Search

We present a parametrization of the chiral even generalized parton distributions, $H$, $E$, $\widetilde{H}$, $\widetilde{E}$, for the quark, antiquark and gluon, in the perturbative QCD-parton framework. Parametric analytic forms are given as a function of two equivalent sets of variables $x,\xi,t$ (symmetric frame) and $X,\zeta,t$ (asymmetric frame), at an initial scale, $Q_o^2$. In the $X>\zeta$ region a convenient and flexible form is obtained as the product of a Regge term $\propto X^{-\alpha + \alpha' t}$, describing the low $X$ behavior, times a spectator model-based functional form depending on various mass parameters; the behavior at $X, Comment: 28 pages, 19 figures

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental programs at the next generation of lepton accelerators. In the context of the hadronic physics program at Jefferson Lab (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of nucleons and nuclei, in both the elastic and deep-inelastic regimes. For instance, elastic scattering of polarized and unpolarized electrons and positrons from the nucleon enables a model independent determination of its electromagnetic form factors. Also, the deeply-virtual scattering of polarized and unpolarized electrons and positrons allows unambiguous separation of the different contributions to the cross section of the lepto-production of photons and of lepton-pairs, enabling an accurate determination of the nucleons and nuclei generalized parton distributions, and providing an access to the gravitational form factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model of particle physics through the search of a dark photon, the precise measurement of electroweak couplings, and the investigation of charged lepton flavor violation. This document discusses the perspectives of an experimental program with high duty-cycle positron beams at JLab., Comment: 18 pages, 7 figures This version superseeds the previous version which scientific content was decomposed into several more elaborated articles. All of these articles will be collected in the EPJ A Topical Issue about "Positron beam and physics at Jefferson Lab (e+@Jlab)"

The strong force which binds hadrons is described by the theory of Quantum Chromodynamics (QCD). Determining the character and manifestations of QCD is one of the most important and challenging outstanding issues necessary for a comprehensive understanding of the structure of hadrons. Within the context of the QCD parton picture, the Parton Distribution Functions (PDFs) have been remarkably successful in describing a wide variety of processes. However, these PDFs have generally been confined to the description of collinear partons within the hadron. New experiments and facilities provide the opportunity to additionally explore the transverse structure of hadrons which is described by Generalized Parton Distributions (GPDs) and Transverse Momentum Dependent Parton Distribution Functions (TMD PDFs). In our previous review, we compared and contrasted the two main approaches used to determine the collinear PDFs: the first based on perturbative QCD factorization theorems, and the second based on lattice QCD calculations. In the present report, we provide an update of recent progress on the collinear PDFs, and also expand the scope to encompass the generalized PDFs (GPDs and TMD PDFs). We review the current state of the various calculations, and consider what new data might be available in the near future. We also examine how a shared effort can foster dialog between the PDF and Lattice QCD communities, and yield improvements for these generalized PDFs., 2020 PDFLattice Report, 64 pages, 34 figures

The last decade has seen a marked shift in how the internal structure of hadrons is understood. Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularised QCD have provided strong indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent hadronic mass that is responsible for almost all visible mass in the universe; experiment has uncovered signals for such correlations in the flavour-separation of the proton's electromagnetic form factors; and phenomenology suggests that diquark correlations might be critical to the formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such information is evaluated herein, with a view to consolidating the facts and therefrom moving toward a coherent, unified picture of hadron structure and the role that diquark correlations might play., 113 pages, 41 figures, 8 tables

Generalized transverse momentum-dependent parton distributions (GTMDs) provide a comprehensive framework for imaging the internal structure of the proton. In particular, by encoding the simultaneous distribution of quark transverse positions and momenta, they allow one to directly access longitudinal quark orbital angular momentum, and, moreover, to correlate it with the quark helicity. The relevant GTMD is evaluated through a lattice calculation of a proton matrix element of a quark bilocal operator (the separation in which is Fourier conjugate to the quark momentum) featuring a momentum transfer (which is Fourier conjugate to the quark position), as well as the Dirac structure appropriate for capturing the quark helicity. The weighting by quark transverse position requires a derivative with respect to momentum transfer, which is obtained in unbiased fashion using a direct derivative method. The lattice calculation is performed directly at the physical pion mass, using domain wall fermions to mitigate operator mixing effects. Both the Jaffe-Manohar as well as the Ji quark spin-orbit correlations are extracted, yielding evidence for a strong quark spin-orbit coupling in the proton., Comment: 8 pages, 2 figures, to appear in the proceedings of the 38th International Symposium on Lattice Field Theory, LATTICE2021, PoS LATTICE2021, 413

