2021 Seminars & News Archive

Back to Archive Home

10 AM – 11:30 AM EST  Mar 16th

GPDs and angular momentum of the proton with basis light-front quantization

Speaker: Chandan Mondal (CAS Institute of Modern Physics)

In this talk, we will discuss the generalized parton distributions (GPDs) of the proton in both momentum space and position space within the basis light-front quantization (BLFQ) framework. The GPDs for the valence quarks are computed from the eigenvectors of a light-front effective Hamiltonian in the valence Fock sector consisting of a three-dimensional confinement potential and a one-gluon exchange interaction with fixed coupling. We will also discuss the gluon GPDs when one dynamical gluon is retained in the system. Employing these GPDs, we obtain the spatial distributions of quark angular momentum inside the proton. In our BLFQ approach, we explore various definitions of angular momentum density and illustrate the differences between them arising from terms that integrate to zero. We will also discuss the flavor contributions to the quark angular momentum densities.

 

10 AM – 11:30 AM EST  Mar 9th 

GPDs of sea quarks from nonlocal chiral effective theory

Speaker: Fangcheng He (CAS Institute of Theoretical Physics)

Generalized parton distributions (GPDs) provide three-dimension imaging of quark and gluon substructure in hadrons. In this talk, I will present our recent calculations about the unpolarized GPDs of sea quark in the proton from nonlocal chiral EFT. A relativistic regulator is generated from the nonlocal Lagrangian where a gauge link is introduced to guarantee local gauge invariance, with additional diagrams from the expansion of the gauge link ensuring conservation of electric charge and strangeness. Flavor asymmetries for sea quarks at zero and finite momentum transfer, as well as strange form factors, are obtained from the calculated GPDs, and results compared with phenomenological extractions and lattice QCD.

 

Date: Nov 17 10 AM ET

Seminar Talk: Deeply Virtual and Timelike Compton scattering at JLab and the EIC: mapping the nucleon in 3D

Speaker: Daria Sokhan (CEA Saclay)

The electron scattering kinematics accessible at Jefferson Lab (JLab) reveal the valence quark region of the nucleon. Accessing the quark-gluon sea with high precision awaits the Electron-Ion Collider (EIC), soon to start construction at Brookhaven National Lab. The high luminosities of JLab and EIC enable measurements of exclusive processes which are sensitive to the 3D structure of the nucleon encoded in Generalised Parton Distributions. Two such related processes are Deeply Virtual Compton Scattering (DVCS) and Timelike Compton Scattering (TCS) -- both involve the scattering of a photon from a quark in a minimally-perturbed nucleon. In DVCS the initial photon is highly virtual and the momentum transfer results in the production of a high energy real photon. In TCS, a real photon scatters to produce a highly virtual photon in the final state. The seminar discusses measurements of both processes at JLab and the EIC, as well as what we can learn from them about nucleon structure.T

Date: Nov 10 10AM ET

Seminar Talk: Physics of the nucleon's energy-momentum tensor form factor C (or the D-term)

Speaker: Xiangdong Ji (UMD/CNF)

In this talk, I will discuss the physics of the nucleon's energy-momentum tensor form factor C, which can be measured in DVCS-like processes and calculable through lattice QCD. The form factor contributes to the energy/mass structure as well as the gluon-scalar density, and the latter acts as a "confinement potential" for the quarks and gluons.  The form factor also generates a gravitational potential inside the nucleon through the momentum density current, which can be characterized by the gravitational tensor monopole moment (the D-term) similar to the well-known magnetic moment.  Contrary to the common wisdom, I will argue that the form factor does not give a usual sense of pressure distribution and the sign of monopole moment has little to do with mechanical stability.

Date: Nov 3 10AM ET

Seminar Talk: Parton distribution uncertainties and their correlations

Speaker: Stefano Forte (U. Milan)

I will discuss the problem of accurately estimating the uncertainty on parton distributions (PDFs). I will show that data do not determine fully parton distribution. I will present several techniques that have been developed for the assessment of PDF uncertainties, both in the region where experimental information is available, and when extrapolating outside it: closure tests and future tests. I will finally discuss the correlation between different PDFs determined from the same underlying data, their use as a methdological assessment tool, and also their possible use for PDF combination.

