Bulletin of the American Physical Society
2013 Fall Meeting of the APS Division of Nuclear Physics
Volume 58, Number 13
Wednesday–Saturday, October 23–26, 2013; Newport News, Virginia
Session NE: Nucelon Resonance Structure |
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Chair: Volker Crede, Florida State University Room: Grand Ballroom V |
Saturday, October 26, 2013 8:30AM - 8:42AM |
NE.00001: Polarization Observables in the $\vec p\,\vec \gamma \to \pi^+\pi^-p$ Reaction from FROST Steffen Strauch, Hao Jiang, Yuqing Mao, Aneta Net Double-pion photoproduction is an important tool in the study of baryon resonances. It dominates the total photoabsorption cross section above the second resonance region and allows the study of resonance decay properties. The $\vec p\,\vec \gamma \to \pi^+\pi^-p$ reaction has been measured in Hall B at Jefferson Lab with linearly and circularly polarized tagged photon beams incident on longitudinally and transversely polarized protons. The experiment covered center-of-mass energies between 1.4 GeV and 2.3 GeV. The target was a FROzen Spin Target (FROST) and the final-state particles were detected by the CEBAF Large Acceptance Spectrometer (CLAS). Linear combinations of polarized yields have been used for the extraction of the single-, $P_z$, $I^\odot$, and double- polarization observables, $P^\odot_{x,y}$, $P_z^{c,s}$. Most of these observables were extracted for the first time. Calculations of an effective Lagrangian model have been compared with the preliminary results and resemble some features of the data. The data will constrain model calculations and test our understanding of nucleon structure by establishing reaction amplitudes. The data will thus improve our knowledge of photocouplings, hadronic resonance decays, and possibly help to identify new baryon resonances. [Preview Abstract] |
Saturday, October 26, 2013 8:42AM - 8:54AM |
NE.00002: Preliminary Results on Polarization Observables for Double-Pion Photoproduction from FROST P. Roy, S. Park, V. Crede Hadron spectroscopy is essential to understand nucleon structure in the low energy regime. Many low mass resonances have been observed via $\pi N$ scattering but only a few resonances above 1.7 GeV have been seen so far. It is speculated that photoproduction could be the key to detect the {\it missing} resonances, many of which probably decay into multi-particle final states. Double-pion photoproduction with $p\pi^{+}\pi^{-}$ final state becomes the biggest contributor to the total cross section for center-of-mass energies above 1.7 GeV, making it an indispensable channel to explore. It is imperative to measure polarization observables to isolate the resonant and non-resonant contributions to this reaction for the extraction of $N^{*}$ parameters. Here we report on the preliminary results obtained for polarization observables $I^{\odot}$, $P_{z}$, $P_{z}^{\odot}$ from the study of $\pi^{+}\pi^{-}$ photoproduction using circularly polarized photons ($E_{\gamma}$ up to 2.3 GeV) incident on a longitudinally polarized butanol target and discuss the ongoing analysis to extract $P_{x}^{s,c}$, $P_{y}^{s,c}$ using linearly polarized photons (coherent edge up to 2.1 GeV) and a transversely polarized target. The experiments were conducted at Jefferson Lab using the CLAS spectrometer. [Preview Abstract] |
Saturday, October 26, 2013 8:54AM - 9:06AM |
NE.00003: Cross section measurement of the exclusive single pion electroproduction for $Q^2<5$ GeV$^2$ from CLAS Kijun Park The study of nucleon excitations is the key to our understanding of the effective degrees of freedom at different distance scales where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance takes place. The exclusive meson electroproduction off protons is a powerful tool to probe these effective degrees of freedom in excited nucleon states. During the last decade, a broad experimental program has been executed with the CLAS detector in Hall B at Jefferson Lab, to study the excited states of the proton using polarized electron beams and (un)polarized proton targets. In this talk, results of our latest analysis work will be presented for the reaction of $ep\to e^{\prime}n\pi^+$ at W=1.6-2.0GeV, and Q2=1.8-4.0GeV2. A preliminary extraction of the helicity amplitudes, A(1/2), A(3/2), and S(1/2) will be also presented for the N(1680)5/2+ and the N(1720)3/2+ states, in particular. Result of this new analysis as well as ongoing work on the $p\pi^0$ and $p\pi^+\pi^-$ channels in similar kinematics, will allow for the determination of electro-couplings of several high-lying excited proton states (W$>$1.6GeV) at distances where quark degrees of freedom become increasingly important and eventually dominant as photon virtuality increase. [Preview Abstract] |
