Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session DQ: Mini-Symposium: Electromagnetic FF of N*'s I |
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Chair: Ken Hicks, Ohio U |
Friday, October 30, 2020 8:30AM - 9:06AM |
DQ.00001: Studies of Excited Nucleon Structure with CLAS and CLAS12 Invited Speaker: Kyungseon Joo Significant progress has been realized in studies of excited nucleon state structure (N$^{\mathrm{\ast }}$ program) from the data on exclusive meson electroproduction measured with the CLAS detector in Hall B at Jefferson Laboratory (JLab). Electrocouplings of most N$^{\mathrm{\ast }}$ with masses \textless 1.8 GeV have become available at Q$^{\mathrm{2}}$ \textless 5 GeV$^{\mathrm{2}}$. These results give us the unique opportunity to explore the interplay of quark-gluon core and meson-baryon cloud in the N$^{\mathrm{\ast }}$ structure and to shed light on the hadron mass generation. New precision experimental data with the CLAS12 detector have been taken to extract the N$^{\mathrm{\ast }}$ electrocouplings at high photon vitalities (Q$^{\mathrm{2}})$ ever achieved up to 10-12 GeV$^{\mathrm{2}}$. This high-Q$^{\mathrm{2}}$ reach will shed light on the emergence of the dominant part of the hadron mass from QCD based on the data on electrocouplings of all prominent nucleon resonances of different structure at the distances where the transition from quark-gluon confinement to perturbative QCD is expected. This talk will review the current status of N$^{\mathrm{\ast }}$ program with CLAS and discuss on-going efforts with CLAS12. [Preview Abstract] |
Friday, October 30, 2020 9:06AM - 9:18AM |
DQ.00002: New N'(1720)3/2$^+$ resonance from combined studies of $\pi^+\pi^-p$ photo-/electroproduction Victor Mokeev Observation of a new N’(1720)3/2$^+$ baryon state from combined analysis of the CLAS $\pi^+\pi^-p$ photo- and electroproduction data in the resonance region will be presented. This new resonance was discovered, in addition to several ``missing” resonances already observed and included in the PDG, from a global multi-channel analysis of exclusive meson photoproduction channels with decisive impact from the CLAS K$^+$Y data. Contributions from both the N(1720)3/2$^+$ PDG state of mass 1.74 GeV and the new N’(1720)3/2$^+$ state of mass 1.72 GeV, which have different decay widths to the $\pi\Delta$ and $\rho p$ final states, are needed in order to describe the $\pi^+\pi^-p$ photo- and electroproduction data with Q$^2$-independent resonance masses, total and partial hadronic decay widths of the new N’(1720)3/2$^+$ state, and all well-established resonances in the third resonance region. This provides strong evidence for the existence of the new N’(1720)3/2$^+$ resonance. The results on the Q$^2$ evolution of the resonance electroexcitation amplitudes of the N’(1720)3/2$^+$ have become available, offering insight into the structure of this new resonance for the first time. [Preview Abstract] |
Friday, October 30, 2020 9:18AM - 9:30AM |
DQ.00003: N* and hybrid baryon spectrum from LQCD Robert Edwards The suggestion that there are multiplets of hybrid gluonic excitations within the baryon and meson spectrum of QCD is a key result from lattice QCD calculations. Since these initial calculations, there has been progress in the determination of scattering amplitudes and resonance properties of hadrons. This talk will give a summary of this progress and outline some of the objectives. [Preview Abstract] |
Friday, October 30, 2020 9:30AM - 9:42AM |
DQ.00004: Mechanical properties of the nucleon with the CLAS12 Francois-Xavier Girod The second Mellin moments of Generalized Parton Distributions (GPDs) are parameterized by the Energy Momentum Form Factors of partonic confined dynamics. A pair of photons in the Bjorken regime, one in the initial and one in the final state as in Deeply Virtual Compton Scattering (DVCS), can unravel mechanical properties of the nucleon otherwise only accessible through Graviton Scattering, such as the Pressure and Shear Force Distributions. We demonstrate the feasibility of this measurement, with a dispersion analysis of the DVCS cross-sections and beam spin asymmetries where the D-term is a subtraction constant. We will improve the uncertainties of this analysis by separating the DVCS squared amplitude from the interference term with the Bethe-Heitler process, using their beam energy dependences and new data from CLAS12. Preliminary results demonstrate high statistics samples of exclusive events up to $x_B=0.7$ and $Q^2=10\text{ GeV}^2$. We will extend this measurement employing Charge Asymmetries with Polarized Positrons, which will crucially constrain global analysis of GPDs, and provide a clean test of universality and factorization in the valence region, a crucial step towards an extension of this program to the sea quarks and gluons at Electron Ion Collider. [Preview Abstract] |
Friday, October 30, 2020 9:42AM - 9:54AM |
DQ.00005: 3D Nucleon Structure and Deeply Virtual Meson Production Valery Kubarovsky Deeply virtual Compton scattering and deeply virtual meson electroproduction were recognized as important contributors to the study of the three-dimensional structure of the nucleon in terms of the quark and gluon fields. Most reactions studied, such as DVCS or vector meson electroproduction, are primarily sensitive to the chiral-even GPDs. The chiral-odd GPDs are difficult to access since hard subprocesses with the quark spin-flip are suppressed. It turns out that pseudoscalar meson electroproduction, and especially $\pi^0$ and $\eta$ production, were identified as especially sensitive to the parton helicity-flip subprocesses. Dedicated experiments to study Deeply Virtual Meson Production have been carried out at Jefferson Lab. The cross sections and asymmetries of the exclusive pseudoscalar meson electroproduction processes in a very wide kinematic range of $Q_2$, $x_B$ and $t$ have been measured with CLAS. The comparison of these data with the theoretical models will be presented. The extraction of the transversity GPDs parameters using global fit of the world data will be discussed in the report. The review of the current 12-GeV Jlab program and expected results will be discussed in the presentation. [Preview Abstract] |
Friday, October 30, 2020 9:54AM - 10:06AM |
DQ.00006: Nucleon and Resonance Structure: Windows onto Emergent Hadronic Mass Craig Roberts Atomic nuclei are the core of everything we can see. At the first level of approximation, their atomic weights are simply the sum of the masses of all the nucleons they contain. Each nucleon has a mass mN $\approx $1GeV, i.e. approximately 2000-times the electron mass. The Higgs boson produces the latter, but what produces the nucleon mass? This is the crux: the vast bulk of the mass of a nucleon is lodged with the energy needed to hold quarks together inside it; and that is supposed to be explained by quantum chromodynamics (QCD), the strong-interaction piece within the Standard Model. Critically, one can argue that revealing the origin of mass will also explain the nature of confinement; and the problem of confinement is the focus of a "Millennium Problem" posed by the Clay Mathematics Institute. This contribution canvasses the potential for a coherent effort in QCD phenomenology and theory, coupled with experiments at existing and planned facilities, to reveal the origin and distribution of mass by focusing on the properties of the nucleon and its excitations, particularly as they are revealed in the baryon spectrum and through nucleon-resonance electroproduction. [Preview Abstract] |
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