861 research outputs found
Electromagnetic form factors of the nucleon in effective field theory
We calculate the electromagnetic form factors of the nucleon to third chiral
order in manifestly Lorentz-invariant effective field theory. The rho and omega
mesons as well as the Delta(1232) resonance are included as explicit dynamical
degrees of freedom. To obtain a self-consistent theory with respect to
constraints we consider the proper relations among the couplings of the
effective Lagrangian. For the purpose of generating a systematic power
counting, the extended on-mass-shell renormalization scheme is applied in
combination with the small-scale expansion. The results for the electric and
magnetic Sachs form factors are analyzed in terms of experimental data and
compared to previous findings in the framework of chiral perturbation theory.
The pion-mass dependence of the form factors is briefly discussed.Comment: 26 pages, 9 figure
Positron Beams and Two-Photon Exchange: The Key to Precision Form Factors
The proton elastic form factor ratio can be measured either via Rosenbluth
separation in an unpolarized beam and target experiment, or via the use of
polarization degrees of freedom. However, data produced by these two approaches
show a discrepancy, increasing with . The proposed explanation of this
discrepancy---two-photon exchange---has been tested recently by three
experiments. The results support the existence of a small two-photon exchange
effect but cannot establish that theoretical treatment at the measured momentum
transfers are valid. At larger momentum transfers, theory remains untested.
This paper investigates the possibilities of measurements at DESY and Jefferson
Lab to measure the effect at larger momentum transfers.Comment: 6 pages, 5 figures. Conference proceedings from JPOS17
(https://www.jlab.org/conferences/JPos2017/
Two-Photon Exchange: Future experimental prospects
The proton elastic form factor ratio is accessible in unpolarized
Rosenbluth-type experiments as well as experiments which make use of
polarization degrees of freedom. The extracted values show a distinct
discrepancy, growing with . Three recent experiments tested the proposed
explanation, two-photon exchange, by measuring the positron-proton to
electron-proton cross section ratio. In the results, a small two-photon
exchange effect is visible, significantly different from theoretical
calculation. Theory at larger momentum transfer remains untested. This paper
discusses the possibilities for future measurements at larger momentum
transfer.Comment: 7 pages, 4 figures. Contribution to the proceedings of the 11th
International Workshop on the Physics of Excited Nucleons (NSTAR 2017
The effect of nine days of recumbency, with and without exercise, on the redistribution of body fluids and electrolytes, renal function and metabolism
Effect of nine days of recumbency with and without exercise on redistribution of body fluids and electrolytes, renal functions and metabolis
Polarization transfer in scattering using the Super BigBite Spectrometer
The effects of multi-photon-exchange and other higher-order QED corrections
on elastic electron-proton scattering have been a subject of high experimental
and theoretical interest since the polarization transfer measurements of the
proton electromagnetic form factor ratio at large momentum
transfer conclusively established the strong decrease of this ratio with
for GeV. This result is incompatible with previous
extractions of this quantity from cross section measurements using the
Rosenbluth Separation technique. Much experimental attention has been focused
on extracting the two-photon exchange (TPE) effect through the unpolarized
cross section ratio, but polarization transfer in polarized elastic
scattering can also reveal evidence of hard two-photon exchange. Furthermore,
it has a different sensitivity to the generalized TPE form factors, meaning
that measurements provide new information that cannot be gleaned from
unpolarized scattering alone. Both -dependence of polarization
transfer at fixed , and deviations between electron-proton and
positron-proton scattering are key signatures of hard TPE. A polarized positron
beam at Jefferson Lab would present a unique opportunity to make the first
measurement of positron polarization transfer, and comparison with
electron-scattering data would place valuable constraints on hard TPE. Here, we
propose a measurement program in Hall A that combines the Super BigBite
Spectrometer for measuring recoil proton polarization, with a non-magnetic
calorimetric detector for triggering on elastically scattered positrons. Though
the reduced beam current of the positron beam will restrict the kinematic
reach, this measurement will have very small systematic uncertainties, making
it a clean probe of TPE.Comment: 6 pages, 3 figures. Contribution to the EPJA topical issue, "An
Experimental Program with Positron Beams at Jefferson Lab." arXiv admin note:
substantial text overlap with arXiv:2007.15081, arXiv:1906.0941
The RMS Charge Radius of the Proton and Zemach Moments
On the basis of recent precise measurements of the electric form factor of
the proton, the Zemach moments, needed as input parameters for the
determination of the proton rms radius from the measurement of the Lamb shift
in muonic hydrogen, are calculated. It turns out that the new moments give an
uncertainty as large as the presently stated error of the recent Lamb shift
measurement of Pohl et al.. De Rujula's idea of a large Zemach moment in order
to reconcile the five standard deviation discrepancy between the muonic Lamb
shift determination and the result of electronic experiments is shown to be in
clear contradiction with experiment. Alternative explanations are touched upon.Comment: 6 pages, 4 figures, final version includes discussion of systematic
and numerical error
The OLYMPUS Internal Hydrogen Target
An internal hydrogen target system was developed for the OLYMPUS experiment
at DESY, in Hamburg, Germany. The target consisted of a long, thin-walled,
tubular cell within an aluminum scattering chamber. Hydrogen entered at the
center of the cell and exited through the ends, where it was removed from the
beamline by a multistage pumping system. A cryogenic coldhead cooled the target
cell to counteract heating from the beam and increase the density of hydrogen
in the target. A fixed collimator protected the cell from synchrotron radiation
and the beam halo. A series of wakefield suppressors reduced heating from beam
wakefields. The target system was installed within the DORIS storage ring and
was successfully operated during the course of the OLYMPUS experiment in 2012.
Information on the design, fabrication, and performance of the target system is
reported.Comment: 9 pages, 13 figure
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