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Past seminars

11AM - 12:30PM EDT May 22nd, 2023

Accessing GPDs using the dilepton final state: results and perspectives with CLAS12

Pierre Chatagnon (Jefferson Lab)

Generalized Parton Distributions (GPDs) describe the correlations between the longitudinal momentum and the transverse position of the partons inside the nucleon. They are nowadays the subject of an intense effort of research, in the perspective of understanding nucleon spin and the nucleon Gravitational Form Factors (GFFs). Experimentally, the main reaction to measure GPDs have been the Deeply Virtual Compton Scattering (DVCS) reaction. In this talk, I will present complementary ways to access GPDs using the dilepton final state with the CLAS12 detector. First, I will present the first measurement of the Timelike Compton Scattering reaction (the hard photoproduction of a lepton pair), that gives access to the GPDs via the beam polarization asymmetry and angular asymmetry of the electron/positron pair. This measurement was done using data taken in 2018 by the CLAS12 detector with a 10.6 GeV electron beam impinging on a liquid-hydrogen target. I will then present the current effort to extract the near-threshold J/ψ photoproduction cross section using the same dataset. This later measurement is expected to provide direct insight on the gluons GPDs of the proton.

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11AM - 12:30PM EDT May 8th, 2023

Modeling and extraction of GPDs from exclusive processes

Hervé Dutrieux (The College of William and Mary)

Generalized parton distributions (GPDs) encompass crucial information on the three-dimensional structure of hadrons and their mechanical properties via the gravitational form factors. I will discuss how their scale dependence determines the propagation of uncertainty in the deconvolution problem of hard exclusive observables, and offers at the same time a great opportunity to model GPDs in the small Bjorken-x limit. I will present a modeling strategy to implement theoretical constraints and give a more comprehensive picture of extraction uncertainties.

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11AM - 12:30PM EDT April 3rd, 2023

New methods to access GPDs from lattice QCD and synergies for nuclear tomography

Martha Constantinou (Temple University)

Traditionally, lattice QCD computations of GPDs have been carried out in a frame where the transferred momentum is symmetrically distributed between the incoming and outgoing hadrons. However, such frames are inconvenient for lattice QCD calculations since each value of the momentum transfer requires a separate calculation, increasing the computational cost. In recent work (PRD 106 (2022) 11, 114512), we lay the foundation for more effective calculations of GPDs applicable for any frame, with freedom in the transferred momentum distribution. An important aspect of the approach is the Lorentz covariant parameterization of the matrix elements in terms of Lorentz-invariant amplitudes, which allows one to relate matrix elements in different frames. We demonstrate the efficacy of the formalism through numerical calculations using one ensemble of Nf=2+1+1 twisted mass fermions with a pion mass of about 260 MeV. Concentrating on the proton and zero skewness, we extract the invariant amplitudes from matrix element calculations in both the symmetric and asymmetric frame and obtain results for the twist-2 light-cone GPDs for unpolarized quarks, H and E.

We discuss the necessity of lattice QCD calculation of GPDs in the pre-EIC era. Also, we address and complementarity to the theoretical and phenomenological communities within the scientific program of the DOE-funded QGT Topical Collaboration on the 3D quark and gluon tomography.

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10AM - 11:30AM EDT May 25th, 2022

Building Identity in Physics through Teaching

Narbe Kalantarians (Virginia Union University)

Over the past 9 years I’ve embraced the mission of recruiting Black physicists, motivated by the fact that physics has been largely monotone in culture for the majority of its existence. What I’ve been learning continually for my role as a faculty member and researcher in physics, is that it entails empathy, patience, and recognition. In order to be successful, students from underrepresented groups need a sense of belonging in a field that has been predominantly White. Helping their communities is also significant to them. Time and time again, we have seen that teaching and mentoring are crucial for these. It is also invaluable that they see physicists who look like them. Virginia Union University brought back its physics program in 2016 with a planning grant from the National Science Foundation. We started with 7 students and since then, it has grown to 25 majors at present [1]. In this talk, I will go in to detail about how we have grown this program and the lessons we have learned (and continue to do so) along the way.



