Session LJ: Electromagnetic Interactions

Studies of the $(e,e'p)$ reaction on $^{208}$Pb, $^{209}$Bi and $^{12}$C at quasielastic kinematics at large values of $Q^2$

Experiment E06-007 took place in Hall A at JLab (Virginia, USA) in 2007 and 2008. It measured the $^{208}$Pb$(e,e'p)^{207}$Tl, $^{209}$Bi$(e,e'p)^{208}$Pb and $^{12}$C$(e,e'p)^{11}$B reaction cross sections at true quasielastic kinematics with $(\textbf{q},\omega)$ constant ($x_B = 1$, $q$ = 1 GeV/c, $\omega = 0.433$ GeV) and at both sides of $\textbf{q}$ over a wide range of missing momenta ($0 < p_m < 500$ MeV/c). The experiment addresses several issues concerning our understanding of nuclear structure like the role of relativity and of long-range correlations in the description of nuclei and a possible dependence on $Q^2$ of spectroscopic factors. The existence of long range correlations is checked looking for excess of strength with respect to the predictions of the mean field approximation at high missing momentum. The presence of relativistic effects in nuclei is checked measuring the asymmetry $A_{TL}$, that is the ratio between the difference of the cross sections of both sides of $\textbf{q}$ divided by the sum. The possible dependence on $Q^2$ of spectroscopic factors is investigated with the measurements of the cross sections at low missing momentum and at $Q^2$ from 0.81 to 1.97 GeV/c$^2$.   

A precise extraction of the proton recoil polarization in $^4$He($\vec e,e^{\prime} \vec p\,$)$^3$H

I will present final results from the experiment E03-104 at Jefferson Lab where the proton recoil polarization in the $^4$He($\vec e,e^{\prime} \vec p\,$)$^3$H reaction was measured with unprecedented precision at $Q^2$ values of 0.8 (GeV/$c$)$^2$ and 1.3 (GeV/$c$)$^2$. We extracted both the polarization-transfer coefficients and the induced polarization. The precise extraction of the latter was only possible after extensive work to minimize false asymmetries, and provides a measure of final-state interactions. The measured ratio of polarization-transfer coefficients differs from a fully relativistic calculation by Udias {\it et al.}. The inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model or a chiral quark-soliton model brings the calculation in agreement with the data. Our data are equally well described by the prediction of Schiavilla {\it et al.} which instead uses free proton form factors but incorporates meson-exchange current effects and strong charge-exchange final state interactions. Neither theoretical calculation offers a satisfactory description of our induced polarization results.   

Light Vector Meson Photoproduction off of $^1$H at Jefferson Lab

Modification of light vector mesons in the nuclear medium continues to be a topic of high interest to the nuclear physics community. Results from recent photoproduction and heavy ion collision experiments have shown a clear broadening of the width in medium, but there remains no consensus on whether the meson undergoes a definite mass shift. An experimental study of the elementary free-space processes is necessary for a precise interpretation of prior in-medium analysis, and is valuable input for theoretical models and calculations. In experiment E04-005, high statistics photoproduction data has been taken in Jefferson Lab's CLAS detector with tagged photon energies up to 5 GeV incident on a LH$_2$ target. Preliminary results of the $e^{+}e^{-}$ decay channel, with emphasis on the $\rho$-$\omega$ interference region, will be shown and compared to similar experimental data of photoproduction off of heavier nuclear targets ($^2$H to Pb) from Jefferson Lab experiment E01-112.   

Scalar and Spin-Polarisabilities of the Nucleon from Deuteron Compton Scattering

We present progress in elastic deuteron Compton scattering in Chiral Effective Field Theory. Including the $\Delta(1232)$ as explicit degrees of freedom is particularly improtant for deuteron Compton scattering at $\ga90$~MeV as measured at SAL and MAXlab. Consistency arguments dictate including the $np$-rescattering states and automatically render the correct Thomson limit, shedding new light on Weinberg's power-counting of nuclear forces. We show that the static electric and magnetic scalar polarisabilities of the proton and neutron are identical within the accuracy of available data. In view of proposals at HI$\gamma$S and ongoing effort at MAXlab, we address in detail single- and doubly-polarised observables with linearly or circularly polarised photons on both un- and vector-polarised deuterons. Several observables can be used to extract not only spin-independent nucleon polarisabilities, but also the so-far practically un-determined spin-dependent polarisabilities which parameterise the stiffness of the nucleon spin in external electro-magnetic fields. Amongst the questions addressed are: convergence of the expansion when including the $\Delta$, the r\^ole of $np$-rescattering, and sensitivity to the deuteron wave function.   

Spin Asymmetry on the Nucleon Experiment

The Spin Asymmetry on the Nucleon Experiment (SANE) is a measurement of the spin structure function $g^{p}_{2}$ and $A^{p}_{1}$ over a broad range of Bjorken scaling variable \textit{x} from 0.3 to 0.8, for four-momentum transfers from 2.5 GeV$^2$ to 6.5 GeV$^2$. The experiment measured inclusive double spin asymmetries using TJNAF polarized electron beams of about 4.7 and 5.9 GeV energies,scattered off UVA solid polarized NH$_{3}$ target. The experiment took place from January to March of 2009. We will discuss the physics motivation for SANE and current status of the analysis, energy resolution and kinematic coverage.   

Vibrational and Rotational as well as Linear Kinetic Energies Should be Included in Compton Efect Energy Formula

In Compton scattering the incident photon will affect the vibration and rotation of the impacted particle as well as its linear motion. Therefore, the energy equation for the Compton Effect must be modified to include the change in vibrational and rotational kinetic energy of the particle before and after photon impact. The Compton Effect equation equation should then be as follows. $hc/\lambda_1 + m_0c^2 + 1/2mv_1^2 + 1/2I{\omega_r}_1^2 + 1/2k_1{x_0}_1^2 = hc/\lambda_2 + m_0c^2 + 1/2mv_2^2 + 1/2I\omega_2^2 + 1/2k_2{x_0}_2^2$.   

Lattice QCD in Background Fields

The response of hadrons to electromagnetic probes is highly constrained by chiral dynamics; but, in some cases, predictions have not compared well with experimental data. Electromagnetic properties of hadrons can be computed by lattice simulations of QCD in background fields. Focusing on calculations in background electric fields, we demonstrate new techniques to determine electric polarizabilities. We argue that the lattice can be used to test the chiral electromagnetism of hadrons, and ultimately confront experiment.