4,268 research outputs found
Proton recoil polarization in exclusive (e,e'pp) reactions
The general formalism of nucleon recoil polarization in the () reaction is given. Numerical predictions are presented for the
components of the outgoing proton polarization and of the polarization transfer
coefficient in the specific case of the exclusive O()C knockout reaction leading to discrete states in the residual
nucleus. Reaction calculations are performed in a direct knockout framework
where final-state interactions and one-body and two-body currents are included.
The two-nucleon overlap integrals are obtained from a calculation of the
two-proton spectral function of O where long-range and short-range
correlations are consistently included. The comparison of results obtained in
different kinematics confirms that resolution of different final states in the
O()C reaction may act as a filter to
disentangle and separately investigate the reaction processes due to
short-range correlations and two-body currents and indicates that measurements
of the components of the outgoing proton polarization may offer good
opportunities to study short-range correlations.Comment: 12 pages, 6 figure
Correlation functions at small quark masses with overlap fermions
We report on recent work on the determination of low-energy constants
describing Delta{S}=1 weak transitions, in order to investigate the origins of
the Delta{I}=1/2 rule. We focus on numerical techniques designed to enhance the
statistical signal in three-point correlation functions computed with overlap
fermions near the chiral limit.Comment: Talk presented at Lattice2004(weak), Fermilab, 21-26 June 2004, 3
pages, 2 figure
Spontaneous chiral symmetry breaking in QCD:a finite-size scaling study on the lattice
Spontaneous chiral symmetry breaking in QCD with massless quarks at infinite
volume can be seen in a finite box by studying, for instance, the dependence of
the chiral condensate from the volume and the quark mass. We perform a
feasibility study of this program by computing the quark condensate on the
lattice in the quenched approximation of QCD at small quark masses. We carry
out simulations in various topological sectors of the theory at several
volumes, quark masses and lattice spacings by employing fermions with an exact
chiral symmetry, and we focus on observables which are infrared stable and free
from mass-dependent ultraviolet divergences. The numerical calculation is
carried out with an exact variance-reduction technique, which is designed to be
particularly efficient when spontaneous symmetry breaking is at work in
generating a few very small low-lying eigenvalues of the Dirac operator. The
finite-size scaling behaviour of the condensate in the topological sectors
considered agrees, within our statistical accuracy, with the expectations of
the chiral effective theory. Close to the chiral limit we observe a detailed
agreement with the first Leutwyler-Smilga sum rule. By comparing the mass, the
volume and the topology dependence of our results with the predictions of the
chiral effective theory, we extract the corresponding low-energy constant.Comment: 24 pages, 8 figure
Non-perturbative renormalisation of left-left four-fermion operators with Neuberger fermions
We outline a general strategy for the non-perturbative renormalisation of
composite operators in discretisations based on Neuberger fermions, via a
matching to results obtained with Wilson-type fermions. As an application, we
consider the renormalisation of the four-quark operators entering the Delta S=1
and Delta S=2 effective Hamiltonians. Our results are an essential ingredient
for the determination of the low-energy constants governing non-leptonic kaon
decays.Comment: 14 pages, 3 figure
Topological susceptibility in the SU(3) gauge theory
We compute the topological susceptibility for the SU(3) Yang--Mills theory by
employing the expression of the topological charge density operator suggested
by Neuberger's fermions. In the continuum limit we find r_0^4 chi = 0.059(3),
which corresponds to chi=(191 +/- 5 MeV)^4 if F_K is used to set the scale. Our
result supports the Witten--Veneziano explanation for the large mass of the
eta'.Comment: Final version to appear on Phys. Rev. Let
Relativistic descriptions of quasielastic charged-current neutrino-nucleus scattering: application to scaling and superscaling ideas
The analysis of the recent experimental data on charged-current
neutrino-nucleus scattering cross sections measured at MiniBooNE requires fully
relativistic theoretical descriptions also accounting for the role of final
state interactions. In this work we evaluate inclusive quasielastic
differential neutrino cross sections within the framework of the relativistic
impulse approximation. Results based on the relativistic mean field potential
are compared with the ones corresponding to the relativistic Green function
approach. An analysis of scaling and superscaling properties provided by both
models is also presented.Comment: 11 pages, 8 figures, version accepted for publication in Physical
Review
Short-range and tensor correlations in the O(e,epn) reaction
The cross sections for electron induced two-nucleon knockout reactions are
evaluated for the example of the O(e,epn)N reaction leading to
discrete states in the residual nucleus N. These calculations account
for the effects of nucleon-nucleon correlations and include the contributions
of two-body meson exchange currents as the pion seagull, pion in flight and the
isobar current contribution. The effects of short-range as well as tensor
correlations are calculated within the framework of the coupled cluster method
employing the Argonne V14 potential as a model for a realistic nucleon-nucleon
interaction. The relative importance of correlation effects as compared to the
contribution of the meson exchange currents depends on the final state of the
residual nucleus. The cross section leading to specific states, like e.g. the
ground state of N, is rather sensitive to the details of the correlated
wave function.Comment: 16 pages, 9 figures include
Polar Varieties and Efficient Real Elimination
Let be a smooth and compact real variety given by a reduced regular
sequence of polynomials . This paper is devoted to the
algorithmic problem of finding {\em efficiently} a representative point for
each connected component of . For this purpose we exhibit explicit
polynomial equations that describe the generic polar varieties of . This
leads to a procedure which solves our algorithmic problem in time that is
polynomial in the (extrinsic) description length of the input equations and in a suitably introduced, intrinsic geometric parameter, called
the {\em degree} of the real interpretation of the given equation system .Comment: 32 page
Polar Varieties, Real Equation Solving and Data-Structures: The hypersurface case
In this paper we apply for the first time a new method for multivariate
equation solving which was developed in \cite{gh1}, \cite{gh2}, \cite{gh3} for
complex root determination to the {\em real} case. Our main result concerns the
problem of finding at least one representative point for each connected
component of a real compact and smooth hypersurface. The basic algorithm of
\cite{gh1}, \cite{gh2}, \cite{gh3} yields a new method for symbolically solving
zero-dimensional polynomial equation systems over the complex numbers. One
feature of central importance of this algorithm is the use of a
problem--adapted data type represented by the data structures arithmetic
network and straight-line program (arithmetic circuit). The algorithm finds the
complex solutions of any affine zero-dimensional equation system in non-uniform
sequential time that is {\em polynomial} in the length of the input (given in
straight--line program representation) and an adequately defined {\em geometric
degree of the equation system}. Replacing the notion of geometric degree of the
given polynomial equation system by a suitably defined {\em real (or complex)
degree} of certain polar varieties associated to the input equation of the real
hypersurface under consideration, we are able to find for each connected
component of the hypersurface a representative point (this point will be given
in a suitable encoding). The input equation is supposed to be given by a
straight-line program and the (sequential time) complexity of the algorithm is
polynomial in the input length and the degree of the polar varieties mentioned
above.Comment: Late
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