Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session S7: Mini-Symposium on Electromagnetic Reactions Involving Light Nuclei III |
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Sponsoring Units: DNP GFB Chair: Mohammad Waseem Ahmed, Duke University Room: Delaware A |
Monday, February 15, 2010 3:30PM - 3:42PM |
S7.00001: Electromagnetic properties of hadrons from lattice QCD Brian Tiburzi, William Detmold, Andre Walker-Loud 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 and magnetic moments. Results for the nucleon are presented. We argue that the lattice can be used to test the chiral electromagnetism of hadrons, and ultimately confront experiment. [Preview Abstract] |
Monday, February 15, 2010 3:42PM - 3:54PM |
S7.00002: Hard Photodisintegration of a Proton Pair Douglas Higinbotham, Ishay Pomerantz The energy dependence of the high energy 90 degree center of mass photodisintegration of proton-pairs in kinematics corresponding to the proton pair (and the spectator neutron) nearly at rest have been measured in Hall A at Jefferson Lab. Cross-section measurements were taken for eight photon energies in the range of 0.8 - 4.7 GeV. Scaling of the cross section by s$^{-11}$ was observed, in agreement with the constituent counting rule prediction, but commencing at E$_{\gamma}$ $\sim$ 2.2 GeV, rather than 1 GeV as in the deuteron (pn pair) breakup. The magnitude of the scaled cross section for pp pair breakup was found to be dramatically lower than for the breakup of pn pairs and theoretical predictions. At energies below the scaling region, the scaled cross section was found to present a strong energy-dependent structure not observed in the pn breakup. The data indicate a transition from three-nucleon hadronic photodisintegration processes at low energies to two-nucleon quark-dominated photodisintegration processes at high energies. [Preview Abstract] |
Monday, February 15, 2010 3:54PM - 4:06PM |
S7.00003: Deeply Virtual Compton Scattering off ${}^4He$ nucleus Ahmed El Alaoui The recently developed formalism of generalized parton distributions (GPDs) provides a theoretical tool to reveal the internal structure of the nucleon. These objects can be obtained via Deeply Virtual Compton Scattering (DVCS) processes. They contain informations on the transverse spatial position and the longitudinal momentum of quarks inside the nucleon and they also give access to the contribution of the quark orbital angular momentum to the nucleon. \noindent In contrast to many DVCS experiments using a proton target, only few experiments are devoted to study GPDs in a nuclear target which is important because it allows to address medium modification of bound nucleon GPDs compared to free nucleon GPDs. \noindent One of the goals of the new EG6 experiment at Thomas Jefferson Laboratory is to extract the real and imaginary parts of the ${}^4He$ Compton form factor $({\cal {H}}_A(x_B,t))$ from measurement of beam spin asymmetries by scattering a polarized 6 GeV electron beam off a ${}^4He$ pressurized gaz target. Details on the experiment will be presented here. [Preview Abstract] |
Monday, February 15, 2010 4:06PM - 4:18PM |
S7.00004: Measurements of the Electric Form Factor of the Neutron at High Momentum Transfer Seamus Riordan The electromagnetic form factors of the nucleon provide experimental access to the underlying charge and magnetic moment distributions of quarks. These form factors provide excellent testing grounds for QCD and QCD-inspired models and are of fundamental importance in our understanding of non-perturbative QCD. Of the four nucleon form factors, the electric form factor of the neutron, $G_E^n$, has been measured in the smallest range of momentum transfer. We have measured the electric form factor of the neutron at four $Q^2$ points between 1.2 and 3.5~$\mathrm{GeV}^2$ in Hall A at Jefferson Lab. This more than doubles the momentum transfer region for which this quantity has previously been measured, providing new information on the structure of the neutron. Final results for $G_E^n$ at three $Q^2$ points, 1.7, 2.5, and 3.5~$\mathrm{GeV}^2$, will be presented and compared with QCD-based models and phenomenological approaches. Separated form factors for $u$ and $d$ quarks will also be presented. [Preview Abstract] |
Monday, February 15, 2010 4:18PM - 4:30PM |