Oct 6th, 10 AM - 11:30 AM ET:

Seminar Talk: DVCS in EIC Era: Precision Frontier

Speaker: Vladimir Braun (U. Regensberg)

I will give a review of our results and ongoing projects on DVCS, aimed at making the theory description as precise as possible to match high statistical accuracy of the expected data. I will cover two subjects:

(1) Next-to-next-to-leading accuracy at leading twist, with some new results on two-loop coefficient functions, and

(2) Frame dependence of the leading-twist approximation and how it is repaired by kinematical power corrections. Here, I will remind of the existing results to 1/Q^2 accuracy, and explain how one  may hope to re-sum such corrections to all orders in the power expansion (ongoing project). One goal is to show that target mass corrections do not spoil QCD factorization for coherent DVCS from nuclei

Sep 29th, 10 AM - 11:30 AM ET:

Seminar Talk: Higher Order Kinematical Effects in DVCS

Speaker: Yuxun Guo (UMD)

We study the deeply virtual Compton scattering cross-section in twist-two generalized parton distribution (GPD) approximation, and show that different choices of light-cone vectors and gauges for the final photon polarization will lead to different higher-order kinematical corrections to the cross-section formula. The choice of light cone vectors affects kinematic corrections at the twist-three level, accounting for the differences between the cross-section formulas in the literatures. Those differences are studied systematically and are found quite significant at JLab 12 GeV and future EIC kinematics. They serve as the intrinsic systematic uncertainties in extracting the Compton form factors through the cross-section formula.  More importantly, they are also necessary for understanding cross-section measurements with higher-twist precision and to reconstruct higher-order Compton form factors.

Sep 15th, 11 AM - 12:30 PM ET:

Seminar Talk: Gluon distribution in the nucleon from lattice QCD

Speaker: Raza Sufian (JLab)

We present a calculation of lattice QCD non-local matrix elements that can be used to determine the unpolarized gluon Ioffe-time distribution and the corresponding parton distribution functions using QCD short distance factorization. We construct the nucleon interpolation fields using the distillation technique and smear the gauge fields using the gradient-flow. We obtain the flow time-independent reduced Ioffe-time pseudo-distribution, and calculate the light-cone Ioffe-time distribution and unpolarized gluon distribution function in the MS-bar scheme at $\mu = 2$ GeV.  We also present progress towards determining gluon helicity distribution from lattice QCD calculation. 

 

June 2nd, 10AM - 11:30 AM ET:

Seminar Talk: Energy-momentum tensor form factors and mechanical properties

Speaker: Peter Schweitzer (U. Conn.)

Studies of hard exclusive processes allow us to access information on the form factors of the energy-momentum tensor opening new ways to investigate the structure of hadrons. Through an interpretation of the form factors, we get among others the very first insights into the distribution of internal forces inside hadrons. The purpose of this talk is to give an overview over the recent advances in theory, experiment, and interpretation of the energy-momentum tensor form factors.

 

May 26th, 10AM - 11:30 AM ET:

Seminar Talk: CORE Compact Detector for EIC

Speaker: Charles Hyde (ODU)

A description of the guiding concepts of CORE (Slides)

 

May 19th, 10AM - 11:30 AM ET:

Seminar Talk: A fully relativistic description of forces in hadrons on the light front

Speaker: Adam Freese (U. Washington)

Since up and down quarks are nearly massless, hadrons are manifestly relativistic systems, so care must be taken to account for relativistic effects when calculating densities within them. Relativistic effects can be incorporated easily by using light front coordinates and integrating out the longitudinal spatial coordinate, giving a transverse density at fixed light front time. We demonstrate how transverse momentum, spin, and force densities can be obtained through 2D Fourier transforms of form factors associated with the energy-momentum tensor. We contrast these fully relativistic 2D densities with non-relativistic 3D densities and explore the relationship between both.

