1,077 research outputs found

### Quantum Monte Carlo calculations of symmetric nuclear matter

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

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

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

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

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

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

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

We have obtained the one--body density matrix and the momentum distribution
$n(p)$ of liquid $^4$He at $T=3D0^o$K 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 $^3$He, using fixed--node DMC, with nodes and
trial functions provided by the EMC. In particular, we analyze the momentum
distribution $n(p)$ with respect to the discontinuity $Z$ as well as the
singular behavior, at the Fermi surface. We also show that an approximate
factorization of the one-body density matrix $\rho(r)\simeq \rho_0(r)\rho_B(r)$
holds, with $\rho_0(r)$ and $\rho_B(r)$ respectively the density matrix of the
ideal Fermi gas and the density matrix of a Bose $^3$He.Comment: 10 pages, REVTeX, 12 figure

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