Saturday, October 26, 2013 9:06AM - 9:18AM |
NE.00004: Exclusive $p$$\pi^{+}$$\pi^{-}$ electroproduction in the resonance region at high $Q^{2}$ Evgeny Isupov, Volker Burkert, Victor Mokeev We report on the analysis of charged $2$-pion electroproduction in the kinematical region of nucleon resonances. The data were taken with a $5.75$ GeV continuous electron beam at Jefferson Lab impinging on a liquid hydrogen target to measure the process $e p \rightarrow e p\pi^{+} \pi^{-}$. Scattered electrons and at least two of the final state hadrons were detected in the CEBAF Large Acceptance Spectrometer (CLAS), and the fully exclusive process was determined using kinematical constraints from the over-determined reaction. The data were used to measure nine differential cross sections and the fully integrated cross section in the invariant mass range of the hadronic final state from $1.4$ to $2.0$ and at photon virtualities $2.0 < Q^{2} < 5~GeV^{2}$. Phenomenological analysis of these data within the framework of reaction model JM is in progress with the goal of establishing all essential contributing mechanisms from their manifestation in observables. New information on ${N^{*}}$ electrocouplings at high ${Q^{2}}$ will give access to the ${N^{*}}$ structure at distance scales in the transition from the domain where contributions of both meson-baryon and quark degrees of freedom are essential, to the domain where contributions of the dressed quark core dominate. [Preview Abstract] |
Saturday, October 26, 2013 9:18AM - 9:30AM |
NE.00005: Studies of $\pi^+\pi^-p$ electroproduction at $W$ from 1.4 to 1.8 GeV and $Q^2$ from 0.4 to 1.1 GeV$^{2}$ with CLAS Gleb Fedotov, Ralf Gothe, Victor Mokeev High precision data for the reaction $e p \rightarrow e'p'\pi^+\pi^-$ have been taken with the CLAS detector at Jefferson Lab covering the kinematic range in the hadronic invariant mass region 1.45 GeV $< W <$ 1.8 GeV and for photon virtualities 0.4 GeV$^2$ $< Q^2 <$ 1.1 GeV$^2$. Nine single-differential cross sections, composed of the angular distribution of all hadrons, the angular distributions given by all combinations of hadron pairs, and the invariant masses of all hadron pairs in the final state, have been measured. The data have been subjected to a phenomenological analysis using the reaction model [1,2], and the electrocouplings of several nucleon resonances in the mass range up to 1.8 GeV have been extracted. The data analysis and preliminary results of the resonance analysis study will be discussed. The kinematic coverage of earlier experiments [3,4] was extended. Special attention will be paid to the previously seen candidate state with $J^P=3/2^+$ at a mass of 1720 MeV.\\[4pt] [1] Mokeev, V.I. and others, Phys.Rev. C86 035203.\\[0pt] [2] Mokeev V. I. and and others, Phys.Rev. C80 045212.\\[0pt] [3] Fedotov G.V. and others, Phys.Rev. C79 015204.\\[0pt] [4] Ripani, M. and others, Phys.Rev.Lett. 91 022002. [Preview Abstract] |
Saturday, October 26, 2013 9:30AM - 9:42AM |
NE.00006: Beam spin asymmetry observables from electroproduction of $p\pi^{+}\pi^{-}$ off protons Arjun Trivedi, Ralf Gothe, Gleb Fedotov, Evan Phelps, Ye Tian One of the goals of the N* program at JLab is to understand the evolution of the dominant degrees of freedom of QCD at varying length scales ($Q^{2}$). A method of probing this evolution is by studying how the electrocouplings - couplings of a virtual photon to the nucleon excited nucleon vertex ($\gamma_{v}NN^{*}$) - change with $Q^{2}$. In my presentation, I will provide an update on the first attempt to obtain beam spin asymmetry observables from the reaction $\gamma_{v}p \rightarrow p\pi^{+}\pi^{-}$ in the Q$^{2}$ range of 2 GeV$^{2}$ $<$ Q$^{2}$ $<$ 5 GeV$^{2}$ and W in the range of 1.3 GeV $<$ W $<$ 3 GeV. These observables will be used to extract the electrocouplings for the $Q^{2}$ and W range of this analysis. The update will be preceded by a short introduction to the electrocouplings motivated observables and followed by a summary outlining the steps remaining before my analysis is complete. [Preview Abstract] |
Saturday, October 26, 2013 9:42AM - 9:54AM |