The strong force which binds hadrons is described by the theory of Quantum Chromodynamics (QCD). Determining the character and manifestations of QCD is one of the most important and challenging outstanding issues necessary for a comprehensive understanding of the structure of hadrons. Within the context of the QCD parton picture, the Parton Distribution Functions (PDFs) have been remarkably successful in describing a wide variety of processes. However, these PDFs have generally been confined to the description of collinear partons within the hadron. New experiments and facilities provide the opportunity to additionally explore the transverse structure of hadrons which is described by Generalized Parton Distributions (GPDs) and Transverse Momentum Dependent Parton Distribution Functions (TMD PDFs). In our previous review, we compared and contrasted the two main approaches used to determine the collinear PDFs: the first based on perturbative QCD factorization theorems, and the second based on lattice QCD calculations. In the present report, we provide an update of recent progress on the collinear PDFs, and also expand the scope to encompass the generalized PDFs (GPDs and TMD PDFs). We review the current state of the various calculations, and consider what new data might be available in the near future. We also examine how a shared effort can foster dialog between the PDF and Lattice QCD communities, and yield improvements for these generalized PDFs.

Twist three Generalized Parton Distributions and Generalized Transverse Momentum Distributions are central to the description of partonic orbital angular momentum. We discuss the origin of the genuine twist three contribution, described by the quark gluon quark correlator, to the twist three Generalized Parton Distribution E˜2T and how it connects to calculations of the kT2 moments of the Generalized Transverse Momentum Distribution F14.

Talk from the Strong QCD from Hadron Structure Experiments Workshop 2019

Progress in particle and nuclear physics 100, 107 - 160 (2018). doi:10.1016/j.ppnp.2018.01.007, In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs., Published by Elsevier, Oxford [u.a.]

New Lorentz Invariance Relations (LIRs) were presented between twist-three Generalized Parton Distributions (GPDs) and transverse momentum, $k_T$, moments of twist-two Generalized Transverse Momentum-Dependent Distributions (GTMDs). By implementing both these LIRs and the QCD Equations of Motion in the quark quark correlation function, we generated a new series of Wandzura Wilczek (WW) relations in the off-forward sector. Two of these WW relations take on a particularly interesting physical meaning in that they provide a clear interpretation of the QCD structure of Orbital Angular Momentum (OAM) in the nucleon. In particular, they provide a solution to the outstanding puzzle of how OAM could be simultaneously described by twist-two GTMDs and twist-three GPDs. Additional relations were discussed, in particular, for the helicity configurations that can be detected analyzing specific spin asymmetries: one corresponding to a longitudinally polarized quark in an unpolarized proton, associated with spin-orbit correlations, and one for transverse proton polarization, as a generalization of the relation obeyed by the $g_2$ structure function; finally, we defined a relation connecting the off-forward extension of the Sivers function to an off-forward Qiu-Sterman term., 5 pages, talk presented at QCD Evolution 2017, 22-16 May 2017, Jefferson Lab, Newport News, VA-USA

Spin and transverse momentum dependent Generalized Parton Distributions (GPDs) exist at the interface between the non-perturbative regime of QCD hadron structure and observable quantities. The distributions appear as linear superpositions and convolutions within helicity amplitudes for parton-nucleon scattering processes, which, in turn, occur in amplitudes for leptoproduction processes. We have developed a “flexible model" of quark and gluon GPDs that incorporates diquark and other spectators, Regge behavior and evolution. Chiral even GPDs determine deeply virtual Compton scattering amplitudes and are compared with cross section and polarization data. The chiral odd GPDs can be generated from these via parity relations. Those chiral odd GPDs, including “transversity", lead to predictions for pseudoscalar leptoproduction. We will present relations between crucial quark-nucleon or gluon-nucleon GPDs and the rich array of angular distributions in Deeply Virtual Scattering processes.

Spin and transverse momentum dependent parton distributions - Generalized Parton Distributions (GPDs) - are at the interface between the QCD structure of the hadrons and observable quantities. The GPDs are linear superpositions within helicity amplitudes. The amplitudes are probed in high energy leptoproduction processes through angular dependent cross sections and polarization asymmetries. Phenomenological extraction of the amplitudes and the distributions is a challenging task. We present observables that connect particularly with the chiral odd quark-nucleon helicity amplitudes for Deeply Virtual p 0 Production.

