43 research outputs found
Helicity asymmetries in double pion photoproduction on the proton
Based on a prior model on double pion photoproduction on the proton,
successfully tested in total cross sections and invariant mass distributions,
we make a theoretical study of the angular dependence of helicity asymmetries
from the interaction of circularly polarized photons with unpolarized protons.
We show that this observable is sensitive to details of the internal mechanisms
and, thus, represents a complementary test of the theoretical model.Comment: 16 pages, 6 figures, version accepted for publication in Nuclear
Physics
Model independent analysis of polarization effects in elastic electron deuteron scattering in presence of two--photon exchange
The general spin structure of the matrix element, taking into account the
two--photon exchange contribution, for the elastic electron (positron)
--deuteron scattering has been derived using general symmetry properties of the
hadron electromagnetic interaction, such as P--, C-- and T--invariances as well
as lepton helicity conservation in QED at high energy. Taking into account also
crossing symmetry, the amplitudes of scattering can be parametrized
in terms of fifteen real functions. The expressions for the differential cross
section and for all polarization observables are given in terms of these
functions. We consider the case of an arbitrary polarized deuteron target and
polarized electron beam (both longitudinal and transverse). The transverse
polarization of the electron beam induces a single--spin asymmetry which is
non--zero in presence of two--photon exchange. It is shown that elastic
deuteron electromagnetic form factors can still be extracted in presence of two
photon exchange, from the measurements of the differential cross section and of
one polarization observable (for example, the tensor asymmetry) for electron
and positron deuteron elastic scattering, in the same kinematical conditions.Comment: 28 page
Precision Calculation of n+p->d+gamma Cross Section for Big-Bang Nucleosynthesis
An effective field theory calculation of the n+p->d+gamma cross section
accurate to 1% for center of mass energy E up to ~1 MeV is presented. At these
energies, which are relevant for big-bang nucleosynthesis, isovector magnetic
transitions M1 and isovector electric transitions E1 give the dominant
contributions. The M1 amplitude is calculated up to next-to-next-to-leading
order (NNLO) and the contribution from the associated four-nucleon-one-photon
operator is determined from the cold neutron capture rate. The E1 amplitude is
calculated up to NNNNLO. The four-nucleon-one-photon operator contribution to
E1 is determined from the related deuteron photodisintegration reaction
gamma+d->n+p.Comment: 16 pages, 3 figures; References added, errors re-evaluated,
conclusions unchange
Dispersion Effects in Nucleon Polarisabilities
We present a formalism to extract the dynamical nucleon polarisabilities
defined via a multipole expansion of the structure amplitudes in nucleon
Compton scattering. In contradistinction to the static polarisabilities,
dynamical polarisabilities gauge the response of the internal degrees of
freedom of a composed object to an external, real photon field of arbitrary
energy. Being energy dependent, they therefore contain additional information
about dispersive effects induced by internal relaxation mechanisms, baryonic
resonances and meson production thresholds of the nucleon. We give explicit
formulae to extract the dynamical electric and magnetic dipole as well as
quadrupole polarisabilities from low energy nucleon Compton scattering up to
the one pion production threshold and discuss the connection to the definition
of static nucleon polarisabilities. As a concrete example, we examine the
results of leading order Heavy Baryon Chiral Perturbation Theory for the four
leading spin independent iso-scalar polarisabilities of the nucleon. Finally,
we consider the possible r{\^o}le of energy dependent effects in low energy
extractions of the iso-scalar dipole polarisabilities from Compton scattering
on the deuteron.Comment: 17 pages LaTeX2e with 2 figures, using includegraphicx (5 .eps
files). Minor corrections, references updated. Contents identical to version
to appear in Phys. Rev. C 65, spelling differen
Selected Topics in High Energy Semi-Exclusive Electro-Nuclear Reactions
We review the present status of the theory of high energy reactions with
semi-exclusive nucleon electro-production from nuclear targets. We demonstrate
how the increase of transferred energies in these reactions opens a complete
new window in studying the microscopic nuclear structure at small distances.
The simplifications in theoretical descriptions associated with the increase of
the energies are discussed. The theoretical framework for calculation of high
energy nuclear reactions based on the effective Feynman diagram rules is
described in details. The result of this approach is the generalized eikonal
approximation (GEA), which is reduced to Glauber approximation when nucleon
recoil is neglected. The method of GEA is demonstrated in the calculation of
high energy electro-disintegration of the deuteron and A=3 targets.