 

Apr 28th, 10AM - 11:30 AM ET:

Seminar Talk: Twist-3 Effects in Forward Photoproduction of a Quarkonium

Speaker: J.P. Ma (ITP)

Photoproduction of a Quarkonium in forward region can be used to extract generalized gluon distributions.  At the leading power, twist-2 gluon GPD’s are involved. To correctly extract these GPD’s, power corrections or twist-3 contributions  need to be studied. In this talk I present the results of twist-3 contributions and discuss their implications. From the results we find that the corrections in the high-energy limit are only suppressed by the inverse of the heavy quark mass.

 

Apr 23rd, 10AM - 11:30 AM ET:

Talk: Sparse Reconstruction of GPDs

Speaker: Yuesheng Xu (ODU)

Abstract: I will report recent results on sparse reconstruction of GPDs by using orthogonal polynomial bases enhanced with singular functions estimated from CFF data. This is a joint work with Jie Jiang (Sun Yat-sen University, Guangzhou) and Charles Hybe (ODU).

 

Apr 21st, 11AM - 12:30 PM ET:

Seminar Talk: Positron Beams and Femtography at JLab

Speaker: Eric Voutier (Paris-Saclay) 

The interest in both polarized and unpolarized intense positron beams for the experimental investigation of the physical world ranges from the macroscopic molecular scale accessible at eV energies down to the most elementary scale of fundamental symmetries probed with hundreds of GeV lepton beams. However, the production of high-quality and high duty-cycle polarized positron beams relevant to these many applications remains a challenging endeavor.

The successful demonstration of the PEPPo (Polarized Electrons for Polarized Positrons) technique at the CEBAF injector opens the door to polarized positron beams at Jefferson Lab energies. An ambitious experimental program supported by ~200 physicists from ~60 institutions has been proposed in the e+ @JLab White Paper. It encompasses the investigation of two-photon exchange physics, nucleon and nuclei tomography, tests of the Standard Model and other specific measurements. This seminar will review the main benefits of positron beams for the hadronic physics program at Jefferson Lab, paying special attention to nucleon femtography in the single and the double deeply virtual Compton scatterings.

 

Apr 7th, 10AM - 11:30 AM ET:

Seminar Talk: QCD real-time dynamics and inverse problems: Highlights from the ACFI 2020 workshop

Speaker: Chris Monahan (W&M)

Abstract:   Realizing the full potential of experimental studies of nuclear matter requires a comprehensive understanding of the dynamics of its microscopic constituents, within the theory of quantum chromodynamics (QCD). Lattice QCD calculations have made significant contributions to our understanding of the strong interaction, but little is known from ab initio calculations about the dynamical properties of quarks and gluons. A central challenge for such calculations is the need to solve ill-posed inverse problems. I will review the themes that emerged from the remote workshop I co-organised with Martha Constantinou and Alexander Rothkopf, hosted by the Amherst Center for Fundamental Interactions, which brought together practitioners in the field of lattice QCD and other communities working with inverse problems.

 

Mar 31st, 11AM - 12:30 PM ET:

Seminar Talk: Near Threshold Heavy Quarkonium Photoproduction at Large Momentum Transfer

Speaker: Feng Yuan (LBNL)

Abstract:   In this talk, we will show that perturbative QCD can provide a powerful tool to study near threshold photoproduction of heavy quarkonium, especially, at large momentum transfer. The conventional power counting method will be modified near the threshold, and the three quark Fock state with nonzero orbital angular momentum provides the dominant contribution. It  carries a power behavior of $1/(-t)^5$ for the differential cross section. We will also comment on the impact of our results on the interpretation of the experiment measurement in terms of the gluonic gravitational form factors of the proton.