The quark orbital angular momentum component of proton spin, $L_q$, can be defined in QCD as the integral of a Wigner phase space distribution weighting the cross product of the quark's transverse position and momentum. It can also be independently defined from the operator product expansion for the off-forward Compton amplitude in terms of a twist-three generalized parton distribution. We provide an explicit link between the two definitions, connecting them through their dependence on partonic intrinsic transverse momentum. Connecting the definitions provides the key for correlating direct experimental determinations of $L_q$, and evaluations through Lattice QCD calculations. The direct observation of quark orbital angular momentum does not require transverse spin polarization, but can occur using longitudinally polarized targets., Comment: 5 pages, 2 figures

We present a physically motivated parametrization of the chiral-odd generalized parton distributions. The parametrization is an extension of our previous one in the chiral-even sector which was based on the Reggeized diquark model. While for chiral-even generalized distributions a quantitative fit with uncertainty estimation can be performed using deep inelastic scattering data, nucleon electromagnetic, axial and pseudoscalar form factors measurements, and all available deeply virtual Compton scattering data, the chiral-odd sector is far less constrained. While awaiting the analysis of measurements on pseudoscalar mesons exclusive electroproduction which are key for the extraction of chiral-odd GPDs, we worked out a connection between the chiral-even and chiral-odd reduced helicity amplitudes using parity transformations. The connection works for quark-parton models including both scalar and axial vector diquark models, and spectator models in general. This relation allows us to estimate the size of the various chiral-odd contributions and it opens the way for future quantitative fits.

We present a critical discussion of the observables that have been recently put forth to describe quarks and gluons orbital angular momentum distributions. Starting from a standard parameterization of the energy momentum tensor in QCD one can single out two forms of angular momentum, a so-called kinetic term, generally associated with the Ji decomposition, and a canonical term from the Jaffe Manohar decomposition. Orbital angular momentum has been connected to a Generalized Transverse Momentum Distribution (GTMD), for the canonical term, and to a twist three Generalized Parton Distribution for the kinetic term. We argue that while the latter appears as an azymuthal angular modulation in the longitudinal target spin asymmetry in deeply virtual Compton scattering, due to parity constraints, the GTMD associated with canonical angular momentum cannot be measured in a similar set of experiments.

We evaluate the impact of recent developments in hadron phenomenology on extracting possible fundamental tensor interactions beyond the standard model. We show that a novel class of observables, including the chiral-odd generalized parton distributions, and the transversity parton distribution function can contribute to the constraints on this quantity. Experimental extractions of the tensor hadronic matrix elements, if sufficiently precise, will provide a so far absent testing ground for lattice QCD calculations., Comment: 5 pages, 2 figures

I present an overview of recent data on Deep Inelastic Scattering at large Bjorken x and low invariant mass, W2, where parton-hadron duality was originally observed. I discuss the concept of parton-hadron duality from the perspective of perturbative QCD. Within this framework, I show that parton-hadron duality is broken for low values of W2, in the \(\Delta\)- and S11-resonance production region. A model that accounts for the anomalous scale dependence ensuing from this situation is developed.

We examine the low Bjorken $x$ gluon distribution in nuclei in the asymptotic region., Comment: 7 pages, 1 figure

Spin and transverse momentum dependent parton distributions - Generalized Parton Distributions, Transverse Momentum Distributions and related distributions - are at the interface between the QCD structure of the hadrons and observable quantities. The distributions are contained as linear superpositions within helicity amplitudes that factorize into universal forms at leading or next to leading order. These amplitudes are probed in high energy leptoproduction processes through angular dependent cross sections and polarization asymmetries. The phenomenological extraction of the amplitudes and the distributions is a challenging task. We will present some models for the distributions in leptoproduction processes, as well as the observables that connect with the quark-nucleon helicity amplitudes for different Deeply Virtual production possibilities.

