Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session BD: Mini-symposium on Compton Scattering from Nucleons and Nuclei |
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Sponsoring Units: DNP JPS Chair: Bira van Kolck Room: Ritz-Carlton Hotel Salon 1 |
Monday, September 19, 2005 7:00PM - 7:30PM |
BD.00001: Compton Scattering from Nucleons and Nuclei Invited Speaker: This talk will introduce the minisymposium by presenting an overview of the physics issues that are addressed by Compton scattering from nucleons/nuclei. At low energies, the scattering determines the generalized polarizabilities, which characterize the response of the system to slowly-varying external electric and magnetic fields. These quantities provide stringent tests of models of nucleon or nuclear structure, such as chiral perturbation theory, lattice QCD, and the effective nucleon-nucleon interaction. The simplest of these are the dipole electric and magnetic polarizabilities, which are well measured for the proton and less well measured for the neutron. With polarized beams and targets, one can measure the spin polarizabilities. With incident virtual photons, one can probe the spatial distribution of charges and currents contributing to the polarizabilities, allowing a separation of short-range (e.g., quark core) from long-range (e.g., pion cloud) contributions. For high energies and momentum transfers, the scattering is sensitive to the spatial distribution of the scattering centers. For nuclei, the photon scatters from nucleons, so that the scattering is sensitive to polarizabilities of bound nucleons as well as exchanged meson. For nucleons, the photon scatters from quarks, giving rise to new form factors that are derived from the same generalized parton distributions as the electron scattering form factors. The present status of these issues will be discussed in light of recent and planned experiments. [Preview Abstract] |
Monday, September 19, 2005 7:30PM - 7:45PM |
BD.00002: Compton scattering on the proton and deuteron in chiral effective field theory Daniel Phillips In this talk I will discuss recent efforts I have been involved in to describe Compton scattering in one- and two-nucleon systems using chiral perturbation theory ($\chi$PT). I will begin by summarizing the status of $\gamma$p scattering in $\chi$PT. For this reaction we fit the two undetermined parameters in the $O(Q^4)$ $\gamma$p amplitude of McGovern~[1] to experimental data in the region $\omega,\sqrt{|t|} \leq 180$ MeV, obtaining a $\chi^2/{\rm d.o.f.}$ of 133/113~[2]. This yields a model-independent extraction of proton polarizabilities based solely on low-energy data: $\alpha_p=(12.1 \pm 1.1~({\rm stat.}))_{-0.5}^{+0.5}~({\rm theory})$ and $\beta_p=(3.4 \pm 1.1~({\rm stat.}))_{-0.1}^{+0.1}~({\rm theory})$, both in units of $10^{-4}~{\rm fm}^3$. I will then discuss how the $\chi$PT formalism can be extended to treat deuteron Compton scattering. In particular, $\chi$PT provides a systematic treatment of the large isoscalar exchange currents that occur in this process, and so it facilitates extractions of nucleon polarizabilities from $\gamma$d data which have a well-motivated theoretical error bar~[2]. \begin{thebibliography}{9} \bibitem{judith} J.A.~McGovern, Phys.\ Rev.\ C {\bf 63}, 064608 (2001). \bibitem{allofus} S.R. Beane, M. Malheiro, J.A. McGovern, D.R. Phillips, and U. van Kolck, Phys. Lett. B {\bf 567}, 200 (2003); Nucl. Phys. A {\bf 747}, 311 (2005). \end{thebibliography} [Preview Abstract] |
Monday, September 19, 2005 7:45PM - 8:00PM |
BD.00003: Compton Scattering from the Deuteron to Investigate Neutron Polarizability Gerald Feldman The electric and magnetic polarizabilities of the neutron can be studied in Compton scattering reactions on deuterium via elastic scattering from the whole nucleus or quasi-free (QF) scattering from the bound neutron. Quasi-free scattering experiments have been performed at Saskatoon and Mainz, and the most precise values to date for the neutron polarizabilities were obtained recently at Mainz. Elastic Compton scattering data from Illinois, Saskatoon and Lund have been analyzed using effective field theories (EFT) and have yielded only moderately precise values, even when combining data sets together. The current EFT extractions are limited by the quality of the data which cover only restricted regions in energy and angle. In order to acquire an extensive and consistent data set for elastic Compton scattering from deuterium, we are initiating a new experimental program at Lund using tagged photons in the energy range 40-110 MeV and the angle range 30\r{ }-150\r{ } using three large-volume high-resolution NaI detectors ($\Delta $E/E $\sim $ 2{\%} at 100 MeV). With a projected statistical accuracy of 5{\%}, these new data will substantially constrain the EFT calculations and enable an experimental precision for the neutron polarizabilities that approaches the proton values. Previous work on deuterium (both QF and elastic) will be reviewed and an overview of the upcoming Lund experiments on elastic Compton scattering will be presented. [Preview Abstract] |
