2022 Seminars: Upcoming
Proton Mass Discussion
Zoom Link: https://us02web.zoom.us/j/4241669197
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.
Dec 16th, 11AM - 12:30 PM EST:
Seminar: Parton quasi-distributions for GPDs and twist-3 PDFs
Speaker: A. Metz (TU)
Abstract: Parton quasi-distributions (quasi-PDFs) offer novel opportunities for studying the structure of hadrons in lattice QCD. At present the impact of this approach can be largest for parton correlators which are difficult to measure. This applies to both GPDs and twist-3 PDFs. The seminar addresses the following specific topics: First, we discuss quasi-GPDs in a diquark spectator model which can provide qualitative results for the prospects of the quasi-GPD approach. We pay particular attention to higher-twist corrections related to the parton momentum and the longitudinal momentum transfer to the target. Second, we address several model-independent results for quasi-GPDs as well as quasi-PDFs for twist-3, including Burkhardt-Cottingham-type sum rules. Third, we will address the role of singular zero mode contributions in the process of relating, in perturbative QCD, quasi-PDFs and light-cone PDFs for twist-3.
Dec 2nd, 11AM - 12:30 PM EST
Seminar: Long-range processes in QCD
Speaker: R. Briceno
A rich variety of phenomena in the Standard Model and its extensions manifest in long-range processes involving bound states of quantum chromodynamics (QCD), namely hadrons. These are processes where intermediate hadronic states propagate over a long distance, between electroweak interactions. A key example includes deeply virtual Compton scattering. Such processes are at the cusp of what can be systematically studied given two challenges. First, these reactions involve hadrons, and as a result one must use a non-perturbative tool to access their amplitudes. Currently lattice QCD is the only systematically improvable way we have for doing just this. Second, lattice QCD is defined in a finite, Euclidean spacetime. This introduces its own specific challenges, time in purely imaginary in lattice QCD, and by truncating the space one looses the notion of asymptotic states. In this talk I explain how these issues can all be resolved systematically for a relatively large kinematic region. Lastly, I will touch on novel ideas for studying these reactions using future quantum computers.
Nov 25th, 11AM - 12:30 PM EST
Seminar: Beyond leading-twist PDFs from lattice QCD: GPDs and twist-3 PDFs
Speaker: M. Constantinou (Temple University)
"Lattice QCD (LQCD) is a theoretical non-perturbative approach for studying QCD dynamics numerically from first principles. LQCD is widely used for hadron structure calculations and is becoming a reliable tool, striving to control various sources of systematic
uncertainties. Parton distribution functions (PDFs) have a central role in understanding the hadron structure and have been calculated in lattice QCD mainly via their Mellin moments.
In this talk, we will present selected results using a novel pioneering approach to access PDFs proposed by X. Ji in 2013. This is the so-called quasi-distribution method, which relies on matrix elements of fast-moving hadrons coupled to non-local operators. These are matched to the light-cone PDFs using Large Momentum Effective Theory (LaMET). The main part of the talk is dedicated to the chiral-even unpolarized and helicity quark generalized parton distributions (GPDs), extracted from numerical simulations of lattice QCD. We will also demonstrate the feasibility of the approach for twist-3 distributions and present results for gT(x), explored in lattice QCD for the first time. The calculation is performed on one ensemble of two degenerate light, a strange and a charm quark (Nf=2+1+1) of maximally twisted mass fermions with a clover term, reproducing a pion mass of 260 MeV."
11:00am - 12:30pm EST Nov 11th
Seminar: "Nucleon and pion structure from lattice QCD: progress and challenges"
Abstract: I describe recent work by the hadstruc collaboration at understanding the structure of the pion and nucleon from lattice QCD. In particular, I describe the steps in the extraction of the x-dependent PDFs from the matrix elements computed on the lattice. I then review some recent theoretical and algorithmic developments needed for future computations of three-dimensional structure encoded through GPDs, notably the need at attain high spatial momenta.
