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
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.