Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session CG: Low Energy QCD |
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Chair: George Fai, Kent State University Room: Jewett Ballroom C |
Friday, October 24, 2008 10:30AM - 10:42AM |
CG.00001: A New Approach to Effective Field Theory for Few-Nucleon Physics David Kaplan, Silas Beane, Aleksi Vuorinen I describe an alternate formulation of the effective field theory expansion for nucleon-nucleon interactions, which is a hybrid between the approach of Weinberg and that of Kaplan, Savage and Wise (KSW). Like the KSW approach, pion exchange is treated perturbatively, and amplitudes may be computed analytically, yet without the convergence problems previously caused by the tensor force. [Preview Abstract] |
Friday, October 24, 2008 10:42AM - 10:54AM |
CG.00002: Bayesian parameter estimation in effective field theories Matthias R. Schindler, Daniel R. Phillips Estimation of low-energy constants (LECs) is an important component of the effective field theory (EFT) program for low-energy QCD. So far the calculation of LECs from QCD is only possible in very few cases, and in practice the LECs are determined by fits to experimental data. There are several questions that need to be considered regarding such a fit: How does one incorporate the information that the LECs are of natural size? At which order in the EFT expansion should the fit be performed? And which data should be used to determine the LECs? We propose a method to address these questions that is based on Bayesian probability theory. The Bayesian framework allows us to incorporate the naturalness assumption by use of a prior probability density. Our method also accounts for the uncertainty due to the vagueness in the definition of ``naturalness''. In addition, the choice of the order of the EFT calculation is addressed by marginalization over the order, again systematically accounting for uncertainties. To demonstrate our method we present the application to a ``toy'' problem as well as a problem in chiral perturbation theory. [Preview Abstract] |
Friday, October 24, 2008 10:54AM - 11:06AM |
CG.00003: Cross Sections for $\gamma p \rightarrow p \eta$ and $\gamma p \rightarrow p \eta\prime$ using data from CLAS at Jefferson Lab. Mike Williams Studying $\eta$ and $\eta\prime$ photoproduction presents a good opportunity to search for missing $N^*$ states, since both mesons act as isospin filters. Previous searches for these states in $\gamma p \rightarrow p \eta\prime$ have been hindered by limited statistics in the available experimental data. I will present differential cross sections obtained from the CLAS g11a dataset. Measurements have been made in $\cos{\theta_{CM}^{\eta(\eta\prime)}}$ bins of width 0.1 for $\eta$ photoproduction in 64 $\sqrt{s}$ bins over the energy range $1.68$~GeV $<\sqrt{s}<$ 2.84~GeV and for $\eta\prime$ photoproduction in 40 $\sqrt{s}$ bins over the energy range $1.92$~GeV $<\sqrt{s}<$ 2.84~GeV. The width of the center-of-mass energy bins is from 10~MeV$-$50~MeV, depending on $\sqrt{s}$. In total, 1082 $\eta$ and 682 $\eta\prime$ cross section points are reported. These results represent a tremendous increase in the precision of the world's $\eta\prime$ photoproduction data and extend the energy coverage by $\sim 500$~MeV in $\sqrt{s}$. [Preview Abstract] |
Friday, October 24, 2008 11:06AM - 11:18AM |
CG.00004: Chiral symmetry restoration and deconfinement in Coulomb Gauge QCD Peng Guo, Adam Szczepaniak Phenomenology of QGP has attracted a lot of attention in recent years. In the framework of Coulomb Gauge QCD we explore dynamical spontaneously breaking of chiral symmetry of QCD vacuum and confinement. In the extension of quasi-quark model at finite temperature and density, we investigate the relation of chiral symmetry restoration and deconfinement as a function of temperature and density. We will show how the confinement potential behaves depending on screening effects once the temperature and density is increased and also at what condition a de-confined potential will generate the chiral symmetry restoration. Possible appearance of the quarkyonic phase will be discussed. [Preview Abstract] |
Friday, October 24, 2008 11:18AM - 11:30AM |
