Bulletin of the American Physical Society
2011 Fall Meeting of the APS Division of Nuclear Physics
Volume 56, Number 12
Wednesday–Saturday, October 26–29, 2011; East Lansing, Michigan
Session GE: Electromagnetic Interactions |
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Chair: Bill Briscoe, The George Washington University Room: 103AB |
Friday, October 28, 2011 8:30AM - 8:42AM |
GE.00001: Near-Threshold Measurement of $\gamma n \rightarrow p \pi^-$ at MAX-lab Grant O'Rielly One of the important questions in nuclear science is to describe the properties of the nucleon using the framework provided by QCD. Pion photoproduction is one process where both experiment and theory can produce valid and useful results. Consequently, high-quality measurements of this fundamental process can be used to test the predictions of approaches such as chiral effective-field theories and dispersion relations, as well as provide additional data for the SAID and MAID partial-wave analyses. The Photon Tagging Facility at MAX-lab in Lund, Sweden is uniquely suited to perform measurements of pion photoproduction at energies between threshold and the $\Delta$-resonance. The MAX-TAGG collaboration is undertaking a comprehensive program to investigate the $\gamma p \rightarrow n \pi^+$, $\gamma n \rightarrow p \pi^-$ and, eventually, $\gamma n \rightarrow n \pi^0$ channels to complement the existing large data set on the $\gamma p \rightarrow p \pi^0$ channel. The first measurement of the $\gamma n \rightarrow p \pi^-$ channel has just been completed. Using a LD$_2$ target and the reaction $\gamma d \rightarrow p p \pi^-$, the $\pi^-$ is captured on another deuteron creating a high-energy $\gamma$-ray. These $\gamma$-rays were detected using three very large NaI spectrometers. These new near-threshold data will be used to better evaluate the threshold $E_{0^{+}}\left(\pi^- p\right)$ amplitude, which can be compared with the predictions from Chiral Perturbation Theory and other quark-based theories. Initial results from this measurement will be presented. [Preview Abstract] |
Friday, October 28, 2011 8:42AM - 8:54AM |
GE.00002: Compton Scattering on $^{6}$Li at 60 MeV L. Myers, M.W. Ahmed, S.S. Henshaw, J.M. Mueller, H.R. Weller, G. Feldman, T. Balint, K. Sykora, M.A. Kovash We have measured the Compton scattering cross section from $^6$Li for the first time using the High Intensity Gamma Source (HI$\gamma$S) at the Triangle Universities Nuclear Laboratory (TUNL). This measurement is intended to provide a new method for obtaining the electric and magnetic polarizabilities ($\alpha$ and $\beta$) of the nucleon. The Z = 3 target gives a higher Compton cross section compared to a proton or deuteron target, and as long as nuclear effects can be theoretically modeled, it should be feasible to extract information about $\alpha$ and $\beta$. The experiment was conducted using a 60 MeV $\gamma$-ray beam with an intensity of $10^{7}$ Hz and the eight-element HI$\gamma$S NaI Detector Array (HINDA) covering an angular range of $55^{\circ}$-$159^{\circ}$. A phenomenological Compton scattering model has been developed [1] which utilizes known photoabsorption data to infer the Compton scattering amplitudes -- with these fixed inputs, $\alpha$ and $\beta$ are free parameters that can be fit to the angular distribution of the scattering data. Our preliminary data will be presented and compared to initial calculations using the phenomenological model which includes nucleon electric and magnetic polarizabilities. Partially supported by the USDOE under grant numbers DG-FG02-97ER41033 and DE-FG02-06ER41422. [1] D.H. Wright et al., Phys. Rev. C 32, 1174 (1985). [Preview Abstract] |
Friday, October 28, 2011 8:54AM - 9:06AM |
GE.00003: Measuring Proton Spin Polarizabilities with Polarized Compton Scattering Philippe Martel, William Barnes, Rory Miskimen, Alexander Mushkarenkov An important test of low-energy QCD theories is the extraction of the proton spin polarizabilities (SPs), which describe the response of the proton spin to a polarized photon. The SPs arise as third order terms in the energy expansion of the Compton scattering amplitude, with theoretical values provided by dispersion and effective field theories, and in the future by lattice calculations. Extraction of these values is possible by measuring two beam-target asymmetries of a circularly polarized photon beam on a transversely polarized target, $\Sigma_{2x}$, and on a longitudinally polarized target, $\Sigma_{2z}$, and a beam asymmetry of a linearly polarized photon beam on an unpolarized target, $\Sigma_{3}$, at photon energies between $\pi$ and $2\pi$ threshold. The MAMI A2 Bremsstrahlung beam is used in conjunction with either a frozen-spin butanol or an unpolarized hydrogen target, and the Crystal Ball and TAPS detectors which combined provide 97\% coverage of $4\pi$. We will report on $\Sigma_{2x}$ measurements, supplemented by initial $\Sigma_{3}$ measurements, both just below and above $2\pi$ threshold. [Preview Abstract] |
