1,077 research outputs found

    Quantum Monte Carlo calculations of symmetric nuclear matter

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    We present an accurate numerical study of the equation of state of nuclear matter based on realistic nucleon--nucleon interactions by means of Auxiliary Field Diffusion Monte Carlo (AFDMC) calculations. The AFDMC method samples the spin and isospin degrees of freedom allowing for quantum simulations of large nucleonic systems and can provide quantitative understanding of problems in nuclear structure and astrophysics.Comment: Final version published in the Phys. Rev. Let

    Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters

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    The structural and dynamical properties of OCS molecules solvated in Helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system, and provide a more complex and detailed microscopic picture of this system than inferred from experiments.Comment: 4 pages. TeX (requires revtex4) + 3 ps figures (1 color

    S-pairing in neutron matter. I. Correlated Basis Function Theory

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    S-wave pairing in neutron matter is studied within an extension of correlated basis function (CBF) theory to include the strong, short range spatial correlations due to realistic nuclear forces and the pairing correlations of the Bardeen, Cooper and Schrieffer (BCS) approach. The correlation operator contains central as well as tensor components. The correlated BCS scheme of Ref. [Nucl. Phys. A363 (1981) 383], developed for simple scalar correlations, is generalized to this more realistic case. The energy of the correlated pair condensed phase of neutron matter is evaluated at the two--body order of the cluster expansion, but considering the one--body density and the corresponding energy vertex corrections at the first order of the Power Series expansion. Based on these approximations, we have derived a system of Euler equations for the correlation factors and for the BCS amplitudes, resulting in correlated non linear gap equations, formally close to the standard BCS ones. These equations have been solved for the momentum independent part of several realistic potentials (Reid, Argonne v_{14} and Argonne v_{8'}) to stress the role of the tensor correlations and of the many--body effects. Simple Jastrow correlations and/or the lack of the density corrections enhance the gap with respect to uncorrelated BCS, whereas it is reduced according to the strength of the tensor interaction and following the inclusion of many--body contributions.Comment: 20 pages, 8 figures, 1 tabl

    Microscopic calculations of the enhancement factor in the electric dipole sum rule

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    Correlated basis function perturbation theory with state-dependent correlations is used to calculate the nuclear photoabsorp- tion enhancement factor K in the electric dipole sum rule for some realistic models of nuclear matter. The contribution due to 2p-2h admixtures in the ground state wave function turns out to be only a few percent of the unperturbed value. The values obtained for K are about 1.8 at experimental equilibrium density and increase almost linearly with density. We also give estimates of K for finite nuclei, obtained within the local density approximation framework. The surface effects give a contribution which is - 20% of the volume term. state of the non-relativistic hamiltonian having V as nuclear potential and D z = ~Ei= 1 ,A rizZi is the z component of the electric dipole operator, with riz being the third component of the isospin opertor for the ith nucleon. The theoretical estimates (2) of K do not depend very much on the realistic interaction adopted, and are more than a factor of two larger than the experimental value (3), Kex p = 0.76 + 0.10, obtained from the integrated photo- nuclear cross sections up to the rr-meson production threshold. It is important to know how much of this discre- pancy is due to effects not explicitly taken into account in the Bethe-Levinger sum rule, like tail corrections of the integrated cross section, higher multipoles and dipole retardation effects, and how much is due to the in- adequacy of the variational wave function used in the calculation. In this letter we present the results obtained for K when the variational ground state is corrected by adding 2p2h correlated basis functions (CBF) components to it. The 2p2h admixtures are calculated by using second order CBF perturbation theory (4--6). The CBF states are normalized but not orthogonal, and are given by (koi) = F( (bi)/(cb i (F+Fltbi )1/2, (2) where I(I)i) are Fermi gas states and F = S H

    Comparative study of three-nucleon potentials in nuclear matter

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    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 figure

    Evaluation of the effects of a Twin Spark ignition system on combustion stability of a high performance PFI engine

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    The continuous demand for high performances and low emissions engines leads the engine manufactures to set the operating range of combustion devices near to their stability limit. Combustion stability is closely related to the formation of the first ignition kernel: an effective way of lowering Cycle-by-Cycle Variation (CCV) is to enhance the start of combustion by means of multiple sparks. A Ducati engine was equipped with a Twin Spark ignition system and a consistent improvement in combustion stability arised for both part load and full load conditions. At part load a sensible reduction of cycle-by-cycle variability of indicated mean effective pressure was found, while at full load condition the twin spark configuration showed an increase of power, but with higher knocking tendency. The aim of this work is to better understand the root causes of the increased level of knock and to make a critical evaluation of most used knock indexes, by means of an accurate analysis of the experimental and simulated pressure signals. The numerical methodology based on a perturbation of the initial kernel by a statistical evaluation of mixture condition at ignition location. A lagrangian ignition model developed at University of Bologna was used, here modified to take into account the statistical distribution of mixture around the spark plugs. The RANS simulations proved to be accurate in representing all the main information related to combustion efficiency and knocking events. © 2015 The Authors. Published by Elsevier Ltd

    Recent progress on the accurate determination of the equation of state of neutron and nuclear matter

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    The problem of accurately determining the equation of state of nuclear and neutron matter at density near and beyond saturation is still an open challenge. In this paper we will review the most recent progress made by means of Quantum Monte Carlo calculations, which are at present the only ab-inito method capable to treat a sufficiently large number of particles to give meaningful estimates depending only on the choice of the nucleon-nucleon interaction. In particular, we will discuss the introduction of density-dependent interactions, the study of the temperature dependence of the equation of state, and the possibility of accurately studying the effect of the onset of hyperons by developing an accurate hyperon-nucleon and hyperon-nucleon-nucleon interaction.Comment: 3 figures, 1 table, to appear in the Proceedings of "XIII Convegno di Cortona su Problemi di Fisica Nucleare Teorica", Cortona (Italy), April 6-8, 201

    Momentum distribution of liquid helium

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    We have obtained the one--body density matrix and the momentum distribution n(p)n(p) of liquid 4^4He at T=3D0oT=3D0^oK from Diffusion Monte Carlo (DMC) simulations, using trial functions optimized via the Euler Monte Carlo (EMC) method. We find a condensate fraction smaller than in previous calculations. Though we do not explicitly include long--range correlations in our calculations, we get a momentum distribution at long wavelength which is compatible with the presence of long--range correlations in the exact wave function. We have also studied 3^3He, using fixed--node DMC, with nodes and trial functions provided by the EMC. In particular, we analyze the momentum distribution n(p)n(p) with respect to the discontinuity ZZ as well as the singular behavior, at the Fermi surface. We also show that an approximate factorization of the one-body density matrix ŌĀ(r)‚ČÉŌĀ0(r)ŌĀB(r)\rho(r)\simeq \rho_0(r)\rho_B(r) holds, with ŌĀ0(r)\rho_0(r) and ŌĀB(r)\rho_B(r) respectively the density matrix of the ideal Fermi gas and the density matrix of a Bose 3^3He.Comment: 10 pages, REVTeX, 12 figure
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