NE.00007: New results in the studies of the high-lying nucleon resonances from the CLAS data on $\pi^+\pi^-p$ electroproduction Victor Mokeev, Volker Burkert, Ralf Gothe The exclusive double-pion electroproduction represents the primary source of information on the structure of excited proton states in the mass region $M_{N^*}$ $>$ 1.6 GeV. Preliminary results on electrocouplings and $\pi \Delta$ and $\rho p$ hadronic decays of prominent $N^*$ states in this mass range will be presented. They were obtained from the analysis of CLAS data on $\pi^+\pi^-p$ electroproduction at photon virtualities 0.5$<$Q$^2$$<$1.5 GeV$^2$ [1] within the framework of the reaction model [2]. Our results confirmed the electrocouplings of N(1680)5/2$^+$ resonance determined in the studies of $N\pi$ exclusive channels [3]. The electrocouplings of $\Delta$(1620)1/2$^-$, $\Delta$(1700)3/2$^-$, and N(1720)3/2$^+$ resonances were determined for the first time from analysis of $\pi^+\pi^-p$ electroproduction with good accuracy. The results on electrocouplings of proton states in the mass range up to 1.8 GeV provide stringent constraints for the development of QCD-based approaches aimed at describing the complex structure of excited nucleon states. \\[4pt] [1] M.Ripani et al., CLAS Collaboration, Phys. Rev. Lett. 91, 022002 (2003).\\[0pt] [2] V.I.Mokeev et al., Phys. Rev. C80, 045212 (2009).\\[0pt] [3] L.Tiator et al., Eur. Phys. J. ST198, 141 (2011). [Preview Abstract] |
Saturday, October 26, 2013 9:54AM - 10:06AM |
NE.00008: Exclusive $\omega^0$ Electroproduction off the Proton as a Probe of High-Lying N* States Evan Phelps, Ralf Gothe, Gleb Fedotov, Ye Tian, Arjun Trivedi Differential cross-sections of $\omega^0(783)$ electroproduction in the exclusive $3\pi$ reaction channel off the proton will be reported over a kinematic range from the production threshold to $W = 3.0\,GeV$ and from $Q^2= 1.0\,GeV^2$ to $5.5\,GeV^2$. This kinematic region is composed of many overlapping $N^*$ states and is strongly affected by the final state interaction between many open exclusive channels, so the information from different exclusive channels is of particular importance for reliable extraction of resonance parameters. Exclusive $\omega^0$ electroproduction provides an excellent probe of potential high-lying resonances due to the narrow $9$-$MeV$ $\omega^0$ decay width and its isoscalar nature that restricts it to isospin-$1/2$ resonance couplings. Extending our view of the $\omega^0$ channel into the virtual photon domain, via exclusive electroproduction measurements, provides an opportunity to further explore potential resonance contributions and offers the natural next step toward extracting $\gamma_{v}NN*$ transition form factors at different photon virtualities. In this work, preliminary differential cross-sections were obtained from data taken with the CLAS detector at Jefferson Lab during run periods E1F and E1-6. [Preview Abstract] |
Saturday, October 26, 2013 10:06AM - 10:18AM |
NE.00009: Exclusive $\pi^{-}$ Electroproduction off the Neutron in Deuterium in the Resonance Region Ye Tian, Ralf W. Gothe, Gleb V. Fedotov, Evan Phelps, Arjun Trivedi The nucleon resonance ($N^{\ast}$) studies is crucial to our understanding of the structure and interaction of hadrons. Although the excited states of the proton have been studied in great detail, there are very few data available for the neutron excitations because of the inherent difficulty in obtaining a free neutron target. The deuterium target is the best alternate target for neutrons. The ``e1e" CLAS data that we analyze includes both a hydrogen and deuterium target run period, which allows a combined analysis of pion electroproduction off the free proton, bound proton, and bound neutron. Hence it will provide the experimentally best possible information about the off-shell and final state interaction effects in deuterium, which must be considered in order to extract the neutron information. The goal of our research is to provide the exclusive ${\gamma}^{\ast}(n) \rightarrow p^{+} {\pi}^{-}$ reaction cross section. The good agreement of the inclusive cross section off deuterium with the world data not only verified that the electron identification and fiducial cut are proper, but also confirmed that the normalization used in the exclusive channel is right. In this talk, I will present a current data analysis status, and potentially preliminary cross-section results. [Preview Abstract] |
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