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 MF: Baryon Resonances in Hadronic Physics |
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Chair: Latifa Elouadrhiri, Thomas Jefferson National Accelerator Facility Room: Simmons Ballroom 4 |
Sunday, October 26, 2008 10:30AM - 10:42AM |
MF.00001: Coupled-channel analysis of hadronic and electromagnetic $\pi$, $\eta$, two-$\pi$ production reactions H. Kamano, B. Julia-Diaz, T.-S. H. Lee, A. Matsuyama, T. Sato Recent high precision data of the electromagnetic meson production reactions off nucleons from Bonn, GRAAL, JLab, Mainz and Spring-8 make possible to extract $Q^2$ dependence of the electromagnetic $N$-$N^\ast$ transition form factors. To extract such information on the quark-gluon structures of the $N^\ast$ states, a comprehensive analysis of the hadronic and electromagnetic meson production reactions is ongoing in Excited Baryon Analysis Center (EBAC) at JLab. The analysis is based on the dynamical coupled-channel model which properly describes correlations among all relevant reaction channels required from unitarity and can treat non-resonant and resonant processes in a unified manner. In this talk, we will report current status on the analysis of $\gamma N \to \pi N$, $\gamma N \to \eta N$ and $e N \to e' \pi N$ reactions. The main purpose is to determine parameters associated with the electromagnetic interactions by this combined analysis of the electromagnetic meson production reactions. We will also discuss the $N$-$N^\ast$ electromagnetic form factors extracted from the analysis of the $e N \to e' \pi N$ reaction. [Preview Abstract] |
Sunday, October 26, 2008 10:42AM - 10:54AM |
MF.00002: $\pi^+$ photoproduction on the proton from 0.675 to 2.875 GeV Barry Ritchie Differential cross sections for the reaction $\gamma + p \rightarrow n + \pi^+$ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS), a tagged photon beam with energies from 0.675 to 2.875~GeV, and a cryogenic hydrogen target. The reaction channel was isolated by detecting the photoproduced pion and identifying the recoil neutron through the missing mass technique, assuming $\gamma + p \rightarrow \pi^{+} + X$. Photon energy bin widths were 50 MeV, and absolute normalization uncertainties for these differential cross sections were less than 5\% at all energies studied. These cross sections complement and extend the existing data for the process. Data from this experiment were included in a SAID fit and compared with MAID and previous experiments. The impact of this new data set will be discussed. [Preview Abstract] |
Sunday, October 26, 2008 10:54AM - 11:06AM |
MF.00003: Photon beam asymmetries for $\pi^0$ and $\pi^+$ photoproduction from the proton Michael Dugger Pion photoproduction data have been vital to uncovering details of the nucleon resonance spectrum. The pions, as the lightest mesons, are copiously produced in the strong interaction. However, while pion photoproduction data is an important fundamental tool in baryon spectroscopy, the existing data set still remains relatively limited, and the existing database is dominated by measurements of the differential cross sections. I will present preliminary Jefferson Lab data from CLAS on photon beam asymmetry for both the $\pi^0$ and $\pi^+$ reactions for energies up to about E = 2.1 GeV. The kinematic range of these measurements both complements and extends the world database for these reactions. [Preview Abstract] |
Sunday, October 26, 2008 11:06AM - 11:18AM |
MF.00004: Beam asymmetry in $\eta$ meson photoproduction from the proton Patrick Collins The excitation spectrum of the proton is comprised of many broad overlapping resonances. Due to this feature, investigations of individual resonances are challenging. One excellent tool in helping understand the spectrum is $\eta$ meson photoproduction from the proton. Because this meson has isospin zero, it can be seen as an ``isospin filter'' for the nucleon resonance spectrum. Differential cross section data has been the primary tool used to study $\eta$ meson photoproduction. There have been a comparatively smaller number of beam asymmetry measurements for $\eta$ photoproduction. However, these beam asymmetries cover the energy range up to only about $E_{\gamma}$ = 1.5 GeV. I will present preliminary Jefferson Lab CLAS data on beam asymmetry for the $\eta$ meson for energies up to about $E_{\gamma}$ = 2.1 GeV. I will also discuss how the new measurements will be useful in understanding the structure and excited states of the proton. [Preview Abstract] |
Sunday, October 26, 2008 11:18AM - 11:30AM |
