Investigation of the conjectured nucleon deformation at low momentum transfer.

We report new precise H$(e,e^\prime p)\pi^0$ measurements at the $\Delta(1232)$ resonance at $Q^2= 0.127$ (GeV/c)$^2$ using the MIT/Bates out-of-plane scattering (OOPS) facility. The data reported here are particularly sensitive to the transverse electric amplitude ($E2$) of the $\gamma^* N\to\Delta$ transition. Analyzed together with previous data yield precise quadrupole to dipole amplitude ratios $EMR = (-2.3 \pm 0.3_{stat+sys} \pm 0.6_{model})%$ and $CMR = (-6.1 \pm 0.2_{stat+sys}\pm 0.5_{model})%$ and for $M^{3/2}_{1+} = (41.4 \pm 0.3_{stat+sys}\pm 0.4_{model})(10^{-3}/m_{\pi^+})$. They give credence to the conjecture of deformation in hadronic systems favoring, at low $Q^2$, the domin…

Evidence for Strange-Quark Contributions to the Nucleon’s Form Factors atQ2=0.108   (GeV/c)2

We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of ${Q}^{2}=0.108\text{ }(\mathrm{GeV}/c{)}^{2}$ and at a forward electron scattering angle of $30\ifmmode^\circ\else\textdegree\fi{}l{\ensuremath{\theta}}_{e}l40\ifmmode^\circ\else\textdegree\fi{}$. The measured asymmetry is ${A}_{LR}(\stackrel{\ensuremath{\rightarrow}}{e}p)=[\ensuremath{-}1.36\ifmmode\pm\else\textpm\fi{}0.29(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.13(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. The expectation from the standard model as…

Measurement of the magnetic form factor of the neutron

The [sup 2]H([ital e],[ital e][prime][ital n])[sup 1]H quasielastic cross section was measured at [ital Q][sup 2] values of 0.109, 0.176, and 0.255 (GeV/[ital c])[sup 2]. The neutron detection efficiency was determined by the associated particle technique with the [sup 2]H([gamma],[ital pn]) reaction for each of the three neutron kinetic energies. These [sup 2]H([ital e],[ital e][prime][ital n]) measurements of the coincidence cross sections are the first at low [ital Q][sup 2]. The cross sections are sensitive primarily to the neutron magnetic form factor [ital G][sub [ital M]][sup [ital n]] at these kinematics. The extracted [ital G][sub [ital M]][sup [ital n]] values have smaller uncerta…

Out-of-plane measurements of the fifth response function of the exclusive electronuclear response

The first measurements of f{sub LT}{sup '}, known as the fifth response function, have been made for the {sup 2}H(e(vector sign),e{sup '}p) and {sup 12}C(e(vector sign),e{sup '}p) reactions. This response is directly related to the imaginary part of the interference between the transverse and longitudinal nuclear electromagnetic currents. Its observation requires longitudinally polarized electron beams and out-of-plane detection, the latter made possible by the newly developed out-of-plane spectrometer system. The initial measurements were made by using a 560-MeV polarized electron beam and quasielastic kinematics at Q{sup 2}=3.3 fm{sup -2}. The development of the methodology for out-of-pla…

New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei

We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

We report on a new measurement of the beam transverse single spin asymmetry in electron-proton elastic scattering, $A^{ep}_{\perp}$, at five beam energies from 315.1 MeV to 1508.4 MeV and at a scattering angle of $30^{\circ} < \theta < 40^{\circ}$. 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 $\mathrm{\pi N}$ inelastic intermediate states, suggests that there are ot…

Elastic electron scattering from the multipole moment distributions ofMg25

Measurements of the Generalized Electric and Magnetic Polarizabilities of the Proton at LowQ2Using the Virtual-Compton-Scattering Reaction