S7.00005: Understanding 3He(e,e'p) Reaction Asymmetry Dependence on Missing Momentum Ge Jin Two-body calculations using realistic wave-functions predict that the D(e,e'p) asymmetry will vary strongly as a function of missing momentum. This prediction has been tested in quasi-elastic D(e,e'p)n experiments in which people have observed the predicted sign change of the asymmetry as the missing momentum gets greater than the Fermi momentum. Using state-of-the-art Faddeev calculations, the 3He(e,e'p) reaction channel can also be calculated and it has been shown that the asymmetry as a function of missing momentum is again sensitive to the initial-state wave-function. Jefferson Lab experiment E05-102 measured the polarized-target and polarized-beam asymmetries in the quasi-elastic and x great than one 3He(e,e'p) and 3He(e,e'd) channels. An overview of experiment will be discussed and preliminary (e,e'p) asymmetries as a function of missing momentum will be presented. [Preview Abstract] |
Monday, February 15, 2010 4:30PM - 4:42PM |
S7.00006: A$_y$ Measurement from $^3\mbox{He}^\uparrow(e,e'n)$ Scattering at Jefferson Lab Elena Long Recently A$_y$ asymmetry measurements have been conducted in Jefferson Lab's Hall A through electron scattering from a vertically polarized $^3$He target. Experiment E08-005 measured the target single-spin asymmetry A$_y$ in the quasi-elastic $^3\mbox{He}^\uparrow(e,e'n)$ reaction. Plane wave impulse approximation (PWIA) predicts that A$_y$ should be exactly zero. A previous experiment at Q$^2$ of 0.2 (GeV/c)$^2$, where Laget and Nagorny indicated A$_y$ to be small, showed a large asymmetry as inferred by Faddeev calculations. The recent experiment measured this asymmetry at Q$^2$ of 0.1 (GeV/c)$^2$, 0.5 (GeV/c)$^2$ and 1.0 (GeV/c)$^2$. This is the first measurement of A$_y$ at large Q$^2$, which is another region where A$_y$ is expected to be small. Any non-zero result is an indication of effects beyond simple impulse approximation. This measurement will place new restrictions on form factor extractions from polarized $^3$He at large Q$^2$. Details of the measurement will be presented. [Preview Abstract] |
Monday, February 15, 2010 4:42PM - 4:54PM |
S7.00007: A precise extraction of the proton induced polarization in $^4$He($e,e^\prime \vec p\,$)$^3$H Simona Malace, Michael Paolone, Steffen Strauch I will present final results on the extraction of the induced polarization P$_y$ in $^4$He($e,e^\prime \vec p\,$)$^3$H from experiment E03-104 at Jefferson Lab at $Q^2$ values of 0.8 (GeV/$c$)$^2$ and 1.3 (GeV/$c$)$^2$. False asymmetries complicate the extraction of P$_y$. Our experiment was uniquely set up to offer the same coverage in the proton acceptance in H($e,e^\prime \vec p\,$) as in $^4$He($e,e^\prime \vec p\,$)$^3$H and allow for a significant cancellation of false asymmetries in the difference. An extensive work was carried out to minimize false asymmetries. As a result we were able to reduce the experimental uncertainties in the difference of P$_y$ ($\Delta$P$_y$) extracted from $^4$He($e,e^\prime \vec p\,$)$^3$H and H($e,e^\prime \vec p\,$) by a factor of 3 compared to previous results. The greatly reduced experimental uncertainties result in an increased possible sensitivity of $\Delta$P$_y$ to higher order physics effects that yield a nonzero value for P$_y$ in H($e,e^\prime \vec p\,$) such as two-photon-exchange effects. We compare our results with two recent calculations for P$_y$, those of Udias {\it et al.} and Schiavilla {\it et al.}, which use different approaches to model final state interactions. [Preview Abstract] |
Monday, February 15, 2010 4:54PM - 5:06PM |
S7.00008: Polarization Transfer in $^4$He($\vec{e}$,$e'\vec{p}$)$^3$H Michael Paolone, Simona Malace, Steffen Strauch Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium, including possible modification of the nucleon form factor. In experiment E03-104 at Jefferson Lab we measured the proton recoil polarization in the $^4$He($\vec{e}$,$e'\vec{p}$)$^3$H reaction at a $Q^2$ of 0.8 (GeV$/c$)$^2$ and 1.3 (GeV$/c$)$^2$ with unprecedented precision, allowing the individual polarization transfer coefficients and their ratio to be studied as a function of the missing momentum. The data differ from a fully relativistic distorted wave impulse approximation (RDWIA) calculation which uses free space proton form factors, but strong agreement is obtained if medium modified form factors are used. The data also agree with a model calculation including a charge-exchange final-state interaction with no medium modification of the nucleon form factors. The polarization transfer ratio has also been studied as a function of the virtuality of the proton; the ratio increases in an almost linear fashion from the farthest off-shell measurements of the recoiling proton to the extrapolated value of an unbound proton expected at virtuality equal to zero. [Preview Abstract] |
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