23,723 research outputs found

    Cold neutrons trapped in external fields

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    The properties of inhomogeneous neutron matter are crucial to the physics of neutron-rich nuclei and the crust of neutron stars. Advances in computational techniques now allow us to accurately determine the binding energies and densities of many neutrons interacting via realistic microscopic interactions and confined in external fields. We perform calculations for different external fields and across several shells to place important constraints on inhomogeneous neutron matter, and hence the large isospin limit of the nuclear energy density functionals that are used to predict properties of heavy nuclei and neutron star crusts. We find important differences between microscopic calculations and current density functionals; in particular the isovector gradient terms are significantly more repulsive than in traditional models, and the spin-orbit and pairing forces are comparatively weaker.Comment: 5 pages, 4 figures, final version. Additional material reference added in the published versio

    Electron densities and temperatures in the f-region from backscatter measurements at arecibo

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    Diurnal variation in electron densities and temperatures in F region from backscatter measurements over Puerto Ric

    Quantum Monte Carlo study of inhomogeneous neutron matter

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    We present an ab-initio study of neutron drops. We use Quantum Monte Carlo techniques to calculate the energy up to 54 neutrons in different external potentials, and we compare the results with Skyrme forces. We also calculate the rms radii and radial densities, and we find that a re-adjustment of the gradient term in Skyrme is needed in order to reproduce the properties of these systems given by the ab-initio calculation. By using the ab-initio results for neutron drops for close- and open-shell configurations, we suggest how to improve Skyrme forces when dealing with systems with large isospin-asymmetries like neutron-rich nuclei.Comment: 8 pages, 6 figures, talk given at Horizons on Innovative Theories, Experiments, and Supercomputing in Nuclear Physics 2012, (HITES2012), New Orleans, Louisiana, June 4-7, 2012; to appear in Journal of Physics: Conference Series (JPCS

    Leading-Log Effects in the Resonance Electroweak Form Factors

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    We study log corrections to inelastic scattering at high Bjorken x for Q^2 from 1 to 21 GeV^2. At issue is the presence of log corrections, which can be absent if high x scattering has damped gluon radiation. We find logarithmic correction of the scaling curve extrapolated to low Q^2 improves the duality between it and the resonance plus background data in the Delta region, indicating log corrections exist in the data. However, at W > 2 GeV and high x, the data shows a (1-x)^3 form. Log corrections in one situation but not in another can be reconciled by a W- or Q^2- dependent higher twist correction.Comment: 13 pages, report nos. RPI-94-N90 and WM-94-106, revtex, two figures (available by fax or post

    Quantum Monte Carlo Calculations of A6A\leq6 Nuclei

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    The energies of 3H^{3}H, 3He^{3}He, and 4He^{4}He ground states, the 32{\frac{3}{2}}^{-} and 12{\frac{1}{2}}^{-} scattering states of 5He^{5}He, the ground states of 6He^{6}He, 6Li^{6}Li, and 6Be^{6}Be and the 3+3^{+} and 0+0^{+} excited states of 6Li^{6}Li have been accurately calculated with the Green's function Monte Carlo method using realistic models of two- and three-nucleon interactions. The splitting of the A=3A=3 isospin T=12T=\frac{1}{2} and A=6A=6 isospin T=1T=1, Jπ=0+J^{\pi} = 0^{+} multiplets is also studied. The observed energies and radii are generally well reproduced, however, some definite differences between theory and experiment can be identified.Comment: 12 pages, 1 figur

    Quantum Monte Carlo calculations of excited states in A = 6--8 nuclei

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    A variational Monte Carlo method is used to generate sets of orthogonal trial functions, Psi_T(J^pi,T), for given quantum numbers in various light p-shell nuclei. These Psi_T are then used as input to Green's function Monte Carlo calculations of first, second, and higher excited (J^pi,T) states. Realistic two- and three-nucleon interactions are used. We find that if the physical excited state is reasonably narrow, the GFMC energy converges to a stable result. With the combined Argonne v_18 two-nucleon and Illinois-2 three-nucleon interactions, the results for many second and higher states in A = 6--8 nuclei are close to the experimental values.Comment: Revised version with minor changes as accepted by Phys. Rev. C. 11 page

    Tensor Forces and the Ground-State Structure of Nuclei

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    Two-nucleon momentum distributions are calculated for the ground states of nuclei with mass number A8A\leq 8, using variational Monte Carlo wave functions derived from a realistic Hamiltonian with two- and three-nucleon potentials. The momentum distribution of npnp pairs is found to be much larger than that of pppp pairs for values of the relative momentum in the range (300--600) MeV/c and vanishing total momentum. This order of magnitude difference is seen in all nuclei considered and has a universal character originating from the tensor components present in any realistic nucleon-nucleon potential. The correlations induced by the tensor force strongly influence the structure of npnp pairs, which are predominantly in deuteron-like states, while they are ineffective for pppp pairs, which are mostly in 1^1S0_0 states. These features should be easily observable in two-nucleon knock-out processes, such as A(e,enp)A(e,e^\prime np) and A(e,epp)A(e,e^\prime pp).Comment: 4 pages including 3 figure

    Dependence of two-nucleon momentum densities on total pair momentum

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    Two-nucleon momentum distributions are calculated for the ground states of 3He and 4He as a function of the nucleons' relative and total momenta. We use variational Monte Carlo wave functions derived from a realistic Hamiltonian with two- and three-nucleon potentials. The momentum distribution of pp pairs is found to be much smaller than that of pn pairs for values of the relative momentum in the range (300--500) MeV/c and vanishing total momentum. However, as the total momentum increases to 400 MeV/c, the ratio of pp to pn pairs in this relative momentum range grows and approaches the limit 1/2 for 3He and 1/4 for 4He, corresponding to the ratio of pp to pn pairs in these nuclei. This behavior should be easily observable in two-nucleon knock-out processes, such as A(e,e'pN).Comment: 3 pages, 3 figure
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