5,234 research outputs found
Meson-Exchange Currents and the Strangeness Radius of 4he
Meson-exchange current contributions to the strangeness radius of He are
computed in the one-boson exchange approximation. It is found that these
contributions introduce a \lapp10\% correction to the one-body contribution.
They should not, therefore, hamper the extraction of the nucleon strangeness
radius from the parity-violating electron-He asymmetry.Comment: 9 tex pages and 2 figures (not included, available from authors on
request), CEBAF Preprint #TH-93-16 and MIT Preprint CTP#223
Coincidence charged-current neutrino-induced deuteron disintegration for
Semi-inclusive charge-changing neutrino reactions on targets of heavy water
are investigated with the goal of determining the relative contributions to the
total cross section of deuterium and oxygen in kinematics chosen to emphasize
the former. The study is undertaken for conditions where the typical neutrino
beam energies are in the few GeV region, and hence relativistic modeling is
essential. For this, the previous relativistic approach for the deuteron is
employed, together with a spectral function approach for the case of oxygen.
Upon optimizing the kinematics of the final-state particles assumed to be
detected (typically a muon and a proton) it is shown that the oxygen
contribution to the total cross section is suppressed by roughly an order of
magnitude compared with the deuterium cross section, thereby confirming that
CC studies of heavy water can effectively yield the cross sections for
deuterium, with acceptable backgrounds from oxygen. This opens the possibility
of using deuterium to determine the incident neutrino flux distribution, to
have it serve as a target for which the nuclear structure issues are minimal,
and possibly to use deuterium to provide improved knowledge of specific aspects
of hadronic structure, such as to explore the momentum transfer dependence of
the isovector axial-vector form factor of the nucleon
Semi-inclusive charged-current neutrino-nucleus reactions
The general, universal formalism for semi-inclusive charged-current
(anti)neutrino-nucleus reactions is given for studies of any hadronic system,
namely, either nuclei or the nucleon itself. The detailed developments are
presented with the former in mind and are further specialized to cases where
the final-state charged lepton and an ejected nucleon are presumed to be
detected. General kinematics for such processes are summarized and then
explicit expressions are developed for the leptonic and hadronic tensors
involved and for the corresponding responses according to the usual charge,
longitudinal and transverse projections, keeping finite the masses of all
particles involved. In the case of the hadronic responses, general symmetry
principles are invoked to determine which contributions can occur. Finally, the
general leptonic-hadronic tensor contraction is given as well as the cross
section for the process
Coincidence charged-current neutrino-induced deuteron disintegration
Deuteron disintegration by charged-current neutrino (CC) scattering
offers the possibility to determine the energy of the incident neutrino by
measuring in coincidence two of the three resulting particles: a charged lepton
(usually a muon) and two protons, where we show that this channel can be
isolated from all other, for instance, from those with a pion in the final
state. We discuss the kinematics of the process for several detection
scenarios, both in terms of kinematic variables that are natural from a
theoretical point of view and others that are better matched to experimental
situations. The deuteron structure is obtained from a relativistic model
(involving an approximation to the Bethe-Salpeter equation) as an extension of
a previous, well-tested model used in deuteron electrodisintegration. We
provide inclusive and coincidence (semi-inclusive) cross sections for a variety
of kinematic conditions, using the plane-wave impulse approximation,
introducing final-state hadronic exchange terms (plane-wave Born approximation)
and final-state hadronic interactions (distorted-wave Born approximation).Comment: 31 pages, 14 figure
Parity violation in quasielastic electron-nucleus scattering within the relativistic impulse approximation
We study parity violation in quasielastic (QE) electron-nucleus scattering
using the relativistic impulse approximation. Different fully relativistic
approaches have been considered to estimate the effects associated with the
final-state interactions. We have computed the parity-violating quasielastic
(PVQE) asymmetry and have analyzed its sensitivity to the different ingredients
that enter in the description of the reaction mechanism: final-state
interactions, nucleon off-shellness effects, current gauge ambiguities.
Particular attention has been paid to the description of the weak neutral
current form factors. The PVQE asymmetry is proven to be an excellent
observable when the goal is to get precise information on the axial-vector
sector of the weak neutral current. Specifically, from measurements of the
asymmetry at backward scattering angles good knowledge of the radiative
corrections entering in the isovector axial-vector sector can be gained.
Finally, scaling properties shown by the interference nuclear
responses are also analyzed.Comment: 15 pages, 11 figure
Parity violation and dynamical relativistic effects in reactions
It is well known that coincidence quasielastic reactions are
not appropriate to analyze effects linked to parity violation due the presence
of the fifth electromagnetic (EM) response . Nevertheless, in this
work we develop a fully relativistic approach to be applied to parity-violating
(PV) quasielastic processes. This is of importance as a
preliminary step in the subsequent study of inclusive quasielastic PV
reactions. Moreover, our present analysis allows us to
disentangle effects associated with the off-shell character of nucleons in
nuclei, gauge ambiguities and the role played by the lower components in the
nucleon wave functions, i.e., dynamical relativistic effects. This study can
help in getting clear information on PV effects. Particular attention is paid
to the relativistic plane-wave impulse approximation where the explicit
expressions for the PV single-nucleon responses are shown for the first time.Comment: 39 pages, 9 figure
Global analysis of parity-violating asymmetry data for elastic electron scattering
We perform a statistical analysis of the full set of parity-violating
asymmetry data for elastic electron scattering including the most recent high
precision measurement from -weak. Given the basis of the present analysis,
our estimates appear to favor non-zero vector strangeness, specifically,
positive (negative) values for the electric (magnetic) strange form factors. We
also provide an accurate estimate of the axial-vector nucleon form factor at
zero momentum transfer, . Our study shows to be
importantly reduced with respect to the currently accepted value. We also find
our analysis of data to be compatible with the Standard Model values for the
weak charges of the proton and neutron.Comment: 6 pages, 4 figures, 2 tables. Accepted for publication in PR
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