We report on a comprehensive reinterpretation of the existing cross-section data for elastic electron-proton scattering obtained by the initial-state radiation technique, resulting in a significantly ...improved accuracy of the extracted proton charge radius. By refining the external energy corrections we have achieved an outstanding description of the radiative tail, essential for a detailed investigation of the proton finite-size effects on the measured cross sections. This development, together with a novel framework for determining the radius, based on a regression analysis of the cross sections employing a polynomial model for the form factor, led us to a new value for the charge radius, which is
(
0.878
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0
.
011
stat
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±
0
.
031
sys
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±
0
.
002
mod
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fm
We report the measurement of the ratio of polarization-transfer components, Px/Pz, in the H2(e→,e′p→)n reaction at low and high missing momenta, in search of differences between free and bound ...protons. The observed deviation of Px/Pz from that of a free proton, which is similar to that observed in 4He, indicates that the effect in nuclei is a function of the virtuality of the knock-out proton and the missing momentum direction, but not
the average nuclear density. There is a general agreement between the data and calculations, which assume free proton form factors, however, the measurements are consistently about 10% higher.
A comparison between polarization-transfer to a bound proton in quasi-free kinematics by the A(e→,e′p→) knockout reaction and that in elastic scattering off a free proton can provide information on ...the characteristics of the bound proton. In the past the reported measurements have been compared to those of a free proton with zero initial momentum. We introduce, for the first time, expressions for the polarization-transfer components when the proton is initially in motion and compare them to the 2H data measured at the Mainz Microtron (MAMI). We show the ratios of the transverse (Px) and longitudinal (Pz) components of the polarization transfer in H2(e→,e′p→)n, to those of elastic scattering off a “moving proton”, assuming the proton's initial (Fermi-motion) momentum equals the negative missing momentum in the measured reaction. We found that the correction due to the proton motion is up to 20% at high missing momentum. However the effect on the double ratio (Px/Pz)A(Px/Pz)H1 is largely canceled out, as shown for both 2H and 12C data. This implies that the difference between the resting- and the moving-proton kinematics is not the primary cause for the deviations between quasi-elastic and elastic scattering reported previously.
We report measurements of the induced polarization P→ of protons knocked out from 2H and 12C via the A(e,e′p→) reaction. We have studied the dependence of P→ on two kinematic variables: the missing ...momentum pmiss and the “off-coplanarity” angle ϕpq between the scattering and reaction planes. For the full 360° range in ϕpq, both the normal (Py) and, for the first time, the transverse (Px) components of the induced polarization were measured with respect to the coordinate system associated with the scattering plane. Px vanishes in coplanar kinematics, however in non-coplanar kinematics, it is on the same scale as Py.
We find that the dependence on ϕpq is sine-like for Px and cosine-like for Py. For carbon, the magnitude of the induced polarization is especially large when protons are knocked out from the p3/2 shell at very small pmiss. For the deuteron, the induced polarization is near zero at small |pmiss|, and its magnitude increases with |pmiss|. For both nuclei such behavior is reproduced qualitatively by theoretical results, driven largely by the spin-orbit part of the final-state interactions. However, for both nuclei, sizeable discrepancies exist between experiment and theory.
We present measurements of the polarization-transfer components in the H2(e→,e′p→) reaction, covering a previously unexplored kinematic region with large positive (anti-parallel) missing momentum, ...pmiss, up to 220MeV/c, and Q2=0.65(GeV/c)2. These measurements, performed at the Mainz Microtron (MAMI), were motivated by theoretical calculations which predict small final-state interaction (FSI) effects in these kinematics, making them favorable for searching for medium modifications of bound nucleons in nuclei. We find in this kinematic region that the measured polarization-transfer components Px and Pz and their ratio agree with the theoretical calculations, which use free-proton form factors. Using this, we establish upper limits on possible medium effects that modify the bound proton's form factor ratio GE/GM at the level of a few percent. We also compare the measured polarization-transfer components and their ratio for 2H to those of a free (moving) proton. We find that the universal behavior of 2H, 4He and 12C in the double ratio (Px/Pz)A(Px/Pz)H1 is maintained in the positive missing-momentum region.
The cross section of the p(e,e′π+)n reaction has been measured for five kinematic settings at an invariant mass of W=1094 MeV and for a four-momentum transfer of Q2=0.078 (GeV/c)2. The measurement ...has been performed at MAMI using a new short-orbit spectrometer (SOS) of the A1 collaboration, intended for detection of low-energy pions. The transverse and longitudinal cross section terms were separated using the Rosenbluth method and the transverse-longitudinal interference term has been determined from the left-right asymmetry. The experimental cross section terms are compared with the calculations of three models: DMT2001, MAID2007 and χMAID. The results show that we do not yet understand the dynamics of the fundamental pion.
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