We present recent developments on the role of the running coupling constant at the intersection of perturbative and nonperturbative QCD. A number of experiments show a smooth transition from small to large scales given by the four-momentum transfer in the reactions. This is at variance with perturbative QCD where the running coupling constant becomes infinite when the scale equals $\Lambda_{{\tiny QCD}}$. Approaches using an effective coupling constant could help interpret the opposite trend of data as compared to standard perturbative QCD predictions. We give an overview of the role of the coupling constant in the procedure to match nonperturbative hadronic model to perturbative QCD and we propose an extraction of an effective coupling constant from inclusive electron proton scattering data at large Bjorken x., Comment: To appear in the proceedings of the QCD Evolution Workshop, JLab, May 14-17, 2012. Includes ws-ijmpcs.cls

A physically motivated parameterization of the chiral-even generalized parton distributions (GPDs) has been obtained from a global analysis using available experimental data including nucleon electromagnetic form factors, deep inelastic scattering and DVCS. Additional information provided by lattice calculations of the higher moments of GPDs is also considered. Recently extracted observables from DVCS at JLab and Hermes are reproduced by our fit and predictions are made for higher energy regimes. Our scheme uses a Reggeized diquark model and concomitant helicity amplitude relations, which allow the extension to the chiral-odd sector. Having obtained the chiral-odd GPDs, we are able to explain the features of exclusive neutral meson electroproduction. Our analysis is applied to existing and forthcoming spin-dependent π0 and η data in the kinematical region of intermediate Bjorken x and Q2 in the multi-GeV region, accessible at present and currently planned facilities.

We give an interpretation of the $u$ and $d$ quarks contributions to the nucleon electromagnetic form factors for values of the four-momentum transfer in the multi-GeV region where flavor separated data have been recently made available. The data show, in particular, a suppression of $d$ quarks with respect to $u$ quarks at large momentum transfer. %and constant ratios of the flavor dependent Pauli to Dirac form factors ratios. This trend can be explained using a reggeized diquark model calculation of generalized parton distributions, thus providing a correlation between momentum and coordinate spaces, both of which are necessary in order to interpret the partonic substructure of the form factors. We extend our discussion to the second moments of generalized parton distributions which are believed to contribute to partonic angular momentum., Comment: 22 pages, 19 figures; results and figures added and changed, tables added, formulae added, major rewriting of text

We describe a new method to extract parton distribution functions from hard scattering processes based on Self-Organizing Maps. The extension to a larger, and more complex class of soft matrix elements, including generalized parton distributions is also discussed., 6 pages, 3 figures, to be published in the proceedings of ACAT 2011, 14th International Workshop on Advanced Computing and Analysis Techniques in Physics Research

We propose to investigate a largely unexplored sector that is unique to the formulation of hard exclusive processes in terms of GPDs, namely the electroproduction of strange and charmed mesons in the kinematical ranges of Jefferson Lab's 12 GeV upgrade, and of the proposed Electron Ion Collider (EIC). In this contribution we focus on charmed meson production that is unique to the EIC. Exclusive strange and charmed meson production provides new insights in the connection of the quark/gluon degrees of freedom with the meson-baryon description, both in the unpolarized and polarized sectors. However, as particularly evident in polarized scattering, the underlying mechanisms are still far from being fully understood. We present an approach in terms of generalized parton distributions. As an application, we show that through exclusive electroproduction of pseudoscalar charmed mesons one can uniquely single out the non-perturbative charmed component in the nucleon structure function.

We derive a sum rule for the total quark angular momentum of a spin-one hadronic system within a gauge invariant decomposition of the hadron's spin. We show that the total angular momentum can be measured through deeply virtual Compton scattering experiments using transversely polarized deuteron targets., 4 pages, 2 figures, changes in text, figures changed, references changed

We discuss the extension of a systematic perturbative QCD based analysis to the x -> 1 region. After subtracting a number of effects that transcend NLO pQCD evolution, such as target mass corrections and large x resummation effects, the remaining power corrections can be interpreted as dynamical higher twists. The quantitative outcome of the analysis is dominated by the interplay between the value of alpha_S in the infrared region and the higher twists. We uncover a dual role played by alpha_S at large Bjorken x that can be used to experimentally extract its value in the non-perturbative regime., 8 pages, 3 figures, presented at HiX2010 Workshop, Jefferson Lab, October 2010

