Bulletin of the American Physical Society
2005 APS April Meeting
Saturday–Tuesday, April 16–19, 2005; Tampa, FL
Session K12: Electromagnetic Interactions I |
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Sponsoring Units: DNP Chair: Joerg Reinhold, Florida International University Room: Marriott Tampa Waterside Room 11 |
Sunday, April 17, 2005 1:15PM - 1:27PM |
K12.00001: Beam Asymmetry Measurements from Single Pion Electro-production Kyungseon Joo, Maurizio Ungaro This experiment measures beam spin asymmetries in $p(\vec e,e'p)\pi^o$ and $p(\vec e,e'\pi^+)n$ in the Roper and the second resonance regions. Single pion electroproduction in the resonance region has been studied as a means of exploring the physics underlying the structure of the nucleon. Most previous measurements have focused on unpolarized cross section measurements. A new experiment using CLAS at Jefferson Lab/Hall B has measured beam spin asymmetries over a large kinematic range. The high statistical accuracy of this data set provides new and unique information on small resonant amplitudes such as Roper resonance by utilizing the interference between small resonant and large non-resonant amplitudes which are not available from unpolarized cross section measurements. [Preview Abstract] |
Sunday, April 17, 2005 1:27PM - 1:39PM |
K12.00002: The Spin Correlation Parameters in Electron-Proton Inclusive Scattering in the Delta-Excitation Region Octavian Filoti, John Calarco We present a preliminary report on the measurement of spin correlation parameters in inclusive scattering of longitudinally polarized electrons from nuclear-polarized hydrogen using the BLAST detector at MIT-Bates Linear Accelerator Center. The Bates Large Acceptance Spectrometer Toroid (BLAST) is a detector designed to study in a comprehensive and precise way the spin-dependent electromagnetic response in one and few-body systems over a large kinematic range using the MIT-Bates South Hall Ring. Currently it is used to measure spin-dependent scattering from the elastic to the nucleon resonance region for hydrogen and deuterium using the Bates longitudinally polarized electron beam at beam energies up to 850 MeV and polarized internal gas targets of hydrogen and deuterium. The data provide a stringent test of pion electroproduction models that are sensitive to the scalar and electric quadrupole amplitudes in the Delta-region. This work is supported by DOE grants 181021 (UNH), and DE-FC02-94ER40818 (MIT-Bates). [Preview Abstract] |
Sunday, April 17, 2005 1:39PM - 1:51PM |
K12.00003: A Precision Measurement of $G_E^p/G_M^p$ at BLAST Chris Crawford We have measured $G_{\!E}^p/G_{\!M}^p$ at $Q^2 = 0.1\mbox{--}0.9\,{\rm GeV/c}^2$ in the South Hall Ring of the MIT-Bates Linear Accelerator Facility. This experiment used a polarized electron beam, a pure hydrogen internal polarized target, and the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST) detector. By measuring the spin-dependent elastic $\vec{\rm\,H}(\vec {\rm\,e},{\rm e}'{\rm p})$ asymmetry in both sectors simultaneously, we could extract the form factor ratio independent of beam and target polarization. This was the first experiment to measure $G_{\!E}^p/G_{\!M}^p$ using a polarized target, which is complementary to recoil polarimetry experiments. Preliminary results are presented. [Preview Abstract] |
Sunday, April 17, 2005 1:51PM - 2:03PM |
K12.00004: A new measurement of the neutron magnetic form factor using CLAS. Jeff Lachniet The CEBAF Large Acceptance Spectrometer (CLAS), located in Hall B of Jefferson Lab, has been used to measure quasielastic electron-neutron and electron-proton scattering from a deuterium target at electron beam energies of 2.6 and 4.2 GeV. The neutron magnetic form factor has been extracted from the e-n/e-p ratio with the use of the more accurately determined proton form factors. The experiment employed a novel dual-cell hydrogen/deuterium target that allowed the neutron detection efficiency of the CLAS calorimeters to be measured simultaneously with the quasielastic measurement. The large acceptance of the CLAS detector allowed the e-n/e-p ratio to be measured simultaneously over the range $0.5 < Q^2 < 4.5 GeV^2$. Preliminary results of the ratio measurement and form factor extraction will be presented. [Preview Abstract] |
Sunday, April 17, 2005 2:03PM - 2:15PM |