Monday, September 19, 2005 8:00PM - 8:15PM |
BD.00004: Of Neutron polarizabilities and polarization observables in Compton scattering on deuteron and He-3 Deepshikha Choudhury, Daniel Phillips The HIGS upgrade program has motivated us to investigate how sensitive observables involving polarized beam and/or polarized target are to neutron polarizabilities. Thus, we calculate several polarization observables for Compton scattering on deuteron and He-3 using chiral perturbation theory ($\chi $PT) up to O(Q$^{3})$ at energies of the order of the pion mass. For deuteron we have found that the photon polarization asymmetry, $\Sigma $ is insensitive to neutron electromagnetic polarizabilities. However, one of the double polarization asymmetries, $\Sigma _{x}$ is sensitive to $\gamma _{1n}$ [1]. In the first part of the talk I will present these results. Thereafter, I will discuss the formalism for similar calculations for He-3 and report some preliminary results. \newline \newline [1] http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PRVCAN000071000004044002000001\&idtype=cvips\&gifs=Yes [Preview Abstract] |
Monday, September 19, 2005 8:15PM - 8:30PM |
BD.00005: Polarizabilities of light nuclei Giuseppina Orlandini An overview of the present knowledge of the polarizabilities of light nuclei will be presented. It will be explained why this knowledge is at present rather limited, describing the difficulties in the theoretical calculations and how they may be overcome using integral transform approaches. Results obtained for systems up to A=7 will be presented and their sensitivity to the potential model will be discussed. The need of an experimental determination of these observables in light systems will be emphasized. [Preview Abstract] |
Monday, September 19, 2005 8:30PM - 8:45PM |
BD.00006: Compton scattering from nuclear targets at HI$\vec{\gamma}$S B.A. Perdue, M.A. Ahmed, M.A. Blackston, Y. Parpottas, A.P. Tonchev, H.R. Weller, V.N. Litvinenko, I.V. Pinayev, Y. Wu, R.M. Prior, M.C. Spraker, G. Feldman, B.D. Sawatzky, B. Norum, J.R. Calarco A Compton scattering program is presently being developed at HI$\vec{\gamma}$S utilizing the nearly-monoenergetic beams of 100\% polarized $\gamma$-rays produced by intra-cavity Compton backscattering of FEL photons. Polarization asymmetries of Compton scattering on $^{16}$O have been measured between 25-40 MeV to search for a narrow iso-vector giant quadrupole resonance (IVGQR). A beam with $\Delta$E/E$\sim$10\% was incident on a H$_2$O target. The scattered $\gamma$-rays were detected by four 10''$\times$10'' NaI detectors located between $\theta$=90-150$^\circ$ and $\phi$=0, 90, 180, and 270$^\circ$. The data indicate that significant, narrow concentrations of E2 strength are not present below 40 MeV. Another measurement of the Compton scattering cross section of $^3$He between E$_\gamma$=3-11 MeV is proposed to extract the electric polarizability, $\alpha_E$, of the $^3$He nucleus. A preliminary run has been performed to study the backgrounds, and a high pressure gas target system has been tested. [Preview Abstract] |
Monday, September 19, 2005 8:45PM - 9:00PM |
BD.00007: Double Distributions, Wide-Angle Compton Scattering, and $\gamma \gamma \to \pi\pi$ Anatoly Radyushkin, Ales Psaker Double Distributions (DD) are the basis of a novel approach to describe hadronic structure. They represent a form of Generalized Parton Distributions, which attracted recently a considerable attention both from theoretical and experimental side. We develop a DD-based approach to wide-angle Compton scattering and annihilation processes like $\gamma \gamma \to \pi^+ \pi^-$ in kinematics when all Mandelstam variables $s,t,u$ are large. We take into account both twist-2 and twist-3 effects, and also show that $t$-dependence of the short-distance propagator can be treated as a generalized target mass correction, in a treatment analogous to that developed by Nachtmann and Georgi and Politzer for deep inelastic scattering. We study the dependence of the amplitude on polarization of participating photons and give estimates for cross sections. To this end we propose models for double distributions in spacelike and timelike regions. [Preview Abstract] |
Monday, September 19, 2005 9:00PM - 9:15PM |
