Bulletin of the American Physical Society
2021 Fall Meeting of the APS Division of Nuclear Physics
Volume 66, Number 8
Monday–Thursday, October 11–14, 2021; Virtual; Eastern Daylight Time
Session PC: 3D Parton Structure of Hadrons: Generalized Parton Distributions |
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Chair: Nobuo Sato, Jefferson Lab Room: Studio 2 |
Thursday, October 14, 2021 9:30AM - 9:42AM |
PC.00001: Deeply Virtual Compton Scattering at 6.5 GeV and 7.5 GeV Polarized Electron Beam with CLAS12 Joshua Artem D Tan, Latifa Elouadrhiri, Francois-Xavier Girod-Gard Deeply Virtual Compton Scattering (DVCS) is the cleanest channel providing access to the 3D imaging of the nucleon structure described by Generalized Parton Distributions (GPDs) which correlates the 1D longitudinal momentum fraction of the nucleon’s constituent to its 2D transverse position. In the DVCS reaction, the interaction of the virtual photon from scattered electron and a quark inside the nucleon results in the nucleon’s emission of a high-energy real photon. Detection of DVCS events also means detection of Bethe-Heitler (BH) events, which have the same final-state particles but with the photons emitted by scattered electrons. By conducting DVCS experiments at different beam energies, separation of DVCS and DVCS-BH interference amplitudes can be performed allowing the extraction of GPD H in some kinematics, and eventually the gravitational d1(t) form factor, which provides access to the mechanical properties of the nucleon. |
Thursday, October 14, 2021 9:42AM - 9:54AM |
PC.00002: N→N* transition GPD measurements with CLAS12 at JLAB Stefan Diehl, Kyungseon Joo The measurement of transition GPDs is a unique tool to understand the 3 dimensional structure and the mechanical properties of nucleon resonances for the first time. To extract transition GPDs, the non-diagonal DVCS process can be used. In this process a pion-nucleon system, originating from the decay of an N* resonance, is produced in addition to a real photon. Because the factorisation of this process amplitude requires the constraints on Mandelstam variable t and photon virtuality Q² (-t/Q² << 1) and several final state particles have to be detected for a clean identification, CLAS12 in combination with the upgraded CEBAF accelerator at JLAB provides a unique opportunity to study this process. The talk will introduce the topic and present an on-going analysis effort on the recently collected CLAS12 data. The analysis will be discussed in the context of theoretical predictions for the p → delta transition GPDs. |
Thursday, October 14, 2021 9:54AM - 10:06AM |
PC.00003: Cross Sections for Deeply Virtual Exclusive Experiments from Jefferson Lab at 6 GeV to an EIC Brandon Kriesten, Simonetta Liuti Imaging the 3D partonic structure of the nucleon is a fundamental goal of nuclear experimental programs ranging from Jefferson Lab (JLAB) at 6 GeV to the Electron Ion Collider (EIC). Deeply Virtual Compton Scattering (DVCS) was first proposed as a way to probe the spatial distribution of partons in nuclei [1] by Fourier transform of the exchanged momentum transfer between the initial and final. The extraction of observables from deeply virtual exclusive reactions in a clear and concise formalism is a necessity. We recently presented a completely covariant description of the DVCS process that can be extended to any kinematics, either fixed target or collider [2,3]. In our helicity formalism, we extract observables such that the dependence on Q2 is clear [4]. Utilizing current data from JLAB at both 6 and 12 GeV as well as pseudo-data of an EIC generated by state of the art machine learning techniques, we show predictions of what such an experimental program will do for our understanding of the physical properties of nuclei. This work was funded by DOE grant DE-SC0016286 and SURA grant C2020-FEMT-002-04 and C2021-FEMT-006-05. |
Thursday, October 14, 2021 10:06AM - 10:18AM |
PC.00004: Generalized Parton Distributions from Hard Exclusive Compton-like and Vector meson production Marie Boer Hard exclusive Compton-like process, involving the exchange of at least one high virtuality photon off a quark of the nucleon, or hard exclusive vector meson production (where a hard scale is provided by the exchange of a virtual photon or by the meson mass), enable access to the partonic structure of the nucleon, in particular it the transverse distribution versus the longitudinal momentum of the partons. The latter can be parametrized by the so-called Generalized Parton Distributions (GPDs). We propose a simulation framework to study these reactions and will present the interest of such studies for accessing GPDs, our methods and simulations, and our projections for potential future experiments. |
Thursday, October 14, 2021 10:18AM - 10:30AM |
