We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, A_{⊥}^{ep}, at five beam energies from 315.1 to 1508.4 MeV and at a scattering angle ...of 30°<θ<40°. The covered Q^{2} values are 0.032, 0.057, 0.082, 0.218, 0.613 (GeV/c)^{2}. The measurement clearly indicates significant inelastic contributions to the two-photon-exchange (TPE) amplitude in the low-Q^{2} kinematic region. No theoretical calculation is able to reproduce our result. Comparison with a calculation based on unitarity, which only takes into account elastic and πN inelastic intermediate states, suggests that there are other inelastic intermediate states such as ππN, KΛ, and ηN. Covering a wide energy range, our new high-precision data provide a benchmark to study those intermediate states.
New measurements of the beam normal single spin asymmetry in the electron elastic and quasielastic scattering on the proton and deuteron, respectively, at large backward angles and at ⟨Q^{2}⟩=0.22 ...(GeV/c)^{2} and ⟨Q^{2}⟩=0.35 ( GeV/c)^{2} are reported. The experimentally observed asymmetries are compared with the theoretical calculation of Pasquini and Vanderhaeghen Phys. Rev. C 70, 045206 (2004).PRVCAN0556-281310.1103/PhysRevC.70.045206. The agreement of the measurements with the theoretical calculations shows a dominance of the inelastic intermediate excited states of the nucleon, πN and the Δ resonance. The measurements explore a new, important parameter region of the exchanged virtual photon virtualities.
A new measurement of the parity violating asymmetry in elastic electron scattering on hydrogen at backward angles and at a four momentum transfer of Q;{2} = 0.22 (Ge V / c);{2} is reported here. The ...measured asymmetry is A_{LR} = (-17.23 +/- 0.82_{stat} +/- 0.89_{syst}) x 10;{-6}. The standard model prediction assuming no strangeness is A_{0} = (-15.87 +/- 1.22) x 10;{-6}. In combination with previous results from measurements at forward angles, it is possible to disentangle for the first time the strange form factors at this momentum transfer, G_{E};{s} = 0.050 +/- 0.038 +/- 0.019 and G_{M};{s} = -0.14 +/- 0.11 +/- 0.11.
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The question of the determination of the proton charge radius
R
p
from electron scattering data led to an unprecedented experimental effort for measurements of the electric form factor of the ...proton at low and very low momentum transfer in electron and muon elastic scattering. On the basis of basic properties of densities and fitting bias considerations, a procedure is developed in order to evaluate the impact of forthcoming data on
R
p
. Particularly, it is shown that a 0.1% precision on these future cross section data is necessary to establish indisputably the
R
p
-value as determined from lepton scattering. An experimental method to meet this stringent constraint is further discussed in the case of the ProRad (Proton Radius) experiment at the PRAE (Platform for Research and Applications with Electrons) facility in Orsay.
We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta ...(e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.
Cross sections for (p,t) two-neutron transfer reactions are calculated in the one-step zero-range distorted-wave Born approximation for the tin isotopes {sup 124}Sn and {sup 136}Sn and for incident ...proton energies from 15 to 35 MeV. Microscopic quasiparticle random-phase approximation form factors are provided for the reaction calculation and phenomenological optical potentials are used in both the entrance and the exit channels. Three different surface/volume mixings of a zero-range density-dependent pairing interaction are employed in the microscopic calculations and the sensitivity of the cross sections to the different mixings is analyzed. Since absolute cross sections cannot be obtained within our model, we compare the positions of the diffraction minima and the shapes of the angular distributions. No differences are found in the position of the diffraction minima for the reaction {sup 124}Sn(p,t){sup 122}Sn. On the other side, the angular distributions obtained for the reaction {sup 136}Sn(p,t){sup 134}Sn with surface and mixed interactions differ at large angles for some values of the incident proton energy. For this reaction, we compare the ratios of the cross sections associated to the ground state and the first excited state transitions. Differences among the three different theoretical predictions are found and they are more important at the incident proton energy of 15 MeV. As a conclusion, we indicate (p,t) two-neutron transfer reactions with very neutron-rich Sn isotopes and at proton energies around 15 MeV as good experimental cases where the surface/volume mixing of the pairing interaction may be probed.
A new measurement of the parity-violating asymmetry in the electron-deuteron quasielastic scattering for backward angles at ⟨Q2⟩=0.224 (GeV/c)2, obtained in the A4 experiment at the Mainz Microtron ...accelerator (MAMI) facility, is presented. The measured asymmetry is APVd=(−20.11±0.87stat±1.03sys)×10−6. A combination of these data with the proton measurements of the parity-violating asymmetry in the A4 experiment yields a value for the effective isovector axial-vector form factor of GAe,(T=1)=−0.19±0.43 and RA(T=1),anap=−0.41±0.35 for the anapole radiative correction. When combined with a reanalysis of measurements obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility, the uncertainties are further reduced to GMs=0.17±0.11 for the magnetic strange form factors, and RA(T=1),anap=−0.54±0.26.
The precision of experimental data and analysis techniques is a key feature of any discovery attempt. A striking example is the proton radius puzzle where the accuracy of the spectroscopy of muonic ...atoms challenges traditional electron scattering measurements. The present work proposes a novel method for the determination of spatial moments from densities expressed in the momentum space. This method provides a direct access to even, odd, and more generally any real, negative and positive moment with order larger than −3. As an illustration, the application of this method to the electric form factor of the proton is discussed in detail.
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A Regge-inspired model is used to discuss the hard exclusive two-body hadronic reactions (
p
,
,
K
-
K
+
,
K
0
for the FAIR facility project at GSI with the PANDA detector. The comparison between ...the differential cross-sections predictions and the available data is shown to determine the values of the few parameters of the model.
Sufficient energy resolution is the key issue for the calorimetry in particle and nuclear physics. The calorimeter of the A4 parity violation experiment at MAMI is a segmented calorimeter where the ...energy of an event is determined by summing the signals of neighboring channels. In this case, the precise matching of the individual modules is crucial to obtain a good energy resolution. We have developed a calibration procedure for our total absorbing electromagnetic calorimeter which consists of 1022 lead fluoride (PbF
2) crystals. This procedure reconstructs the single-module contributions to the events by solving a linear system of equations, involving the inversion of a 1022×1022-matrix. The system has shown its functionality at beam energies between 300 and 1500
MeV and represents a new and fast method to keep the calorimeter permanently in a well-calibrated state.
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