Friday, October 28, 2011 9:06AM - 9:18AM |
GE.00004: Study of dimensional scaling in two-body photodisintegration of 3He Yordanka Ilieva Ever since their formulation in 1973, dimensional scaling laws have been extensively exploited to probe short-distance dynamics in nuclear processes. Despite their overwhelming empirical success in describing a large number of nuclear reactions, many of which were measured at low energies, there has been a long-standing controversy in the interpretation of scaling. Recent theoretical developments within the AdS/CFT approach suggest that in the non-perturbative regime of QCD, scaling is due to the near constancy of the strong coupling constant at very low momentum transfer. Thus, probing exclusive processes in this kinematic regime is instrumental to test the validity of the AdS/CFT approach. In this talk I will present our novel experimental studies of dimensional scaling using two-body photodisintegration of 3He in which we have mapped the invariant cross section of this process over photon-energy range from 0.4 GeV to 1.4 GeV and center-of-mass angles from 40$^\circ$ to 140$^\circ$. The data have been taken with the CLAS at Jefferson Lab. The results of our study support a non-perturbative interpretation of scaling at low momentum transfer. [Preview Abstract] |
Friday, October 28, 2011 9:18AM - 9:30AM |
GE.00005: Short-Range Nucleon-Nucleon Correlations Douglas Higinbotham For short periods of time, the nucleons in the nucleus can form strongly correlated pairs. By probing the nucleus with electrons in xB $>$ 1 and Q2 $>$ 2 [GeV/c]2 kinematics, it seems that researchers have finally cleanly observed the effects of a correlated initial-state and suppressed the other reaction mechanisms which dominated experiments which were done in less extreme kinematics. Recent experimental results in these kinematics, both inclusive and exclusive, will be presented along with a discussion of what the data has taught us. [Preview Abstract] |
Friday, October 28, 2011 9:30AM - 9:42AM |
GE.00006: ABSTRACT WITHDRAWN |
Friday, October 28, 2011 9:42AM - 9:54AM |
GE.00007: Discharge of metastable nuclei during negative muon capture: Energy approach Alexander Glushkov A negative muon captured by a metastable nucleus may accelerate the discharge of the latter by many orders of magnitude. For a certain relation between the energy range of the nuclear and muonic levels a discharge may be followed by muon ejection and muon participates in discharge of other nuclei. We present relativistic energy approach to description of a discharge of nucleus with emission of gamma quantum and further muon conversion. Besides, the external laser (graser) effect on cited processes is studied. The decay probability is linked with imaginary part of the ``nucleons subsystem-photon-muon$^{-}$'' system energy. One should consider 3 channels: 1). radiative purely nuclear 2j-poled transition (probability P1); 2). Non-radiative decay, when a proton transits into the ground state and muon leaves a nucleus with energy E=E(p-N1J1)-E(i), where E(p-N1J1) is an energy of nuclear transition, E(i) is the bond energy of muon in 1s state (P2); 3). A transition of proton to the ground state with muon excitation and emission of gamma quantum with energy E(p-N1J1)-E(nl) (P3). As example, the probabilities for different channels in a case of the Sc, Tl nuclei are presented. The Dirac-Wood-Saxon model is used. The key features of the possible high-power monochromatic $\gamma $ radiation sources on the studied processes basis are analyzed. [Preview Abstract] |
Friday, October 28, 2011 9:54AM - 10:06AM |
GE.00008: The Charge Radius of the Proton, a 5 Sigma Discrepancy? Gil Paz Recently, the charge radius of the proton was extracted, for the first time, from muonic hydrogen. The value was 5 sigma away from similar measurement of regular hydrogen. The extraction of the charge radius depends on a theoretical input. Together with Richard J. Hill, we are studying the hadronic uncertainty in the theoretical prediction, using the tool of an effective field theory, namely NRQED. In the talk I will report on the results of this study. I will also report on a previous study of the model- independent extraction of the charge radius from electron-proton scattering, which found that previous extractions have typically underestimated their errors. [Preview Abstract] |
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