CG.00005: Generalized Sum Rules of the Nucleon Mikhail Gorshteyn, Adam Szczepaniak We consider doubly virtual Compton scattering (VVCS) off the nucleon with the photon virtualities $q_1^2=q_2^2=-Q^2$ and formulate the low energy theorem (LET) for this process. We show that the LET can only be defined at finite momentum transfer $t=-2Q^2$ which is at variance with existing studies in the literature. Combining LET with dispersion relations for the forward VVCS amplitude, we obtain the new, correct version of the generalized sum rules of the nucleon that state a correspondence between the low energy constants of VVCS and the moments of the DIS structure functions. We notice that the $t$-channel unitarity is necessary to translate the forward dispersion relations to the low energy limit. This approach leads to a substantial modification of the generalized GDH sum rule at finite $Q^2$ that undergoes extensive studies at JLab. For the spin-independent VVCS amplitude, the new sum rule for the generalized magnetic susceptibility $\beta(Q^2)$ is obtained. Our approach provides a consistent, Lorentz invariant formulation of LET for the most general VVCS process that removes inconsistencies that stain the previous studies of the generalized polarizabilities of virtual Compton scattering and the generalized sum rules of the nucleon. [Preview Abstract] |
Friday, October 24, 2008 11:30AM - 11:42AM |
CG.00006: ABSTRACT WITHDRAWN |
Friday, October 24, 2008 11:42AM - 11:54AM |
CG.00007: N to Delta Transitions from QCD Sum Rules Lai Wang, Frank Lee We present a calculation of the N to $\Delta$ electromagnetic transition amplitudes from the method of QCD Sum Rules. A complete set of QCD sum rules is derived using the external field method and generalized interpolating fields for the entire family of transitions from baryon octet to decuplet. For each transition, thirteen sum rules are constructed from thirteen independent tensor structures. They are analyzed by a Monte-carlo procedure. Valid sum rules are identified from which the magnetic dipole $G_{M1}$ and the electric quadrupole $G_{E2}$ are determined. The results are compared with calculations from other models and experiment from JLab and other accelerators. [Preview Abstract] |
Friday, October 24, 2008 11:54AM - 12:06PM |
CG.00008: Precision Measurement of $\pi ^0$ Electroproduction Cross Section Near Threshold Mitra Shabestari A high precision measurement of the reaction $H\left( {e,e'p} \right)\pi^0 $ was performed near threshold (experiment E04- 007) at Jefferson Laboratory. Measurements were made in a fine grid of $Q^2 $, in the range $0.045\left( {{{{\rm{GeV}}} \mathord{\left/ {\vphantom {{{\rm{GeV}}} c}} \right. \kern-\nulldelimiterspace} c}} \right)^2 \le Q^2 \le 0.15 \left( {{{{\rm{GeV}}} \mathord{\left/ {\vphantom {{{\rm{GeV}}} c}} \right. \kern-\nulldelimiterspace} c}} \right)^2 $, and $\Delta W$ in the range $0{\rm{MeV}} \le \Delta W \le 30 {\rm{ MeV}}$. The data were taken in Hall A. Polarized electron beams at energies of 1194 and 2232 MeV were used to bombard a liquid hydrogen target. The target was contained in a new and very small aluminum cell with a thin beam entrance window, and thin side walls to minimize the energy loss of low- energy protons recoiling out of the target. The pion was identified by detecting the electron in one of the high- resolution spectrometers in coincidence with the recoiled proton, in the large acceptance ``BigBite'' spectrometer. These coincidence data allow us to test chiral QCD dynamics; a test which has become more critical as earlier measurements showed disagreement with the predictions of chiral perturbation theory. The experimental details will be discussed, and progress in data analysis will be presented. [Preview Abstract] |
Friday, October 24, 2008 12:06PM - 12:18PM |
CG.00009: Electroproduction of soft pions at high $Q^2$ near the threshold from CLAS Kijun Park, Ralf Gothe, Paul Stoler Threshold pion electoproduction has long been a topic of interest for both experimental and theoretical studies. In {\em chiral symmetry}, the approximately zero pion mass allows one to make exact predictions for threshold cross sections using {\em low energy theorems} (LET). The LET established the connection between charged pion electroproduction and the axial form factor in the nucleon. Thanks to studies based on the light-cone sum rules (LCSR), one can calculate the hadron form factors in terms of distribution amplitudes that approach perturbative quantum chromodynamics (pQCD) without other non-perturbative parameters. The extraction of the axial form factor near the pion threshold is dominated by the S- wave multipole $E_{0+}$ in the LCSR framework~[VBraun08]. Preliminary extracted axial form factor close to the threshold region will be presented. This analysis is based on the exclusive electroproduction process $ep \to e'n\pi^+$,~[KPark08] which was measured in the interval of $Q^2$ from $2.0$ to $4.5~\rm{GeV^2}$, and an invariant mass range in the $n\pi^+$ system from $W = 1.11$ to $1.15~\rm{GeV}$. The CEBAF Large Acceptance Spectrometer (CLAS) was used to carry out these measurement. [Preview Abstract] |
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