PC.00005: A new class of exclusive processes to better measure the x-dependence of DAs and GPDs Jianwei Qiu, Zhite Yu We propose a new class of exclusive processes for extracting generalized parton distribution functions (GPDs), which provide a powerful theoretical tool to quantify the spatial distribution of quarks and gluons inside a bound hadron. We argue that these processes can be systematically studied in terms of QCD factorization approach, and demonstrate that these processes offer an enhanced sensitivity to the momentum fraction $x$ of GPDs, for which existinig process, such as DVCS, provides almost no sensitivity. Using the exclusive massive-photon-pair production in meson-baryon collisions, and exclusive massive photon-pion production in photon-baryon collisions, as examples, we show that the leading non-perturbative contribution to these processes can be systematically factorized into pion distribution amplitude (DA) and baryon's GPDs. We demonstrate quantitatively the capability of this new class of exclusive processes to provide an enhanced sensitivity to the momentum fraction $x$-dependence of both DAs and GPDs. |
Thursday, October 14, 2021 10:30AM - 10:42AM |
PC.00006: Beam Spin Asymmetry for Deeply Virtual π0 Electroproduction with CLAS12 at Jefferson Lab. Andrey Kim Recently, the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab carried out experimental measurements of longitudinally polarized 10.6 GeV electrons scattering on unpolarized liquid hydrogen target. These new data allow us to extract beam spin asymmetries for deeply virtual exclusive reactions in a wide kinematic region with Q2 up to 8 GeV2, providing us an opportunity to access and constrain the Generalized Parton Distributons (GPDs). Among the variety of these exclusive reactions, π0 electroproduction channel was shown to be particularly sensitive to the largely unknown chiral-odd GPDs \bar{E}T and HT which contain information on quark transverse spin densities in unpolarized and polarized nucleons. In this talk we will report the current status of the DVπ0P analysis and present preliminary results from CLAS12 first experiment data. |
Thursday, October 14, 2021 10:42AM - 10:54AM |
PC.00007: Exclusive Phi Electroproduction Cross Section Measurement off the Proton at CLAS12 Patrick H Moran A central goal of the Jefferson Lab 12 GeV upgrade is the imaging of the 3-dimensional internal structure of the nucleon at high resolution. The exclusive phi electroproduction process off of a hydrogen target probes the proton's gluon GPD in the valence region, providing insight into the spatial distribution of the gluons within the proton. An update is given on the exclusive phi meson electroproduction cross section measurement at 10.6 GeV by Jefferson Lab's CLAS12 detector. |
Thursday, October 14, 2021 10:54AM - 11:06AM |
PC.00008: Double Spin Asymmetry in Wide-Angle Charged Pion Photoproduction - $A_{LL}$ experiment Arun Tadepalli, Rachel Montgomery, Bogdan B Wojtsekhowski, Gordon Cates The measured differential cross sections for the single neutral/charge pion photoproduction from a nucleon in the wide angle regime (s,-t,-u much larger hadronic scale) disagree with theoretical calculations by almost two orders of magnitude in spite of many years of investigation. A solution was proposed by P. Kroll and K. Passek-Kumericki, whose GPD-based theory includes both twist-2 and twist-3 amplitudes. The signatures of the twist-3 amplitude (compared to twist-2) is a increase in the cross section and has opposite signs of the double polarization observables $K_{LL}$ and $A_{LL}$ at backward production angles. An experimental check of the prediction would provide valuable information on the validity of the handbag mechanism in the GPD framework in the accessible energy range. The proposed experiment will measure the double spin asymmetry $A_{LL}$~for charged pion photoproduction in the wide angle regime using the reaction ($\vec \gamma \vec n \rightarrow \pi^- p$). |
Thursday, October 14, 2021 11:06AM - 11:18AM |
PC.00009: Extraction of CFFs observables from DVCS Liliet Calero Diaz, Dustin Keller Over the last 20 years there has been intense experimental activity dedicated to the measurement of observables to help build a 3D description of the nucleon. Generalized parton distributions (GPDs) describe complementary aspects of the structure of hadrons, providing qualitative and quantitative information about the partonic structure and dynamics such as orbital angular momentum. The formulation of the DVCS cross section is parametrized in terms of Compton Form Factors (CFFs) which are convolutions of GPDs with coefficient functions computed from perturbative QCD. This talk presents fitting techniques aimed at extracting CFFs using the most recent DVCS helicity amplitude formulations. |
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