56 research outputs found
Parity-violating longitudinal response
The longitudinal quasielastic parity-violating electron scattering response
is explored within the context of a model that builds antisymmetrized RPA-HF
correlations on a relativistic Fermi gas basis. The large sensitivity to
nuclear dynamics of this observable, found in previous studies where only
pionic correlations were included, is shown to survive in the present model
where the effects from pion, rho, sigma and omega exchange in a version of the
Bonn potential are incorporated. Through an intricate diagrammatic
cancellation/filtration mechanism the longitudinal parity-violating response
turns out to be close to the one obtained in first-order perturbation theory
with the pion alone. Finally, in accord with our previous work, the
parity-violating response is seen to display appreciable sensitivity to the
electric strangeness content of the nucleon, especially at high momentum
transfer.Comment: 13 pages, uses REVTeX and epsfig, 10 postscript figures; a postscript
version of the paper is available by anonymous ftp at
ftp://carmen.to.infn.it/pub/barbaro/papers/951
Meson-exchange currents and quasielastic neutrino cross sections in the SuperScaling Approximation model
We evaluate the quasielastic double differential neutrino cross sections
obtained in a phenomenological model based on the superscaling behavior of
electron scattering data. We compare our results with the recent experimental
data for neutrinos of MiniBooNE and estimate the contribution of the vector
meson-exchange currents in the 2p-2h sector.Comment: 6 pages, 4 figure
Parity-Violating Electron Scattering from the Pion-Correlated Relativistic Fermi Gas
Parity-violating quasielastic electron scattering is studied within the
context of the relativistic Fermi gas and its extensions to include the effects
of pionic correlations and meson-exchange currents. The work builds on previous
studies using the same model; here the part of the parity-violating asymmetry
that contains axial-vector hadronic currents is developed in detail using those
previous studies and a link is provided to the transverse vector-isovector
response. Various integrated observables are constructed from the differential
asymmetry. These include an asymmetry averaged over the quasielastic peak, as
well as the difference of the asymmetry integrated to the left and right of the
peak -- the latter is shown to be optimal for bringing out the nature of the
pionic correlations. Special weighted integrals involving the differential
asymmetry and electromagnetic cross section, based on the concepts of y-scaling
and sum rules, are constructed and shown to be suited to studies of the
single-nucleon form factor content in the problem, in particular, to
determinations of the isovector/axial-vector and electric strangeness form
factors. Comparisons are also made with recent predictions made on the basis of
relativistic mean-field theory.Comment: 28 pages, LATeX, 13 figures (tar-compressed postscript files,
available from the authors), MIT preprint CTP#222
The 2p-2h electromagnetic response in the quasielastic peak and beyond
The contribution to the nuclear transverse response function R_T arising from
two particle-two hole (2p-2h) states excited through the action of
electromagnetic meson exchange currents (MEC) is computed in a fully
relativistic framework. The MEC considered are those carried by the pion and by
Delta degrees of freedom, the latter being viewed as a virtual nucleonic
resonance. The calculation is performed in the relativistic Fermi gas model in
which Lorentz covariance can be maintained. All 2p-2h many-body diagrams
containing two pionic lines that contribute to R_T are taken into account and
the relative impact of the various components of the MEC on R_T is addressed.
The non-relativistic limit of the MEC contributions is also discussed and
compared with the relativistic results to explore the role played by relativity
in obtaining the 2p-2h nuclear response.Comment: 27 pages, 12 figures, revtex4; minor modifications in the discussion
of the results, references adde
The Pion in Electromagnetic and Weak Neutral Current Nuclear Response Functions
The impact of pionic correlations and meson--exchange currents in determining
the (vector) response functions for electroweak quasielastic lepton scattering
from nuclei is discussed. The approach taken builds on previous work where the
Fermi gas model is used to maintain consistency in treating forces and currents
(gauge invariance) and to provide a Lorentz covariant framework. Results
obtained in first-order perturbation theory are compared with infinite-order
summation schemes (HF and RPA) and found to provide quite successful
approximations for the quasielastic response functions. The role of pionic
correlations in hardening the responses R_L and R_T is investigated in some
detail, including studies of the relative importance of central and tensor
pieces of the force and of exchange and self-energy diagrams; in addition,
their role in significantly modifying the longitudinal parity-violating
response R_{AV}^L is explored. The MEC are shown to provide a small, but
non-negligible, contribution in determining the vector responses.Comment: TeX, 21 figures (Postscript, available from the authors), MIT
preprint CTP\#219
Role of 2p-2h MEC excitations in superscaling
Following recent studies of inclusive electron scattering from nuclei at high
energies which focused on two-nucleon emission mediated by meson-exchange
currents, in this work the superscaling behavior of such contributions is
investigated. Comparisons are made with existing data below the quasielastic
peak where at high momentum transfers scaling of the second kind is known to be
excellent and scaling of the first kind is good, in the proximity of the peak
where both 1p-1h and 2p-2h contributions come into play, and above the peak
where inelasticity becomes important and one finds scaling violations of the
two kinds.Comment: 27 pages, 12 figures; references adde
Investigation of the Neutron Form Factors by Inclusive Quasi-Elastic Scattering of Polarized Electrons off Polarized He: A Theoretical Overview
The theory of quasi-elastic inclusive scattering of polarized leptons off
polarized He is critically reviewed and the origin of different expressions
for the polarized nuclear response function appearing in the literature is
explained. The sensitivity of the longitudinal asymmetry upon the neutron form
factors is thoroughly investigated and the role played by the polarization
angle for minimizing the proton contribution is illustrated.Comment: Phys. Rev C in press; 9 figs. (available upon request
Superscaling of Inclusive Electron Scattering from Nuclei
We investigate the degree to which the concept of superscaling, initially
developed within the framework of the relativistic Fermi gas model, applies to
inclusive electron scattering from nuclei. We find that data obtained from the
low energy loss side of the quasielastic peak exhibit the superscaling
property, i.e., the scaling functions f(\psi') are not only independent of
momentum transfer (the usual type of scaling: scaling of the first kind), but
coincide for A \geq 4 when plotted versus a dimensionless scaling variable
\psi' (scaling of the second kind). We use this behavior to study as yet poorly
understood properties of the inclusive response at large electron energy loss.Comment: 33 pages, 12 color EPS figures, LaTeX2e using BoxedEPSF macros; email
to [email protected]
Reguladores vegetais e teores endógenos de poliaminas durante o desenvolvimento de taro cultivado in vitro
Plane-wave impulse approximation extraction of the neutron magnetic form factor from quasielastic 3He(e,e′) at Q2=0.3 to 0.6 (GeV/c)2
A high precision measurement of the transverse spin-dependent asymmetry AT′ in 3He(e,e′) quasielastic scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q2, between 0.1 and 0.6 (GeV/c)2. AT′ is sensitive to the neutron magnetic form factor, GMn. Values of GMn at Q2=0.1 and 0.2 (GeV/c)2, extracted using Faddeev calculations, were reported previously. Here, we report the extraction of GMn for the remaining Q2 values in the range from 0.3 to 0.6 (GeV/c)2 using a plane-wave impulse approximation calculation. The results are in good agreement with recent precision data from experiments using a deuterium target
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