K12.00005: F2 at Low Q$^2$ Edwin Segbefia The nucleon structure function $F_2^N$ has been measured over a broad range in four-momentum transfer $Q^2$ and Bjorken $x$. The structure function data at large values of $Q^2$ are well understood in terms of logarithmic scaling violations. At low values of $Q^2$ and higher $x$, however, the theoretical understanding of $F_2^N$ is less clear. Preliminary results will be presented from Jefferson Lab Hall C experiment E00-002 which ran in Spring 2003. This experiment measured the inclusive electron--nucleon scattering cross sections in both the deep-inelastic and resonance regions on hydrogen and deuterium targets, using beam energies between 2.2 and 5.5 GeV, and spanning the kinematics range $0.05 \leq Q^2 \leq 1.7$~(GeV/c)$^2$. The nucleon structure functions $F_1$ and $F_L$ ($F_2^N$ and $R$) are being extracted from this data, providing the first $L/T$ separated strength measurements in this unexplored kinematic regime. Recent Jefferson Lab data indicate, in this kinematic regime, the onset of a drop in $F_2^p$ with decreasing $Q^2$. This fall--off of the structure function may indicate an insensitivity to the sea quark component of the nucleon at low $Q^2$. [Preview Abstract] |
Sunday, April 17, 2005 2:15PM - 2:27PM |
K12.00006: L/T separation in kaon electroproduction using CLAS at Jefferson Lab Brian A. Raue, Pawel Ambrozewicz, Daniel S. Carman, Mac D. Mestayer, Avtandil Tkabladze Measurements of the cross sections for the $p(e,e^\prime K^+)\Lambda/\Sigma^0$ reaction have been performed using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. This process was selected since the measured observables are sensitive to the resonant and non-resonant amplitudes of the underlying cross section. The data were taken with beam energies of 2.567, 4.056 and 4.247 GeV, and covered $Q^2$ from $\sim$ 0.5 to 2.5 (GeV/c)$^2$ and $W$ from threshold to $\sim$ 2.5 GeV. A substantial overlap in the $Q^2$ range for the data sets with different energies allowed us to perform a separation of the longitudinal and transverse contributions to the unpolarized cross section. The separation was performed using two different approaches, the Rosenbluth technique and simultaneous $\epsilon-\phi$ fit to all of the data. We will present preliminary results on the extracted ratio, $\sigma_L/\sigma_T$, as a function of $W$ and the center-of-momentum scattering angles. We will also discuss implications for the theoretical descriptions of these processes. This measurement is part of a larger CLAS program to measure cross sections and polarization observables for kaon electroproduction in the nucleon resonance region. [Preview Abstract] |
Sunday, April 17, 2005 2:27PM - 2:39PM |
K12.00007: Interference structure functions in $K^+\Lambda(\Sigma^0)$ electroproduction Avto Tkabladze, Pawel Ambrozewicz, Daniel Carman, Mac Mestayer, Brian Raue The $K+\Lambda$ and $K^+\Sigma^0$ electroproduction cross sections were measured using the CLAS spectrometer at Jefferson Lab. Data were collected at beam energies of 2.5, 4.0, and 4.3 GeV, spanning a range of $Q^2$ from 0.5 to 2.5 (GeV/c)$^2$ and $W$ from threshold up to 2.5 GeV. As part of the analysis of these data, the interference structure functions $\sigma_{TT}$ and $\sigma_{LT}$ have been extracted. These structure functions are crucial to separate resonant and non-resonant amplitudes of the underlying cross section. This measurement is a part of a larger CLAS program to measure cross section and polarization observables for strangeness production in the resonance region. A comparison of the results for overlapping $Q^2$ regions for the different beam energies allows us to reduce the model dependence in our acceptance calculations. The extracted cross sections and structure functions will be compared to existing theoretical models. [Preview Abstract] |
Sunday, April 17, 2005 2:39PM - 2:51PM |
K12.00008: Measurements of $C_x$, $C_z$ and $d_\sigma/d\Omega$ for $K^+\Lambda$ and $K^+\Sigma^0$ Photoproduction Reinhard Schumacher, Robert Bradford The photoproduction reactions $\gamma + p \rightarrow K^+ +\Lambda$ and $\gamma + p \rightarrow K^+ +\Sigma^0$ have been measured from threshold to 2.9 GeV photon energy using the CLAS spectrometer at Jefferson Lab. Using a circularly polarized real photon beam, we have obtained first-ever data for the double polarization observables $C_x$ and $C_z$. These correspond to the transfer of polarization from the photon to the produced hyperon. Results for both hyperons span a wide range of kaon production angles. $C_z$ for the $K^+ \Lambda$ reaction shows a striking trend toward complete polarization transfer to the hyperon over a wide range of kaon production angles, while $C_x$ tends towards zero. Several effective Lagrangian reaction models utterly fail to predict these newly-measured observables. We have extended our previous measurements \footnote{J. W. C. McNabb (CLAS Collaboration), {\it et al.}, Phys. Rev. C {\bf 69} 042201(R) (2004).} of the differential cross sections, $d\sigma/d\Omega$, to cover the nearly complete angular range $-0.85\le \cos\theta(K^+_{c.m.})\le+0.95$ and the extended photon energy range from 2.4 to 2.9 GeV. The results support our previous conclusions that multiple resonance-like contributions are present in $K^+ \Lambda$ production between $W$ of 1.8 and 2.0 GeV. $t$-channel scaling is found to be significant in $K^+\Lambda$ production, but is much less significant in $K^+ \Sigma^0$ production. [Preview Abstract] |
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