1,822 research outputs found
Electromagnetic nucleon form factors in instant and point form
We present a study of the electromagnetic structure of the nucleons with
constituent quark models in the framework of relativistic quantum mechanics. In
particular, we address the construction of spectator-model currents in the
instant and point forms. Corresponding results for the elastic nucleon
electromagnetic form factors as well as charge radii and magnetic moments are
presented. We also compare results obtained by different realistic nucleon wave
functions stemming from alternative constituent quark models. Finally, we
discuss the theoretical uncertainties that reside in the construction of
spectator-model transition operators.Comment: 13 pages, 9 figures, updated and extended version for publicatio
The one-pion-exchange three-nucleon force and the puzzle
We consider a new three-nucleon force generated by the exchange of one pion
in the presence of a 2N correlation. The underlying irreducible diagram has
been recently suggested by the authors as a possible candidate to explain the
puzzle of the vector analyzing powers and for nucleon-deuteron
scattering. Herein, we have calculated the elastic neutron-deuteron
differential cross section, , , , , and
below break-up threshold by accurately solving the Alt-Grassberger-Sandhas
equations with realistic interactions. We have also studied how evolves
below 30 MeV. The results indicate that this new 3NF diagram provides one
possible additional contribution, with the correct spin-isospin structure, for
the explanation of the origin of this puzzle.Comment: revised version: We have also studied how Ay evolves below 30 MeV, 4
Pages (twocolumn), 2 figures, uses psfig, RevTe
Why is the three-nucleon force so odd?
By considering a class of diagrams which has been overlooked also in the most
recent literature on three-body forces, we extract a new contribution to the
three-nucleon interaction which specifically acts on the triplet odd states of
the two nucleon subsystem. In the static approximation, this 3N-force
contribution is fixed by the underlying 2N interaction, so in principle there
are no free parameters to adjust. The 2N amplitude however enters in the 3NF
diagram in a form which cannot be directly accessed or constrained by NN
phase-shift analysis. We conclude that this new 3N-force contribution provides
a mechanism which implies that the presence of the third nucleon modifies the
p-wave (and possibly the f-wave) components of the 2N subsystem in the
triplet-isotriplet channels.Comment: 10 Pages, 7 figures, RevTeX, twocolumn, epsf (updated version with
minor changes
Microscopic calculation of the spin-dependent neutron scattering lengths on 3He
We report on the spin.dependent neutron scattering length on 3He from a
microscopic calculation of p-3H, n-3He, and d-2H scattering employing the
Argonne v18 nucleon-nucleon potential with and without additional three-nucleon
force. The results and that of a comprehensive R-matrix analysis are compared
to a recent measurement. The overall agreement for the scattering lengths is
quite good. The imaginary parts of the scattering lengths are very sensitive to
the inclusion of three-nucleon forces, whereas the real parts are almost
insensitive.Comment: 9 pages, 1 figur
A stochastic template bank for gravitational wave searches for precessing neutron star - black hole coalescence events
Gravitational wave searches to date have largely focused on non-precessing systems. Including precession effects greatly increases the number of templates to be searched over. This leads to a corresponding increase in the computational cost and can increase the false alarm rate of a realistic search. On the other hand, there might be astrophysical systems that are entirely missed by non-precessing searches. In this paper we consider the problem of constructing a template bank using stochastic methods for neutron star - black hole binaries allowing for generic precession, but with the restriction that the total angular momentum of the binary is pointing towards the detector. We quantify the number of templates required for the search, and we explicitly construct the template bank. We show that despite the large number of templates, stochastic methods can be adapted to solve the problem. We quantify the parameter space region over which the non-precessing search might miss signals
The pd <--> pi+ t reaction around the Delta resonance
The pd pi+ t process has been calculated in the energy region around the
Delta-resonance with elementary production/absorption mechanisms involving one
and two nucleons. The isobar degrees of freedom have been explicitly included
in the two-nucleon mechanism via pi-- and rho-exchange diagrams. No free
parameters have been employed in the analysis since all the parameters have
been fixed in previous studies on the simpler pp pi+ d process. The
treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of
the reaction, with continuum calculations for the initial p-d state and
accurate solutions of the three-nucleon bound-state equation. The integral
cross-section was found to be quite sensitive to the NN interaction employed
while the angular dependence showed less sensitivity. Approximately a 4% effect
was found for the one-body mechanism, for the three-nucleon dynamics in the p-d
channel, and for the inclusion of a large, possibly converged, number of
three-body partial states, indicating that these different aspects are of
comparable importance in the calculation of the spin-averaged observables.Comment: 40 Pages, RevTex, plus 5 PostScript figure
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