48 research outputs found
Final state interaction effects in neutrino-nucleus quasielastic scattering
We consider the charged-current quasielastic scattering of muon neutrinos on
an Oxygen 16 target, described within a relativistic shell model and, for
comparison, the relativistic Fermi gas. Final state interactions are described
in the distorted wave impulse approximation, using both a relativistic mean
field potential and a relativistic optical potential, with and without
imaginary part. We present results for inclusive cross sections at fixed
neutrino energies in the range 200 MeV - 1 GeV, showing that final
state interaction effects can remain sizable even at large energies.Comment: 4 pages, 4 figures; poster session of the Third International
Workshop on Neutrino-Nucleus Interactions in the Few GeV Region (NuInt04),
Gran Sasso (Italy), March 17-21, 2004; to appear in the proceeding
Relativistic description of 3He(e,e'p)2H
The Relativistic Distorted-Wave Impulse Approximation is used to describe the
He()H process. We describe the He nucleus within the
adiabatic hyperspherical expansion method with realistic nucleon-nucleon
interactions. The overlap between the He and the deuteron wave functions
can be accurately computed from a three-body calculation. The nucleons are
described by solutions of the Dirac equation with scalar and vector (S-V)
potentials. The wave function of the outgoing proton is obtained by solving the
Dirac equation with a S-V optical potential fitted to elastic proton scattering
data on the residual nucleus. Within this theoretical framework, we compute the
cross section of the reaction and other observables like the
transverse-longitudinal asymmetry, and compare them with the available
experimental data measured at JLab.Comment: 4 pages, 3 figures. Proceedings of the 21st European Few Body
Conference held in Salamanca (Spain) in August-September 201
Superscaling analysis of the Coulomb Sum Rule in quasielastic electron-nucleus scattering
The Coulomb sum rule for inclusive quasielastic electron scattering in
C, Ca and Fe is analyzed based on scaling and superscaling
properties. Results obtained in the relativistic impulse approximation with
various descriptions of the final state interactions are shown. A comparison
with experimental data measured at Bates and Saclay is provided. The
theoretical description based on strong scalar and vector terms present in the
relativistic mean field, which has been shown to reproduce the experimental
asymmetric superscaling function, leads to results that are in fair agreement
with Bates data while it sizeably overestimates Saclay data. We find that the
Coulomb sum rule for a momentum transfer saturates to a
value close to 0.9, being very similar for the three nuclear systems
considered. This is in accordance with Bates data, which indicates that these
show no significative quenching in the longitudinal response.Comment: 22 pages, 6 figures. To be published in Phys. Lett.
Scaling and isospin effects in quasielastic lepton-nucleus scattering in the Relativistic Mean Field Approach
The role of isospin in quasielastic electron scattering and charge-changing
neutrino reactions is investigated in the relativistic impulse approximation.
We analyze proton and neutron scaling functions making use of various
theoretical descriptions for the final-state interactions, focusing on the
effects introduced by the presence of strong scalar and vector terms in the
relativistic mean field approach. An explanation for the differences observed
in the scaling functions evaluated from and reactions is
provided by invoking the differences in isoscalar and isovector contributions.Comment: 10 pages, 5 figures, submitted to Phys. Lett.
Strange form factors of the proton: a new analysis of the neutrino (antineutrino) data of the BNL-734 experiment
We consider ratios of elastic neutrino(antineutrino)-proton cross sections
measured by the Brookhaven BNL-734 experiment and use them to obtain the
neutral current (NC) over charged current (CC) neutrino-antineutrino asymmetry.
