Seminars & News

Upcoming Events


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.

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Past seminars


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.

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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.

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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.

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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.

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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.

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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.

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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.

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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. 

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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

S. Fazio

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:  

Transverse Imaging

M. Burkardt

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

F.-X. Girod

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.