Exclusive meson leptoproduction from nucleons in the deeply virtual exchanged boson limit can be described by generalized parton distributions (GPDs). Including spin dependence in the description requires 8 independent quark-parton and gluon-parton functions. The chiral even subset of 4 quark-nucleon GPDs are related to nucleon form factors and to parton distribution functions. The chiral odd set of 4 quark-nucleon GPDs are related to transversity, the tensor charge, and other quantities related to transversity. Different meson or photon production processes access different combinations of GPDs. This is analyzed in terms of $t$-channel exchange quantum numbers, $J^{PC}$ and it is shown that pseudoscalar production can isolate chiral odd GPDs. There is a sensitive dependence in various cross sections and asymmetries on the tensor charge of the nucleon and other transversity parameters. In a second section, analyticity and completeness are shown to limit the partonic interpretation of the GPDs in the ERBL region., 8 pages, 2 figures, presented at Exclusives 2010 Workshop, May 18-20, Jefferson Lab

In this work we derive a sum rule for the angular momentum of a spin 1 hadronic system., Comment: 5 pages, to appear in Proceedings of XVIII International Workshop on Deep-Inelastic Scattering and Related Subjects, April 19 -23, 2010, Convitto della Calza, Firenze, Italy

Generalized Parton Distributions describe, within QCD factorization, the non perturbative component in the amplitudes for deeply virtual exclusive processes. However, in order for a partonic interpretation to hold, semi-disconnected diagrams should not contribute. We show that this condition is not satisfied for non-forward kinematics at leading order, and that gluon mediated re-interactions are essential for a consistent description in terms of parton degrees of freedom., Comment: 4 pages, 3 figures, replaced figure 2, revised conclusions, added references

We present a physically motivated parametrization of the chiral-even generalized parton distributions in the non-singlet sector obtained from a global analysis using a set of available experimental data. Our analysis is valid in the kinematical region of intermediate Bjorken $x$ and for $Q^2$ in the multi-GeV region which is accessible at present and currently planned facilities. Relevant data included in our fit are from the nucleon elastic form factors measurements, from deep inelastic scattering experiments. Additional information provided by lattice calculations of the higher moments of generalized parton distributions, is also considered. Recently extracted observables from Deeply Virtual Compton Scattering on the nucleon are reproduced by our fit., Comment: 25 pages, 21 figures

In most experiments aimed at determining the masses and mixing parameters for neutrino oscillations in different energy regimes there are strong backgrounds to neutrino interactions with matter that can confuse the expected small signals. One particular set of backgrounds is due to neutrino production of mesons, particularly pions, from nucleon or nuclear targets. It is especially important and theoretically interesting in itself to study $coherent$ neutrino production of pions, i.e. wherein the target does not break up. Preserving the target requires that the meson produced in the reaction will be charged for Charged Current (CC) interactions or neutral for Neutral Current (NC) interactions. Coherent scattering is the weak analog of exclusive neutral meson electroproduction for which considerable progress has been made in understanding the intermediate energy, small momentum transfer region by applying the Generalized Parton Distribution (GPD) phenomenology. The GPD perspective will be explained with emphasis on how the understanding gained from electroproduction of neutral pions can carry over to the neutrino processes., Comment: 5 pages, 3 figures, talks presented at Nufact 09, July 20-25, 2009, Illinois Institute of Technology, Chicago

Exclusive pi0 electroproduction from nucleons at large photon virtuality can be described in terms of Generalized Parton Distributions, particularly the chiral odd subset. Chiral odd GPDs are related to transversity and can be accessed experimentally for various special choices of observables. This is accomplished by choosing C-parity odd and chiral odd combinations of t-channel exchange quantum numbers. These GPDs are calculated in a spectator model, and are constrained by boundary functions. Alternatively, the production amplitudes correspond to C-odd Regge exchanges with final state interactions. The helicity structure of the virtual photoproduction amplitudes provides relations between the partonic description via GPDs and the hadronic, Regge description of C-odd processes, all via quark helicity flip. GPDs, in general, are analytic functions of energy variables. Their integrals over unobservable parton momenta thereby satisfy Dispersion Relations (DR). Using DR's could allow the real part of the amplitudes, the integrated GPDs, to be extracted from the more easily measured imaginary parts. However, at non-zero momentum transfer DRs require integration over unphysical regions of the variables. We show that the relevant unphysical region of the non-forward DRs is considerable. This will vitiate the efforts to avoid the actual measurement of the real parts more directly., 5 pages, 4 figures, Spin 2008 contribution, to be published in Proceedings

Exclusive {pi}{sup 0} electroproduction from nucleons is suggested for extracting the tensor charge and other quantities related to transversity from experimental data. This process isolates C-parity odd and chiral-odd combinations of t-channel exchange quantum numbers. In a hadronic picture it connects the meson production amplitudes to C-odd Regge exchanges with final state interactions. In a description based on partonic degrees of freedom, the helicity structure for this C-odd process relates to the quark helicity flip, or chiral-odd generalized parton distributions. This differs markedly from deeply virtual Compton scattering, and both vector meson and charged {pi} electroproduction, where the axial charge can enter the amplitudes. Contrarily, the tensor charge enters the {pi}{sup o} process. The connection through the helicity description of the process to both the partonic and hadronic perspectives is studied and exploited in model calculations to indicate how the tensor charge and other transversity parameters can be related to cross section and spin asymmetry measurements over a broad range of kinematics.