We discuss the sensitivity of these ratios and of the asymmetry to the
electric, magnetic and axial strange form factors of the nucleon and to the
axial cutoff mass M_A. We show that the effects of the nuclear structure and
interactions on the asymmetry and, in general, on ratios of cross sections are
negligible. We find some restrictions on the possible values of the parameters
characterizing the strange form factors. We show that a precise measurement of
the neutrino-antineutrino asymmetry would allow the extraction of the axial and
vector magnetic strange form factors in a model independent way. The
neutrino-antineutrino asymmetry turns out to be almost independent on the
electric strange form factor and on the axial cutoff mass.Comment: 12 page
Systematic study of Coulomb distortion effects in exclusive (e,e'p) reactions
A technique to deal with Coulomb electron distortions in the analysis of
(e,e'p) reactions is presented. Thereby, no approximations are made. The
suggested technique relies on a partial-wave expansion of the electron wave
functions and a multipole decomposition of the electron and nuclear current in
momentum space. In that way, we succeed in keeping the computational times
within reasonable limits. This theoretical framework is used to calculate the
quasielastic (e,e'p) reduced cross sections for proton knockout from the
valence shells in O, Ca, Zr and Pb. The
final-state interaction of the ejected proton with the residual nucleus is
treated within an optical potential model. The role of electron distortion on
the extracted spectroscopic factors is discussed.Comment: 45 pages, 10 encapsulated postscript figures, Revtex, uses epsfig.sty
and fancybox.sty, to be published in Physical Review
Relativistic analysis of the 208Pb(e,e'p)207Tl reaction at high momentum
The recent 208Pb(e,e'p)207Tl data from NIKHEF-K at high missing momentum
(p_m>300 MeV/c) are compared to theoretical results obtained with a fully
relativistic formalism previously applied to analyze data on the low missing
momentum (p_m < 300 MeV/c) region. The same relativistic optical potential and
mean field wave functions are used in the two p_m-regions. The spectroscopic
factors of the various shells are extracted from the analysis of the low-p_m
data and then used in the high-p_m region. In contrast to previous analyses
using a nonrelativistic mean field formalism, we do not find a substantial
deviation from the mean field predictions other than that of the spectroscopic
factors, which appear to be consistent with both low- and high-p_m data. We
find that the difference between results of relativistic and nonrelativistic
formalisms is enhanced in the p_m<0 region that will be interesting to explore
experimentally.Comment: 12 pages, LaTeX+Revtex, included 3 postscript figures. To appear in
the Physical Review C (Rapid Communications
Meson exchange currents in electromagnetic one-nucleon emission
The role of meson exchange currents (MEC) in electron- and photon-induced
one-nucleon emission processes is studied in a nonrelativistic model including
correlations and final state interactions. The nuclear current is the sum of a
one-body and of a two-body part. The two-body current includes pion seagull,
pion-in-flight and the isobar current contributions. Numerical results are
presented for the exclusive 16O(e,e'p)15N and 16O(\gamma,p)15N reactions. MEC
effects are in general rather small in (e,e'p), while in (\gamma,p) they are
always large and important to obtain a consistent description of (e,e'p) and
(\gamma,p) data, with the same spectroscopic factors. The calculated (\gamma,p)
cross sections are sensitive to short-range correlations at high values of the
recoil momentum, where MEC effects are larger and overwhelm the contribution of
correlations.Comment: 9 pages, 6 figure
Improved dead-time correction for PET scanners: Application to small-animal PET
Pile-up and dead-time are two main causes of nonlinearity in the response of a PET scanner as a function of activity in the field of view (FOV). For a given scanner and acquisition system, pile-up effects depend on the material and size of the object being imaged and on the distribution of activity inside and outside the FOV, because these factors change the singles-to-coincidences ratio (SCR). Thus, it is difficult to devise an accurate correction that would be valid for any acquisition. In this work, we demonstrate a linear relationship between SCR and effective dead-time, which measures the effects of both dead-time (losses) and pile-up (gains and losses). This relationship allows us to propose a simple method to accurately estimate dead-time and pile-up corrections using only two calibration acquisitions with, respectively, a high and low SCR. The method has been tested with simulations and experimental data for two different scanner geometries: a scanner with large area detectors and no pile-up rejection, and a scanner composed of two full rings of smaller detectors. Our results show that the SCR correction method is accurate within 7%, even for high activities in the FOV, and avoids the bias of the standard single-parameter method. © 2013 Institute of Physics and Engineering in Medicine.This work was partially funded by AMIT project (CEN-20101014) from the CDTICENIT program, CIBERsam (CB07/09/0031), projects TEC2010-21619-C04-01 and TEC2011-28972-C02-01 from Spanish Ministerio de Ciencia e Innovación, Spanish Government (ENTEPRASE Grant, PSE-300000-2009-5), PRECISION grant IPT-300000- 2010-3, CPAN (CSD-2007-00042@Ingenio2010), MEC (FPA2010-17142) and ARTEMIS program (S2009/DPI-1802) from Spanish Comunidad de Madrid and EU-ERDF program.Peer Reviewe