18 research outputs found
Double polarization experiments at intermediate energy
At modern electron accelerators with highly polarized, intense, high duty
factor beams double polarization coincidence experiments became feasible with
good statistical accuracy. The strong potential towards the precise
determination of small nucleon structure quantities is illustrated by two
recent examples from MAMI. The measurement of in the quasifree reaction
lead to a new parametrization of which is
significantly above the previously preferred one from elastic scattering.
A experiment at the energy of the resonance
yields preliminary results for the longitudinal quadrupole mixing. Both
experimental errors and model uncertainties are complementary to unpolarized
measurements.Comment: 8 pages, 6 figures, plenary talk given at PANIC'9
The role of the pion cloud in electroproduction of the (1232)
We calculate the ratios and of the multipole amplitudes for
electroproduction of the (1232) in the range of photon virtuality
~GeV in a chiral chromodielectric model and a linear
-model. We find that relatively large experimental values can be
explained in terms of the pion contribution alone; the contribution arising
from d-state quark admixture remains below 10\%. We describe the pion cloud as
a coherent state and use spin and isospin projection to obtain the physical
nucleon and the . The and amplitudes are reasonably
well reproduced in the -model; in the chromodielectric model, however,
they are a factor of two too small.Comment: 10 pages LaTeX2e, 3 LaTeX figures within the text; Requires
elsart.cls (included in the self-unpacking uuencoded gzipped file). (Accepted
for publication in Phys. Lett. B
Relativistic effects in electromagnetic nuclear responses in the quasi-elastic delta region
A new non-relativistic expansion in terms of the nucleon's momentum inside
nuclear matter of the current for isobar electro-excitation from the nucleon is
performed. Being exact with respect to the transferred energy and momentum,
this yields new current operators which retain important aspects of relativity
not taken into account in the traditional non-relativistic reductions. The
transition current thus obtained differs from the leading order of the
traditional expansion by simple multiplicative factors. These depend on the
momentum and energy transfer and can be easily included together with
relativistic kinematics in non-relativistic, many-body models of isobar
electro-excitation in nuclei. The merits of the new current are tested by
comparing with the unexpanded electromagnetic nuclear responses in the isobar
peak computed in a relativistic Fermi gas framework. The sensitivity of the
relativistic responses to the isobar's magnetic, electric and Coulomb form
factors and the finite width of the isobar is analyzed.Comment: 26 pages plus 6 figure
The Drell-Hearn-Gerasimov Sum Rule
The Drell-Hearn-Gerasimov (DHG) sum rule relates the helicity structure of
the photoabsorption cross section to the anomalous magnetic moment of the
nucleon. It is based on Lorentz and gauge invariance, crossing symmetry,
causality and unitarity. A generalized DHG sum rule my be derived for virtual
photons. At low momentum transfer this generalized sum rule is saturated by the
resonance region, at high momentum transfer it may be expressed by the parton
spin distributions measured in deep inelastic scattering. The
longitudinal-transverse interference determines the Cottingham sum rule, which
is related to the electric and magnetic form factors over the whole range of
momentum transfer.Comment: 20 pages LATEX plus 12 figures (available by fax from the author
A unitary isobar model for pion photo- and electroproduction on the proton up to 1 GeV
A new operator for pion photo- and electroproduction has been developed for
nuclear applications at photon equivalent energies up to 1 GeV. The model
contains Born terms, vector mesons and nucleon resonances (,
, , , , and
). The resonance contributions are included taking into account
unitarity to provide the correct phases of the pion photoproduction multipoles.
The dependence of electromagnetic resonance vertices is described with
appropriate form factors in the electromagnetic helicity amplitudes. Within
this model we have obtained good agreement with the experimental data for pion
photo- and electroproduction on the nucleon for both differential cross
sections and polarization observables. The model can be used as a starting
point to predict and analyze forthcoming data.Comment: 32 pages LaTeX including 23 postscript figures (a few misprints have
been corrected
Intrinsic quadrupole moment of the nucleon
We address the question of the intrinsic quadrupole moment Q_0 of the nucleon
in various models. All models give a positive intrinsic quadrupole moment for
the proton. This corresponds to a prolate deformation. We also calculate the
intrinsic quadrupole moment of the Delta(1232). All our models lead to a
negative intrinsic quadrupole moment of the Delta corresponding to an oblate
deformation.Comment: 17 pages, 5 figure
(gamma,np) reactions in <sup>12</sup>C , <sup>6</sup>Li and <sup>3,4</sup>He
The emission of neutron-proton pairs is the most probable outcome of photon absorbtion in the energy region above the giant resonance at least up to the pion threshold, but little detailed information on the process has been obtained due to experimental difficulties. Two nucleon emission following photon absorbtion by a correlated pair is favoured
compared to direct knockout of a single nucleon, which is suppressed by the large momentum mismatch between the ingoing photon and a single outgoing fast nucleon. Studies of the (gamma,np) process seek firstly to obtain a quantitative
understanding of the photon interaction mechanism, and through this to open the door to investigations of nucleon
correlations in nuclei [1], information about which is long sought but not readily obtainable
Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the \Delta(1232) resonance
The recoil proton polarization has been measured in the p (\vec e,e'\vec p)
pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2
and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz
Microtron. Due to the spin precession in a magnetic spectrometer, all three
proton polarization components P_x/P_e = (-11.4 \pm 1.3 \pm 1.4) %, P_y =
(-43.1 \pm 1.3 \pm 2.2) %, and P_z/P_e = (56.2 \pm 1.5 \pm 2.6) % could be
measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR =
(-6.4\pm 0.7_{stat}\pm 0.8_{syst}) % was determined from P_x in the framework
of the Mainz Unitary Isobar Model. The consistency among the reduced
polarizations and the extraction of the ratio of longitudinal to transverse
response is discussed.Comment: 5 pages LaTeX, 1 table, 2 eps figure
The electroproduction of the (1232) in the chiral quark-soliton model
We calculate the ratios E2/M1 and C2/M1 for the electroproduction of the
(1232) in the region of photon virtuality GeV. The
magnetic dipole amplitude M1 is also presented. The theory used is the chiral
quark-soliton model, which is based in the instanton vaccum of the QCD. The
calculations are performed in flavor SU(2) and SU(3) taking rotational
() corrections into account. The results for the ratios agree
qualitatively with the available data, although the magnitude of both ratios
seems to underestimate the latest experimental results.Comment: 14 pages, Latex, 3 figures, uses eps
Delta-to-N-gamma Coulomb Quadrupole Amplitude in PQCD
We present a leading-order pQCD calculation of the helicity-flip matrix element (Coulomb quadrupole amplitude ), taking
into account the transverse momenta of the quarks and the contribution from the
gluons. In the large limit, its scaling behavior acquires a
double-logarithmic correction compared with the
standard scaling analysis, due to the contribution from the orbital motion of
the small- partons. Based on this and on the latest JLab experimental
results of the ratio at = 3 4 GeV, we make a
phenomenological prediction for the latter at higher values of .Comment: 9 pages, two figure