Using as basic observable an angular-integrated asymmetry to be measured in Drell-Yan lepton-pair production at the LHC, we discuss the identification reach on the spin-2 of the lowest-lying Randall-Sundrum resonance predicted by gravity in one warped extra dimension, against the spin-1 and spin-0 hypotheses. Numerical results indicate that, depending on the graviton coupling strength to the standard model particles, such a spin-2 identification can extend up to mass scales of 1.0-1.6 TeV and 2.4-3.2 TeV for LHC integrated luminosities of 10 and 100 fb^{-1}, respectively., 6 pages, 2 figures, talk presented at the 18th International Symposium on Spin Physics (SPIN 2008), Charlottesville, VA, October 6-11, 2008

I examine the uncertainty of perturbative QCD factorization for (un)polarized hadron structure functions in deep inelastic scattering at a large value of the Bjorken variable xB. The focus will be on Target Mass Corrections and Jet Mass Corrections in the collinear factorization framework., 4 pages, 1 figure. Talk given at "SPIN2008", University of Virginia, October 6-11 2008

We present first results from our analysis of the most general quark‐quark correlator of the nucleon, which can be parameterized in terms of so‐called generalized transverse momentum dependent parton distributions. These results include the first complete parameterization of the nucleon GPDs and TMDs as well as new results on possible nontrivial relations between them.

The Standard Model is only a few decades old and has been successfully confirmed by experiment, particularly at the high energy frontier. This will continue with renewed vigor at the LHC. However, many important elements of the Standard Model remain poorly understood. In particular, the exploration of the strong interaction theory Quantum Chromodynamics is in its infancy. How does the spin‐1/2 of the proton arise from the fundamental quark and gluon constituents? Can we understand the new QCD world of virtual quarks and gluons in the nucleon? Using precision measurements can we test the limits of the Standard Model and look for new physics? To address these and other important questions, physicists have developed a concept for a new type of accelerator, namely a high luminosity, polarized electron‐ion collider. Here the scientific motivation is summarized and the accelerator concepts are outlined.

We present studies of the transverse momentum dependence of valence quark transverse and longitudinal spin distributions through measurements of spin‐azimuthal asymmetries in Semi‐Inclusive Deep Inelastic Scattering (SIDIS) of kaons using the upgraded JLab 11 GeV polarized electron beam and the CLAS12 detector with longitudinally polarized proton and deuteron targets. Measurements with kaons are complementary to measurements with pions and will provide additional information on the Collins fragmentation mechanism. The PT dependence of the double spin asymmetry provides additional information on the flavor and polarization dependence of transverse‐momentum‐dependent helicity distributions of quarks, which provides complementary to the Single‐Spin Asymmetry (SSA) measurements access to spin‐orbit correlations. The large acceptance of CLAS12 would allow simultaneous detection of scattered electrons and leading hadrons, providing information on the struck quark’s flavor and transverse momentum.

We present a comprehensive formalism for dilepton production from the collision of two polarized spin−12 hadronsby identifying the general angular distribution of the cross section in combination with a complete set of structure functions. The various structure functions are computed in the parton model approximation where we mainly consider the case when the transverse momentum of the dilepton pair is much smaller than its invariant mass. In this kinematical region dilepton production can be described in terms of transverse momentum dependent parton distributions.