Subsequently we generalize the obtained formulae for A>3 nuclei. The relation
of GEA to the Glauber theory is analyzed. Then based on the GEA framework we
discuss some of the phenomena which can be studied in exclusive reactions,
these are: nuclear transparency and short-range correlations in nuclei. We
illustrate how light-cone dynamics of high-energy scattering emerge naturally
in high energy electro-nuclear reactions.Comment: LaTex file with 51 pages and 23 eps figure
Relativistically invariant analysis of polarization effects in exclusive deuteron electrodisintegration process
A general formalism for the calculation of the differential cross section and
polarization observables, for the process of deuteron electrodisintegration, is
developed in the framework of relativistic impulse approximation. A detailed
analysis of the general structure of the differential cross section and
polarization observables for the reaction is derived, using
the formalism of the structure functions. The obtained expressions have a
general nature and they hold in the one--photon--exchange mechanism, assuming
P--invariance of the hadron electromagnetic interaction. The model of
relativistic impulse approximation is introduced and the final state
interaction is taken into account by means of the unitarization of the helicity
amplitudes. A detailed description of the unitarization procedure is also
presented.
Using different parametrizations of the deuteron wave functions, the
following polarization observables are calculated in the kinematical region of
quasi--elastic deuteron electrodisintegration: the asymmetry for the scattering
of longitudinally polarized electrons on a polarized deuteron target, the
proton and neutron polarizations (for longitudinally polarized electron beam or
vector--polarized deuteron target). The sensitivity to the neutron electric
form factor is also thorougly investigated.
The predictions of the model are compared with the results of recent
polarization measurements and a good agreement with the existing experimental
data has been obtained.Comment: 90 pages, 17 figure
Elastic and Raman scattering of 9.0 and 11.4 MeV photons from Au, Dy and In
Monoenergetic photons between 8.8 and 11.4 MeV were scattered elastically and
in elastically (Raman) from natural targets of Au, Dy and In.15 new cross
sections were measured. Evidence is presented for a slight deformation in the
197Au nucleus, generally believed to be spherical. It is predicted, on the
basis of these measurements, that the Giant Dipole Resonance of Dy is very
similar to that of 160Gd. A narrow isolated resonance at 9.0 MeV is observed in
In.Comment: 31 pages, 11 figure
Hard probes of short-range nucleon-nucleon correlations
One of the primary goals of nuclear physics is providing a complete
description of the structure of atomic nuclei. While mean-field calculations
provide detailed information on the nuclear shell structure for a wide range of
nuclei, they do not capture the complete structure of nuclei, in particular the
impact of small, dense structures in nuclei. The strong, short-range component
of the nucleon-nucleon potential yields hard interactions between nucleons
which are close together, generating a high-momentum tail to the nucleon
momentum distribution, with momenta well in excess of the Fermi momentum. This
high-momentum component of the nuclear wave-function is one of the most poorly
understood parts of nuclear structure.
Utilizing high-energy probes, we can isolate scattering from high-momentum
nucleons, and use these measurements to examine the structure and impact of
short-range nucleon-nucleon correlations. Over the last decade we have moved
from looking for evidence of such short-range structures to mapping out their
strength in nuclei and examining their isospin structure. This has been made
possible by high-luminosity and high-energy accelerators, coupled with an
improved understanding of the reaction mechanism issues involved in studying
these structures. We review the general issues related to short-range
correlations, survey recent experiments aimed at probing these short-range
structures, and lay out future possibilities to further these studies.Comment: Review article to appear in Prog.Part.Nucl.Phys. 77 pages, 33 figure
n+p -> d+gamma for Big-Bang Nucleosynthesis
The cross section for n+p -> d+gamma is calculated at energies relevant to
big bang nucleosynthesis using the recently developed effective field theory
that describes the two-nucleon sector. The E1 amplitude is computed up to NNNLO
and depends only upon nucleon-nucleon phase shift data. In contrast, the M1
contribution is computed up to NLO, and the four-nucleon-one-magnetic-photon
counterterm that enters is determined by the cross section for cold neutron
capture. The uncertainty in the calculation for nucleon energies up to E ~ 1
MeV is estimated to be < 4%.Comment: 11 pages, 3 figures, late
Signatures of three-nucleon interactions in few-nucleon systems
Recent experimental results in three-body systems have unambiguously shown
that calculations based only on nucleon-nucleon forces fail to accurately
describe many experimental observables and one needs to include effects which
are beyond the realm of the two-body potentials. This conclusion owes its
significance to the fact that experiments and calculations can both be
performed with a high accuracy. In this review, both theoretical and
experimental achievements of the past decade will be underlined. Selected
results will be presented. The discussion on the effects of the three-nucleon
forces is, however, limited to the hadronic sector. It will be shown that
despite the major successes in describing these seemingly simple systems, there
are still clear discrepancies between data and the state-of-the-art
calculations.Comment: accepted for publication in Rep. Prog. Phy