MF.00005: Dynamical coupled channel calculation of pion and omega production Mark Paris The dynamical coupled channel approach developed at the Excited Baryon Analysis Center is extended to include the $\omega N$ channel to study $\pi$ and $\omega$--meson production induced by scattering pions and photons from the proton. Six intermediate channels, including $\pi N$, $\eta N$, $\pi\Delta$, $\sigma N$, $\rho N$, $\omega N$, are employed to describe unpolarized and polarized data. Bare parameters in an effective hadronic Lagrangian are determined in a fit to the data for $\pi N\to\pi N$, $\gamma N\to\pi N$, $\pi^-p\to\omega N$, and $\gamma p\to\omega p$ reactions at center-of-mass energies from threshold to $W < 2.0$ GeV. The $T$ matrix determined in these fits is used to calculate the photon beam asymmetry for $\omega$-meson production and the $\omega N\to\omega N$ total cross section and $\omega N$ scattering lengths. The calculated beam asymmetry is in good agreement with the observed in the range of energies near threshold to $W < 2.0$ GeV. [Preview Abstract] |
Sunday, October 26, 2008 11:30AM - 11:42AM |
MF.00006: $\Sigma^+$(1385) photoproduction on proton Gagik Gavalian, M. Amarian The search for missing resonances using coupled channel analysis, $N\pi$ and $KY$, has been ongoing project at CLAS (Jefferson Lab). In this analysis the differential cross sections for the rection $\gamma p\rightarrow K^\circ\Sigma^+$(1385) have been measured with CLAS. The differential cross sections and the angular dependence of the $K^{\circ}$ in the reaction center of mass frame are obtained using a tagged photon beam with photon energies from $1.5$ GeV to $3.6$ GeV. The cross sections and angular dependences will be compared with theoretical predictions using $KY$ coupling to known and missing resonances. [Preview Abstract] |
Sunday, October 26, 2008 11:42AM - 11:54AM |
MF.00007: Photoproduction of $\Lambda$(1520) and interference effect in particle production Chandra Nepali, Moskov Amarian, Gagik Gavalian The cross-sections and t-dependence of two decay channels: $\Lambda(1520) \to K^{-} p$ and $\Lambda(1520) \to \bar{K^{o}} n$ are investigated to unravel quantum mechanical interference between $\Lambda(1520) (K^{-} p)$ and $\phi(1020) (K^{+} K^{-})$ decay modes, which is absent in $\Lambda(1520) \to \bar{K^{o}} n$ final state. The data set collected on hydrogen target in the photon energy range from 1.5 - 3.6 GeV on CLAS are used for this analysis. [Preview Abstract] |
Sunday, October 26, 2008 11:54AM - 12:06PM |
MF.00008: Differential cross sections for $\gamma p \rightarrow p\pi^{+}\pi^{-}$ using CLAS Matthew Bellis The Constituent Quark Model predicts multiplets which are absent from the observed spectra. These states may couple strongly to $\Delta\pi$ and $p\rho$, both of which appear as intermediate states in $p\pi^+\pi^-$. The g1c experiment used the CLAS detector to collect data using brehmstrallung photons (0.8-2.4 GeV) directed onto a liquid hydrogen target and collected over 10 million events for this 2$\pi$ final state alone. The challenge with this analysis is not the statistics, but the 3-body nature of the reaction and the overlapping intermediate states produced. The finite coverage of most detectors also introduces holes in the acceptance which can lead to ambiguities in the extrapolation of the physics solution. We attack this problem by observing the final state in multiple ways: by missing any of the three particles (p,$\pi^+,\pi^-$) in the detector and reconstructing it from missing mass as well as detecting all three tracks. We require a physics solution to be consistent amongst all 4 topologies in order to achieve full coverage of the phase space. This allows us to quote differential cross sections for this reaction in two-body masses ($d\sigma/dM_{ij}$), center-of-mass production angles ($d\sigma/d\cos(\theta_i)$) and helicity angles ($d\sigma/d\cos(\theta_{hel}), d\sigma/d\phi_{hel}$) These measurements achieve at a higher sensitivity than has previously been reported and will provide valuable input for extracting resonance information at later stages of analysis. [Preview Abstract] |
Sunday, October 26, 2008 12:06PM - 12:18PM |
MF.00009: $\pi^-p \rightarrow \pi^-p$ and $\pi^-p \rightarrow K\Lambda$ measurements in the 1700 MeV mass region Michael Sadler An experimental program is underway at ITEP in Moscow to measure differential cross sections for $\pi^-p \rightarrow \pi^-p$ and $\pi^-p \rightarrow K\Lambda$ at pion momenta 900 -- 1200 MeV/$c$ ($\sqrt{s}$ = 1610 -- 1770 MeV). For the elastic measurement the outgoing pion and proton will be measured using scintillator hodoscopes and drift chambers over a cm angle range of 40$^{\circ}$ -- 120$^{\circ}$. For the $K\Lambda$ final state, the decay vertices for $K \rightarrow \pi^+\pi^-$ and $\Lambda \rightarrow \pi^-p$ will be reconstructed using drift chambers to identify the events. A statistical precision of $0.5\%$ and $1.0\%$ is planned for the elastic and strangeness production measurements, respectively. The incident beam momenta will be binned in 1 MeV/$c$ increments to provide a mass resolution of 0.5 MeV in order to be sensitive to narrow resonances such as the possible non-strange pentaquark partner that has been predicted to have a mass near 1700 MeV. Irrespective of the pentaquark situation, the measurements will greatly improve the data upon which the properties of the conventional N*(1710) resonance are determined. [Preview Abstract] |
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