BD.00008: Wide-angle Compton scattering Bogdan Wojtsekhowski Real photon Compton scattering from nucleon in hard scattering regime is a powerful tool for study of partonic structure. We review RCS experimental results and a program of future experiments at Jefferson laboratory. Deep inelastic Compton scattering could be investigated by using a beam charge difference and a beam helicity effect. [Preview Abstract] |
Monday, September 19, 2005 9:15PM - 9:30PM |
BD.00009: Virtual Compton Scattering at Q$^2$=0.05 (GeV/c)$^2$ Yoshiyuki Sato, Paul Bourgeois The generalized polarizabilities of the proton describe the response of the proton to a quasistatic electric or magnetic field, and provide a test of our understanding of low-energy QCD. We have measured the virtual Compton scattering (VCS) cross section at Q$^2$=0.05 (GeV)$^2$ at MIT-Bates. Out-going protons were detected using the multiple out-of-plane spectrometer system (OOPS), which allows to suppress the normally dominant Bethe-Heitler radiation. In this talk, the experiment is described in more detail. The cross section and the polarizabilities are presented and discussed. [Preview Abstract] |
Monday, September 19, 2005 9:30PM - 9:45PM |
BD.00010: Study of Generalized Parton Distributions via Deeply Virtual Compton Scattering with the CLAS detector at Jefferson Lab Latifa Elouadrhiri The Generalized Parton Distributions (GPDs) carry new information about the dynamical degrees of freedom of quarks inside the nucleon. The GPDs have been introduced in the recently developed formalism for description of the deeply exclusive leptoproduction reactions. The GPDs are two-parton correlation functions that encode both the transverse spatial dependence and the longitudinal momentum dependence. At the twist-2 level, for each quark species there are two spin-dependent GPDs, $\tilde{E}(x,\xi,t),~\tilde{H}(x,\xi,t)$, and two spin-independent GPDs, $E(x,\xi,t)$, and $H(x,\xi,t)$. The first moments of GPDs in $x$ link them to the proton's form factors, while at t=0, the GPDs $H$ and $\tilde{H}$ reduce to the quark longitudinal momentum $q(x)$ and the helicity distributions $\Delta q(x)$, respectively. Mapping out the GPDs will allow, for the first time, to construct ``tomographic'' images of the nucleon's charge and quark helicity distributions in transverse impact parameter space. I will discuss the experimental program to study the GPDs in the deeply exclusive processes with CEBAF Large Acceptance Spectrometer (CLAS). I will present first results from Deeply Virtual Compton Scattering (DVCS) from the experimental program at 6 GeV. I will also outline the future experimental program at the Jefferson Lab 12 GeV upgrade. [Preview Abstract] |
Monday, September 19, 2005 9:45PM - 10:00PM |
BD.00011: Compton Scattering from a High Pressure Polarized $^3$He Target at HI$\gamma$S and the neutron spin polarizabilities Haiyan Gao The High Intensity Gamma Source (HI$\gamma$S) at Duke Free Electron Laboratory opens new window to the study of fundamental quantities related to the structure of the nucleon through polarized Compton scattering from a polarized nuclear target. We discuss a future measurement of the spin-dependent asymmetries from Compton scattering of circularly polarized photons from a high-pressure polarized $^3$He gas target at both the quasielastic and elastic kinematics. The Compton scattered photons and the recoil neutron will be detected in coincidence for the quasielastic scattering process, and only the scattered photons will be detected for the elastic scattering process. This experiment will be carried out at a photon energy between 100 - 140 MeV. In combination with the forward and backward polarizabilities extracted from existing experiments, the new experiment will allow for the first time the extraction of all four neutron spin polarizabilites, and provide crucial tests of predictions based on effective field theories, and dispersion theories. This work is supported by the U.S. Department of Energy (DE-FG02-03ER41231) and by the Duke University. [Preview Abstract] |
Monday, September 19, 2005 10:00PM - 10:15PM |
BD.00012: Generalized Power Counting Rule Feng Yuan In this talk, I will present a generalized power counting rule for the hard exclusive processes involving parton orbital angular momentum and hadron helicity flip. We introduce a systematic way to write down the Fock components of a hadronic light-cone wave function with $n$ partons and orbital angular momentum projection $l_z$, from which we derive the generalized counting rule. As an example, I will show the power counting result for the nucleon's Pauli form factor $F_2(Q^2)$ in perturbative QCD. [Preview Abstract] |
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