Speaker: D. Richards
11 am-12:30 pm ET, Oct 21, 2020:
Seminar: "GPD Observables: Theory and Extraction Techniques"
Determining the 3D partonic structure of the nucleon is a fundamental goal of current nuclear experimental programs from Jefferson Lab at 12 GeV to the electron ion collider (EIC). It was proposed that deeply virtual Compton scattering can be used as a probe to access the generalized proton distributions (GPDs) that contain this information. The extraction of GPD 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. In the helicity formalism, we extract the Compton form factors H and E separately using a generalization of the Rosenbluth method such that the dependence on Q2 is clear. In addition, using state of the art neural network techniques, we perform an analysis of the DVCS cross section and show initial steps toward a global neural network extraction of Compton form factors.
Speaker: B. Kriesten (UVa)
11am EST, Oct 07, 2020:
DVCS and GPDs at the future EIC
With the project to build a future Electron-Ion Collider (EIC), advancements in theory and further development of phenomenological tools, we are now preparing for the next step in subnuclear tomographic imaging. The collider’s large range of center-of- mass energies in combination with very high luminosity and polarization of both the lepton and the hadron beams, will open a unique opportunity for high precision measurements of both cross sections and spin-asymmetries. This will allow us for a detailed investigation of the partonic substructure of hadrons in multi-dimensions, as well as addressing the role of orbital angular momentum with respect to the nucleon spin. Generalized parton distributions (GPDs) describe the multi-dimensional partonic structure of a nucleon in coordinate space, providing new information about the internal dynamics of quarks and gluons. Measurements hard exclusive processes, like Deeply Virtual Compton Scattering (DVCS), with all related probes, are the best way in constraining GPDs and achieve precision spatial partonic tomographic images. This talk will highlight key experimental challenges, available and upcoming computer simulation tools and finally discuss the EIC’s expected impact over the current knowledge of GPDs.
11am EST, September 30, 2020:
One of the science motivations that drive the physics program both at JLab@12GeV as well as the future EIC is transverse imaging. Generalized Parton Distributions (GPDs) are one of the key ingredients for transverse imaging. While twist-2 GPDs allow for a determination of the distribution of partons on the transverse plane, twist-3 GPDs contain quark-gluon correlations that provide information about the average transverse color Lorentz force acting on quarks. As an example, we use the nonforward generalization of $g_T(x)$, to illustrate how twist-3 GPDs can provide transverse position information about that force. The difference between the quark orbital angular momentum (OAM) defined in light-cone gauge (Jaffe-Manohar) compared to defined using a local manifestly gauge invariant operator (Ji) is interpreted in terms of the change in quark OAM as the quark leaves the target in a DIS experiment. I also discuss the possibility to measure quark OAM directly using twist 3 GPDs, and to calculate quark OAM in lattice QCD.
11am EST, September 16, 2020:
Nucleon GPD studies with CLAS 12
Generalized Parton Distributions (GPDs) unify elastic Form Factors and ordinary Parton Distribution Func-
tions into one framework enabling tomographic images of the nucleon’s constituents simultaneously in transverse
position and longitudinal momentum. The second Mellin moments of GPDs parameterize the Energy Momen-
tum 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 these tomographic images,
and the mechanical properties of the nucleon otherwise only accessible through Graviton Scattering, such as the
Pressure and Shear Force Distributions. Deeply Virtual Meson Production (DVMP) naturally extends and com-
plements DVCS to map out the GPD flavor dependences, including gluons in the valence region. The CLAS12
GPD program includes a complete set of probe and target spin combinations, as well as several beam energies.
We will present preliminary results from this program, demonstrating high statistics samples of exclusive events
up to xB =0.7 and Q =10GeV .
9pm EST, Thursday September 10, 2020:
New treatment of Landau criterion; its uses in QCD
Prof. John Collins (Penn State University)
My talk is about the results in my new e-print arXiv:2007.04085. A large fraction of the phenomenological applications of QCD rely on factorization theorems and related results. These in turn (at several steps removed) rely on the classification of internal momenta in an amplitude into hard, soft and collinear. That classification as a general result was obtained by Libby and Sterman on the basis of the determination of pinch singular points by Landau, and especially by Coleman and Norton, but applied in a massless theory as a way of analyzing large-$Q$ asymptotics. However, the classic derivations by Landau, Coleman, Norton and others have considerable deficiencies, gaps, and even errors, especially in the massless case. In this talk I explain the problems, and how I go about solving them. I also indicate potential future applications of the new methods and results.