 

Mar 24th, 11AM - 12:30 PM EST:

Seminar Talk: Towards the 3-dimensional parton structure of the pion: integrating transverse momentum data into global QCD analysis

Speaker: Patrick Barry

Abstract:  We present a new Monte Carlo global QCD analysis of pion parton distribution functions (PDFs), including, for the first time, transverse momentum dependent pion-nucleus Drell-Yan cross sections together with transverse momentum integrated Drell-Yan and leading neutron electroproduction data from HERA. While the small transverse momentum region of the Drell-Yan spectrum is described by the Collins-Soper-Sterman (CSS) formalism and through nonperturbative transverse momentum dependent distributions (TMDPDFs), the large transverse momentum region is theoretically parametrized only by the collinear PDFs. In this talk, we discuss the results of including the large transverse momentum data in the Jefferson Lab Angular Momentum (JAM) Monte Carlo global analysis framework. We assess the sensitivity of the Monte Carlo fits to kinematic cuts, factorization scale, and parametrization choice, and we discuss the impact of the various data sets on the pion's quark and gluon distributions. As a further application, we apply threshold resummation on the transverse momentum integrated Drell-Yan theory and present the results with a focus on the large x behavior. We have succeeded in a necessary step towards the simultaneous analysis of collinear and transverse momentum dependent pion distributions and the determination of the pion's 3-dimensional structure.

 

Feb 17th, 10AM - 11:15 AM EST:

Seminar Talk: Quarkonium photo- and lepto-production near threshold

Speaker: Yoshitaka Hatta

Abstract:  I will discuss the physics of near-threshold production of heavy quarkonium as a useful tool to study the gluon gravitational form factors of the nucleon.  These include the gluon D-term which is related to the internal pressure distribution exerted by gluons inside the nucleon, and also the gluon condensate related to the QCD trace anomaly. I also briefly mention my ongoing work on phi-meson lepto-production near threshold and its connection to the strangeness contribution to the D-term.

 

Jan 27th, 11AM - 12:30 PM EST:

Title: Novel twist-three transverse-spin sum rule for the proton and related generalized parton distributions

Speaker: Kyle Shiells (CNF)

Abstract: we derive a new twist-3 partonic sum rule for the transverse spin of the proton, which involves the well-know quark spin structure function $g_T(x)=g_1(x)+g_2(x)$, the less-studied but known transverse gluon polarization density $\Delta G_T(x)$, and quark and gluon canonical orbital angular momentum densities associated with transverse polarization. This is the counter part of the sum rule for the longitudinal spin of the proton derived by Jaffe and Manohar previously. We relate the partonic canonical orbital angular momentum densities to a new class of twist-3 generalized parton distribution functions which are potentially measurable in deep-virtual exclusive processes. We also discuss in detail an important technicality related to the transverse polarization in the infinite momentum frame, i.e., separation of intrinsic contributions from the extrinsic ones. We apply our finding to the transverse-space distributions of partons, angular momentum, and magnetic moment, respectively, in a transversely polarized proton.

 

Jan 20th, 11AM - 12:30 PM EST:

Title: Machine learning for QCD global analysis

Speakers: Yaohang Li, Nobuo Sato

Abstract: A new era for the exploration of hadron structure has begun with the Jefferson Lab 12 GeV program and the planned Electron Ion Collider. The new generation of experiments will allow us to probe the quantum correlation function (QCFs) of quarks and gluons that emerges from the theory of strong interactions.  Since these QCFs are not direct physical observables, the experimental data needs to be analyzed within the framework of QCD factorization that stress test in a self consistent manner the predictive power of QCD and the universality of QCFs using Bayesian inference. In this talk we will discuss the building blocks of  QCD global analysis framework and introduce a new tool based on machine learning (ML) that we call “inverse mappers”  that has the potential to change the paradigm of QCD global analysis.  We will discuss technical details of the ML architectures that make it possible to build the inverse mappers.

 

Jan 13th, 11AM - 12:30 PM EST:

Seminar:  "Pion generalized parton distributions from lattice QCD"

Speaker: Jian-hui Zhang (BNU)

Abstract: Generalized parton distributions (GPDs) are important quantities for understanding the multi-dimensional structure of hadrons. In this talk, I'll discuss lattice calculations of pion GPDs using the large-momentum effective theory (LaMET) approach, and present some results at zero skewness. I'll also discuss some renormalization issues in LaMET and new strategies that can be used to improve the extraction of GPDs as well as other partonic quantities from lattice QCD.