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10:00 AM - 11:00 AM EDT May 17th, 2022

JLab 24 GeV Group Discussion

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10:00 AM - 11:30 AM EDT April 27th, 2022

Exploring the Nucleus in 3D with GPDs

Raphaël Dupré (Laboratoire Irène Joliot-Curie Université Paris-Saclay)

In this presentation, I will review contemporaneous issues with our understanding of the nucleus from the hadronic physics point of view. In particular, I will go into details about the present situation on the study of the EMC effect. I will then rapidly review the physics of GPDs and how it can help us to better understand the nucleus and its quark structure. Then, I will present the CLAS collaboration experiment and our measurement of the beam spin asymmetries in both the coherent and incoherent DVCS channels off helium. I will discuss the importance of these results and lay out our projects to extend this experiment at JLab 12 GeV with a new recoil detector (ALERT). Finally, I will present our recently developed MC event generator for nuclear DVCS and perspectives for the field at the EIC to study lower x phenomena such as shadowing.

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11:00 AM - 12:30 PM EDT April 20th, 2022

Fundamental QCD Effects in Nuclei: Diquark Induced Short-Range Correlations

Jennifer Rittenhouse West (Lawrence Berkeley National Laboratory & EIC Center @ Jefferson Lab)

Newly-formed diquark correlations across overlapping nucleon-nucleon pairs are proposed as the underlying fundamental QCD basis for short-range correlations (SRC) in nuclei.  SRC, in turn, have been shown to be at least partially responsible for the EMC effect.  In this model, each nucleon in a highly overlapping SRC pair donates one valence quark to the new diquark; effectively falling into the attractive short-range QCD potential between quarks and forming a bond.  The lowest mass diquark is shown to be the spin-0 isospin-0 color-antitriplet [ud] diquark with a binding energy of nearly 150 MeV.  The flavor structure of the energetically favored [ud] allows for flavor-dependent inequality range predictions for SRC experiments.  A recent experiment at Jefferson Lab investigated the isospin dependence of SRC with results favorable to the diquark formation model.


10:00 AM - 11:30 AM EDT April 6, 2022

Deep Exclusive Electroproduction of π0 at High Q2 in the Quark Valence Regime

Julie Roche (Ohio University)

In this talk, I will present measurements of the exclusive neutral pion electroproduction cross section off protons at large values of xB (0.36, 0.48, and 0.60) and Q2 (3.1 to 8.4   GeV2) obtained from Jefferson Lab Hall A experiment E12-06-014. The corresponding structure functions dσT/dt+εdσL/dt, dσTT/dt, dσLT/dt, and dσLT′/dt are extracted as a function of the proton momentum transfer t-tmin. The results suggest the amplitude for transversely polarized virtual photons continues to dominate the cross section throughout this kinematic range. The data are well described by calculations based on transversity generalized parton distributions coupled to a helicity flip distribution amplitude of the pion, thus providing a unique way to probe the structure of the nucleon.  These results were recently published in PRL under the reference Phys.Rev.Lett. 127 (2021) 15, 152301.


11:00 AM - 12:30 PM EDT March 29th, 2022 

Exclusive massive photon-pair production in pion-nucleon collisions for extracting generalized parton distributions

Speaker: Jianwei Qiu (JLab)

Generalized parton distribution functions (GPDs) are fundamental quantum correlation functions carrying rich information on internal quark-gluon landscape inside a hadron.  In this talk, I will argue that exclusive massive photon-pair production in pion-nucleon collisions can be systematically studied in terms of QCD factorization approach if the photon's transverse momentum pT with respect to the colliding pion is sufficient large.  This exclusive scattering amplitude can be factorized into universal pion's distribution amplitude (DA) and nucleon's GPD, convoluted with an infrared safe and perturbatively calculable short-distance hard part.  The correction to this factorized expression is suppressed by powers of 1/pT.  I will show quantitatively that this new type of exclusive processes is not only complementary to existing processes for extracting GPDs,  but also capable of providing an enhanced sensitivity to the momentum fraction x-dependence of both DAs and GPDs.  I will also introduce a couple of new and related exclusive processes to enhance our ability to extract GPDs.