We review recent progress on lattice calculations of nucleon spin structure, including the parton distribution functions, form factors, generalization parton distributions, and recent developments in lattice techniques., Comment: Suggested talk at The 18th International Symposium on Spin Physics; to appear in the American Institute of Physics conference proceedings. 6 pages, 4 figures

Transverse-momentum-dependent parton distribution and fragmentation functions describe the partonic structure of the nucleon in a three-dimensional momentum space. They are subjects of flourishing theoretical and experimental activity. They provide novel and intriguing information on hadronic structure, including evidence of the presence of partonic orbital angular momentum., Proceedings of SPIN2008, 6-11 October 2008, Charlottesville, VA, USA

We discuss the double-spin asymmetries in transversely polarized Drell-Yan process, calculating all-order gluon resummation corrections up to the next-to-leading logarithmic accuracy. This resummation is relevant when the transverse-momentum $Q_T$ of the produced lepton pair is small, and reproduces the (fixed-order) next-to-leading QCD corrections upon integrating over $Q_T$. The resummation corrections in $p\bar{p}$-collision behave differently compared with $pp$-collision cases, and are small at the kinematics in the proposed GSI experiments. This fact allows us to predict large value of the double-spin asymmetries at GSI, using recent empirical information on the transversity., Comment: 4 pages, 2 figures; to appear in the proceedings of the 18th International Symposium on Spin Physics (SPIN20008), October 6 - 11, 2008, Charlottesville, Virginia, USA

The polarized gluon distribution function Δg(x) is presently studied at various deep inelastic scattering and pp experiments. The goal is to eventually determine the contribution of the gluon spin ΔG to the overall spin‐1/2 of the proton, given that the quark spins are known to only contribute at the level of 30%. A review of the present status and prospects will be given.

The Generalized Parton Distributions (GPDs) are the appropriate framework for a universal description of the partonic structure of the nucleon. They characterize the dynamics of quarks and gluons inside the nucleon and consequently contain information about the spin of the nucleon. The current experimental knowledge about GPDs is reviewed with the emphasis on the determination of E^q(Q^2,x,xi,t), the least known and constrained GPD, of particular importance in the nucleon spin puzzle. The perspectives of this experimental program are also addressed., Comment: 6 pages, 2 figures Proceedings of the XVIIIth Symposium on Spin Physics, Charlottesville (Virginia, USA), October 6-11,2008

We investigate the spectral properties of quark‐gluon‐quark and quark‐quark‐gluon correlators in order to study gluonic pole matrix elements. Such matrix elements appear both for distribution functions such as the Sivers function, and fragmentation functions such as the Collins function. We find that for a large class of spectator models, the contribution of the gluonic pole matrix element for fragmentation functions vanishes. This result is important in the study of universality for fragmentation functions.

We focus on some highlights of a recent, first global Quantum Chromodynamics (QCD) analysis of the helicity parton distributions of the nucleon, mainly the evidence for a rather small gluon polarization over a limited region of momentum fraction and for interesting flavor patterns in the polarized sea. It is examined how the various sets of data obtained in inclusive and semi‐inclusive deep inelastic scattering and polarized proton‐proton collisions help to constrain different aspects of the quark, antiquark, and gluon helicity distributions. Uncertainty estimates are performed using both the robust Lagrange multiplier technique and the standard Hessian approach.

The main properties of the leading-twist transverse momentum dependent parton distributions in a light-cone constituent quark model of the nucleon are reviewed, with focus on the role of the spin-spin and spin-orbit correlations of quarks. Results for azimuthal single spin asymmetries in semi-inclusive deep inelastic scattering are also discussed., Comment: Proceedings of SPIN2008, 6-11 October 2008, Charlottesville, VA, USA

We discuss the disagreement between the theoretical Heavy Baryon Chiral Perturbation Theory (HBϰPT) predictions of the nucleon generalized forward spin polarizabilities and the experimental data of spin structure functions from Jefferson Laboratory.

We discuss the status and prospects of an experimental program of parity‐violating asymmetry measurements in the scattering of longitudinally polarized electrons off unpolarized fixed targets. In particular, we focus on those measurements where judicious choices of target and kinematics allows the theoretical predictions to be made purely in terms of fundamental electroweak couplings. If such asymmetries are measured with sufficient precision, they are sensitive to new physics at the TeV scale. The physics implications of recent results, experiments under construction and plans for the future are discussed.

Measurements of double‐spin asymmetries for single‐inclusive heavy flavor production and for heavy quark correlations are expected to provide valuable information about the spin structure of the nucleon, in particular, the elusive gluon polarization. We discuss progress towards a versatile parton‐level Monte‐Carlo code for the production and decay of heavy quarks in longitudinally polarized proton‐proton collisions at next‐to‐leading order accuracy of Quantum Chromodynamics (QCD). First phenomenological studies are presented for BNL‐RHIC.