses are based use rather crude models of nuclear structure-Fermi gas or local density approximations of the nuclear matter spectral function-as well as simplistic treatments of the reaction mechanism, and do not fit the picture outlined above. Conclusions based on them should therefore be viewed with caution.The present work provides the first step towards a comprehensive study, within the SNPA, of the quasi-elastic electroweak response functions of light nuclei. We report an exact quantum Monte Carlo (QMC) calculation of the elastic form factor and sum rules associated with the longitudi...

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 measured versus calculated longitudinal response. This is further corroborated by a re-analysis 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.PACS numbers: 21.60. De, 25.30.Pt One of the challenges in quantum many-body physics is calculating the electroweak response of a nucleus by fully accounting for the dynamics of its constituent nucleons. In this paper we report the first such calculation for the electromagnetic response of the 12 C nucleus.The nucleons interact with each other via two-and three-body forces and with external electroweak fields via one-and two-body, and smaller many-body, currents. This dynamical picture of the nucleus in which the consequences of the nucleons' substructure on its structure and response are subsumed into effective many-body forces and currents is by now well established. When coupled to numerically exact methods, such as the Green's function Monte Carlo (GFMC) methods adopted in this work, it has led to a quantitative and successful "first-principles" understanding of many nuclear properties: the low-lying energy spectra of nuclei up to 12 C [1] (and references therein); their radii and magnetic moments [2,3]; their elastic and inelastic electromagnetic form factors [4,5]; electroweak transitions between their low-lying states (M 1 and E2 widths [2,3], and β-decay and electroncapture rates [6]); properties of their ground-state structure, such as the momentum distributions of nucleons and nucleon pairs [7]; insights into the role that the dominant features of the nuclear interaction-the short-range repulsion and long-range tensor nature-have in shaping their ground-state structure [8]; and more (for a recent review see [1]). One of the key features of this approach is the assumption that the couplings of the external fields to the nucleons are governed by those in free-space with modifications induced primarily by two-nucleon currents.Here we report calculations of the 12 C electromagnetic longitudinal and transverse response functions, denoted respectively as R L (q, ω) and R T (q, ω), where q and ω are the electron momentum and energy transfers. These response functions are obtained experimentally by Rosenbluth separation of inclusive (e, e ) scattering data [9,10]. The calculations are based on the AV18+IL7 combination of two and three-nucleon potentials [11,12] and accompanying set of two-body electromagnetic currents (for a review see [1] and references therein). GFMC methods are used to compute these responses as functions of imaginary time [13,14], and maximum-entrop...