Experimental details of a virtual Compton scattering (VCS) experiment performed on the proton at the MIT-Bates out-of-plane scattering facility are presented. The VCS response functions ${P}_{LL}\ensuremath{-}{P}_{TT}/\phantom{{P}_{TT}\ensuremath{\varepsilon}}\ensuremath{\varepsilon}$ and ${P}_{LT}$ have been measured at ${Q}^{2}=0.057\phantom{\rule{0.28em}{0ex}}{\mathrm{GeV}}^{2}/{c}^{2}$. The generalized electric and magnetic polarizabilities, $\ensuremath{\alpha}({Q}^{2})$ and $\ensuremath{\beta}({Q}^{2})$, and the mean-square electric polarizability radius$\ensuremath{\langle}{r}_{\ensuremath{\alpha}}^{2}\ensuremath{\rangle}$ are obtained from a dispersion analysis of the data. The resu…

Measurement of the partial cross sectionsσTT,σLT, and(σT+ɛσL)of the1H(e,e′π+)nreaction in theΔ(1232)resonance

We report precision {sup 1}H(e, e{sup '{pi}+})n measurements in the {Delta}(1232) resonance at Q{sup 2}=0.127(GeV/c){sup 2} obtained at the MIT-Bates out-of-plane scattering facility. These are the lowest, but nonzero, Q{sup 2} measurements in the {pi}{sup +} channel. The data offer tests of the theoretical calculations, particularly of the background amplitude contributions. The chiral effective field theory and Sato-Lee model calculations are not in agreement with this experiment.

Measurement of the Transverse Beam Spin Asymmetry in Elastic Electron Proton Scattering and the Inelastic Contribution to the Imaginary Part of the Two-Photon Exchange Amplitude

We report on a measurement of the asymmetry in the scattering of transversely polarized electrons off unpolarized protons, A$_\perp$, at two Q$^2$ values of \qsquaredaveragedlow (GeV/c)$^2$ and \qsquaredaveragedhighII (GeV/c)$^2$ and a scattering angle of $30^\circ &lt; ��_e &lt; 40^\circ$. The measured transverse asymmetries are A$_{\perp}$(Q$^2$ = \qsquaredaveragedlow (GeV/c)$^2$) = (\experimentalasymmetry alulowcorr $\pm$ \statisticalerrorlow$_{\rm stat}$ $\pm$ \combinedsyspolerrorlowalucor$_{\rm sys}$) $\times$ 10$^{-6}$ and A$_{\perp}$(Q$^2$ = \qsquaredaveragedhighII (GeV/c)$^2$) = (\experimentalasymme tryaluhighcorr $\pm$ \statisticalerrorhigh$_{\rm stat}$ $\pm$ \combinedsyspolerrorhi…

Relativistic Effects and Two-Body Currents inH2(e→,e′p)nUsing Out-of-Plane Detection

Measurements of the (2)H((-->)e,e(')p)n reaction were performed with the out-of-plane magnetic spectrometers (OOPS) at the MIT-Bates Linear Accelerator. The longitudinal-transverse, f(LT) and f(')(LT), and the transverse-transverse, f(TT), interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q2 = 0.15 (GeV/c)(2). In comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions and the importance of two-body meson-exchange currents and isobar configurations. We demonstrate that such effects can be disentangled by extracting these responses using the novel out-of…

Measurement of Strange-Quark Contributions to the Nucleon's Form Factors atQ2=0.230   (GeV/c)2

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 t…

Measurement of Strange Quark Contributions to the Vector Form Factors of the Proton atQ2=0.22  (GeV/c)2

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{sup 2}=0.22 (GeV/c){sup 2} is reported here. The measured asymmetry is A{sub LR}=(-17.23{+-}0.82{sub stat}{+-}0.89{sub syst})x10{sup -6}. The standard model prediction assuming no strangeness is A{sub 0}=(-15.87{+-}1.22)x10{sup -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{sub E}{sup s}=0.050{+-}0.038{+-}0.019 and G{sub M}{sup s}=-0.14{+-}0.11{+-}0.11.

New Measurements of the Beam Normal Spin Asymmetries at Large Backward Angles with Hydrogen and Deuterium Targets

International audience; 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 ⟨Q2⟩=0.22  (GeV/c)2 and ⟨Q2⟩=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 exc…

"Table 1" of "Measurement of the magnetic form factor of the neutron"

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