684 research outputs found
Axial form factor of the nucleon at large momentum transfers
Motivated by the emerging possibilities to study threshold pion
electroproduction at large momentum transfers at Jefferson Laboratory following
the 12 GeV upgrade, we provide a short theory summary and an estimate of the
nucleon axial form factor for large virtualities in the range using next-to-leading order light-cone sum rules.Comment: A comparison to the new neutrino data analysis and several references
added. Final version to appear in Phys.Rev.
Electroproduction of Soft Pions at Large Momentum Transfers
We consider pion electroproduction on a proton target close to threshold for
Q^2 in the region 1-10 GeV^2. The momentum transfer dependence of the S-wave
multipoles at threshold, E_{0+} and L_{0+}, is calculated using light-cone sum
rules.Comment: 8 pages, 3 figures; Invited talk at the workshop on Exclusive
Reactions at High Momentum Transfer, 21-24 May 2007, Newport News, Virginia,
U.S.A. and International Conference on hadron Physics TROIA'07, 30 Aug. - 3
Sept. 2007, Canakkale, Turke
Two Photon Exchange for Exclusive Pion Electroproduction
We perform detailed calculations of two-photon-exchange QED corrections to
the cross section of pion electroproduction. The results are obtained with and
without the soft-photon approximation; analytic expressions for the radiative
corrections are derived. The relative importance of the two-photon correction
is analyzed for the kinematics of several experiments at Jefferson Lab. A
significant, over 20%, effect due to two-photon exchange is predicted for the
backward angles of electron scattering at large transferred momenta
A measurement of the axial form factor of the nucleon by the p(e,e'pi+)n reaction at W=1125 MeV
The reaction p(e,e'pi+)n was measured at the Mainz Microtron MAMI at an
invariant mass of W=1125 MeV and four-momentum transfers of Q^2=0.117, 0.195
and 0.273 (GeV/c)^2. For each value of Q^2, a Rosenbluth separation of the
transverse and longitudinal cross sections was performed. An effective
Lagrangian model was used to extract the `axial mass' from experimental data.
We find a value of M_A=(1.077+-0.039) GeV which is (0.051+-0.044) GeV larger
than the axial mass known from neutrino scattering experiments. This is
consistent with recent calculations in chiral perturbation theory.Comment: 14 pages, 5 figures, uses elsart.cl
Incoherent single pion electroproduction on the deuteron with polarization effects
Incoherent pion electroproduction on the deuteron is studied from threshold
up to the second resonance region with special emphasis on the influence of
final state interaction, in particular on polarization observables. The
elementary amplitude is taken from the MAID-2003 model.
Final state interaction is included by considering complete rescattering in the
final and subsystems. Their influence on the structure functions
governing the semi-exclusive differential cross section, where besides the
scattered electron only the produced pion is detected, is investigated in
detail. For charged pion-production the effect of -rescattering is moderate
whereas -rescattering is almost negligible, except very close to
threshold. -rescattering appears much stronger in neutral pion production
for which the primary mechanism is the elimination of a significant spurious
coherent contribution in the impulse approximation. Sizeable effects are also
found in some of the polarization structure functions for beam and/or target
polarizations.Comment: 20 pages including 14 figure
Invariant Amplitudes for Pion Electroproduction
The invariant amplitudes for pion electroproduction on the nucleon are
evaluated by dispersion relations at constant t with MAID as input for the
imaginary parts of these amplitudes. In the threshold region these amplitudes
are confronted with the predictions of several low-energy theorems derived in
the soft-pion limit. In general agreement with Chiral Perturbation Theory, the
dispersive approach yields large corrections to these theorems because of the
finite pion mass.Comment: 18 pages, 8 figure
Theory Support for the Excited Baryon Program at the Jlab 12 GeV Upgrade
This document outlines major directions in theoretical support for the
measurement of nucleon resonance transition form factors at the JLab 12 GeV
upgrade with the CLAS12 detector. Using single and double meson production,
prominent resonances in the mass range up to 2 GeV will be studied in the range
of photon virtuality up to 12 GeV where quark degrees of freedom are
expected to dominate. High level theoretical analysis of these data will open
up opportunities to understand how the interactions of dressed quarks create
the ground and excited nucleon states and how these interactions emerge from
QCD. The paper reviews the current status and the prospects of QCD based model
approaches that relate phenomenological information on transition form factors
to the non-perturbative strong interaction mechanisms, that are responsible for
resonance formation.Comment: 52 pages, 19 figures, White Paper of the Electromagnetic N-N*
Transition Form Factor Workshop at Jefferson Lab, October 13-15, 2008,
Newport News, VA, US
Study of the A(e,e') Reaction on H, H, C, Al, Cu and Au
Cross sections for the p()n process on H, H, C,
Al, Cu and Au targets were measured at the Thomas
Jefferson National Accelerator Facility (Jefferson Lab) in order to extract the
nuclear transparencies. Data were taken for four-momentum transfers ranging
from =1.1 to 4.8 GeV for a fixed center of mass energy of =2.14
GeV. The ratio of and was extracted from the measured
cross sections for H, H, C and Cu targets at = 2.15
and 4.0 GeV allowing for additional studies of the reaction mechanism. The
experimental setup and the analysis of the data are described in detail
including systematic studies needed to obtain the results. The results for the
nuclear transparency and the differential cross sections as a function of the
pion momentum at the different values of are presented. Global features
of the data are discussed and the data are compared with the results of model
calculations for the p()n reaction from nuclear targets.Comment: 28 pages, 19 figures, submited to PR
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