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Register a new userMeasurements of the electron-helicity asymmetry in the quasi-elastic ${rm A}(vec{e},e p)$ process
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We present measurements of the electron helicity asymmetry in quasi-elastic proton knockout from $^{2}$H and $^{12}$C nuclei by polarized electrons. This asymmetry depends on the fifth structure function, is antisymmetric with respect to the scattering plane, and vanishes in the absence of final-state interactions, and thus it provides a sensitive tool for their study. Our kinematics cover the full range in off-coplanarity angle $phi_{pq}$, with a polar angle $theta_{pq}$ coverage up to about 8 degrees. The missing energy resolution enabled us to determine the asymmetries for knock-out resulting in different states of the residual $^{11}$B system. We find that the helicity asymmetry for $p$-shell knockout from $^{12}$C depends on the final state of the residual system and is relatively large (up to $approx 0.16$), especially at low missing momentum. It is considerably smaller (up to $approx 0.01$) for $s$-shell knockout from both $^{12}$C and $^2$H. The data for $^2$H are in very good agreement with theoretical calculations, while the predictions for $^{12}$C exhibit differences with respect to the data.
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We report on a precise measurement of double-polarization asymmetries in electron-induced breakup of $^3mathrm{He}$ proceeding to $mathrm{pd}$ and $mathrm{ppn}$ final states, performed in quasi-elastic kinematics at $Q^2 = 0.25,(mathrm{GeV}/c)^2$ for missing momenta up to $250,mathrm{MeV}/c$. These observables represent highly sensitive tools to investigate the electromagnetic and spin structure of $^3mathrm{He}$ and the relative importance of two- and three-body effects involved in the breakup reaction dynamics. The measured asymmetries cannot be satisfactorily reproduced by state-of-the-art calculations of $^3mathrm{He}$ unless their three-body segment is adjusted, indicating that the spin-dependent part of the nuclear interaction governing the three-body breakup process is much smaller than previously thought.
We present first measurements of the double ratio of the polarization transfer components $(P^{prime}_{!x} !/ P^{prime}_{!z} )_p/ (P^{prime}_{!x} !/ P^{prime}_{!z} )_s$ for knock-out protons from $s$ and $p$ shells in $^{12}{rm C}$ measured by the $^{12}{rm C}(vec{e},{e}vec{p},)$ reaction in quasi-elastic kinematics. The data are compared to theoretical predictions in relativistic distorted-wave impulse approximation. Our results show that differences between $s$- and $p$-shell protons, observed when compared at the same initial momentum (missing momentum) largely disappear when the comparison is done at the same proton virtuality. We observe no density-dependent medium modifications for protons from $s$ and $p$ shells with the same virtuality in spite of the large differences in the respective nuclear densities.
We report measurements of the induced polarization $vec P$ of protons knocked out from $^2$H and $^{12}$C via the $A(e,evec p,)$ reaction. We have studied the dependence of $vec P$ on two kinematic variables: the missing momentum $p_{rm miss}$ and the off-coplanarity angle $phi_{pq}$ between the scattering and reaction planes. For the full 360$degree$ range in $phi_{pq}$, both the normal ($P_y$) and, for the first time, the transverse ($P_x$) components of the induced polarization were measured with respect to the coordinate system associated with the scattering plane. $P_x$ vanishes in coplanar kinematics, however in non-coplanar kinematics, it is on the same scale as $P_y$. We find that the dependence on $phi_{pq}$ is sine-like for $P_x$ and cosine-like for $P_y$. For carbon, the magnitude of the induced polarization is especially large when protons are knocked out from the $p_{3/2}$ shell at very small $p_{rm miss}$. For the deuteron, the induced polarization is near zero at small $|p_{rm miss}|$, and its magnitude increases with $|p_{rm miss}|$. 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 report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio $G^{n}_{E}/G^{n}_{M}$ was extracted from the beam-target vector asymmetry $A_{ed}^{V}$ at four-momentum transfers $Q^{2}=0.14$, 0.20, 0.29 and 0.42 (GeV/c)$^{2}$.
We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry is $A = -15.05 pm 0.98 ({rm stat}) pm 0.56 ({rm syst})$ ppm at the kinematic point $<theta_{rm lab} > = 12.{3^circ} $ and $<Q^2 > = 0.477$ (GeV/c)$^2$. Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factors $G^s_E + 0.392 G^s_M = 0.014 pm 0.020 pm 0.010$ where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.
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