Search

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 Drell-Yan process provides important information on the internal structure of hadrons including transverse momentum dependent parton distribution functions (TMDs). In this work we present calculations for all leading twist structure functions describing the pion induced Drell-Yan process. The non-perturbative input for the TMDs is taken from the light-front constituent quark model, the spectator model, and available parametrizations of TMDs extracted from the experimental data. TMD evolution is implemented at Next-to-Leading Logarithmic precision for the first time for all asymmetries. Our results are compatible with the first experimental information, help to interpret the data from ongoing experiments, and will allow one to quantitatively assess the models in future when more precise data will become available., Comment: The paper is restructured with new discussions and results, TMD evolution is implemented at Next-to-Leading Logarithmic accuracy for all asymmetries. 27 pages and 10 figures

Selected topics related to the physics of the energy-momentum tensor (EMT) form factors are discussed. The topics are: 1) Fundamental mechanical properties of particles and gravity 2) Mechanical properties of non-spherical particles 3) Gravitational form factors of Goldstone bosons 4) Nucleon's seismology?, Comment: Talk given at 23rd International Spin Physics Symposium - SPIN2018 - 10-14 September, 2018 Ferrara, Italy

The chromoelectric polarizability of $J/\psi$ is extracted from lattice QCD data on the nucleon-$J/\psi$ potential in the heavy quark limit. The value of $\alpha(1S) = (1.6 \pm 0.8) \,{\rm GeV}^{-3} $ is obtained. We also comment on the possibility of hadrocharmonia., Comment: 7 pages, 1 figure, expanded discussion, typos fixed, published version

The physics related to the form factors of the energy momentum tensor spans a wide spectrum of problems, and includes gravitational physics, hard exclusive reactions, hadronic decays of heavy quarkonia, and the physics of exotic hadrons described as hadroquarkonia. It also provides access to the "last global unknown property:" the D-term. We review the physics associated with the form factors of the energy-momentum tensor and the D-term, their interpretations in terms of mechanical properties, their applications, and the current experimental status., Comment: typos fixed, published version

The probably most fundamental information about a particle is contained in the matrix elements of its energy momentum tensor (EMT) which are accessible from hard-exclusive reactions via generalized parton distribution functions. The spin decomposition of the nucleon and Ji sum rule are one example. Less prominent but equally important information is encoded in the stress tensor, related to the spatial components of the EMT, which shows in detail how the strong forces inside the nucleon balance to form a bound state. This provides not only unique insights on nucleon structure. It also leads to fascinating new applications to hadron spectroscopy which allow us to formulate new interpretations of the charmonium-nucleon pentaquarks discovered by LHCb. Recent progress is reviewed in this short overview article., 10 pages, 6 figures, based on talk presented at "QCD Evolution 2016", 30 May to 3 June 2016, Amsterdam

In the present talk, we report a recent investigation on the nucleon form factors of the energy-momentum tensor in nuclear matter, based on the in-medium modified chiral soliton model. The results in free space are in agreement with those from other approaches. We have discussed the changes of the energy-momentum tensor form factors in nuclear matter and the modification of the soliton structure due to the surrounding nuclear environment.

We study the spin-flavor structure of the nucleon's chiral-odd generalized parton distributions (transversity GPDs) in the large-${N}_{c}$ limit of QCD. In contrast to the chiral-even case, only three combinations of the four chiral-odd GPDs are nonzero in the leading order of the $1/{N}_{c}$ expansion: ${\overline{E}}_{T}={E}_{T}+2{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{H}}_{T},{H}_{T}$, and ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}_{T}$. The degeneracy is explained by the absence of spin-orbit interactions correlating the transverse momentum transfer with the transverse quark spin. It can also be deduced from the natural ${N}_{c}$ scaling of the quark-nucleon helicity amplitudes associated with the GPDs. In the GPD ${\overline{E}}_{T}$ the flavor-singlet component $u+d$ is leading in the $1/{N}_{c}$ expansion, while in ${H}_{T}$ and ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}_{T}$ it is the flavor-nonsinglet components $u\ensuremath{-}d$. The large-${N}_{c}$ relations are consistent with the spin-flavor structure extracted from hard exclusive ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ electroproduction data, if it is assumed that the processes are mediated by twist-3 amplitudes involving the chiral-odd GPDs and the chiral-odd pseudoscalar meson distribution amplitudes.

The pentaquark ${P}_{c}^{+}(4450)$ recently discovered by the LHCb has been interpreted as a bound state of $\mathrm{\ensuremath{\Psi}}(2S)$ and a nucleon. The charmonium-nucleon interaction which provides the binding mechanism is given, in the heavy-quark limit, in terms of charmonium chromoelectric polarizabilities and densities of the nucleon energy-momentum tensor. In this work, we show in a model-independent way, by exploring general properties of the effective interaction, that $\mathrm{\ensuremath{\Psi}}(2S)$ can form bound states with a nucleon and $\mathrm{\ensuremath{\Delta}}$. Using the Skyrme model to evaluate the effective interaction in the large-${N}_{c}$ limit and estimate $1/{N}_{c}$ corrections, we confirm the results from prior work which were based on a different effective model (chiral quark soliton model). This shows that the interpretation of ${P}_{c}^{+}(4450)$ is remarkably robust and weakly dependent on the details of the effective theories for the nucleon energy-momentum tensor. We explore the formalism further and present robust predictions of isospin-$\frac{3}{2}$ bound states of $\mathrm{\ensuremath{\Psi}}(2S)$ and $\mathrm{\ensuremath{\Delta}}$ with masses around 4.5 GeV and widths around 70 MeV. The approach also predicts broader resonances in the $\mathrm{\ensuremath{\Psi}}(2S)\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}$ channel at 4.9 GeV with widths of the order of 150 MeV. We discuss in which reactions these new isospin-$\frac{3}{2}$ pentaquarks with hidden charm can be observed.

The understanding of the pion structure as described in terms of transverse-momentum dependent parton distribution functions (TMDs) is of importance for the interpretation of currently ongoing Drell-Yan experiments with pion beams. In this work we discuss the description of pion TMDs beyond leading twist in a pion model formulated in the light-front constituent framework. For comparison, we also review and derive new results for pion TMDs in the bag and spectator models., Comment: 17 pages, 7 figures; v2: modified presentation, updated references; matches the journal version

This White Paper presents the science case of an Electron-Ion Collider (EIC), focused on the structure and interactions of gluon-dominated matter, with the intent to articulate it to the broader nuclear science community. It was commissioned by the managements of Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility (JLab) with the objective of presenting a summary of scientific opportunities and goals of the EIC as a follow-up to the 2007 NSAC Long Range plan. This document is a culmination of a community-wide effort in nuclear science following a series of workshops on EIC physics and, in particular, the focused ten-week program on "Gluons and quark sea at high energies" at the Institute for Nuclear Theory in Fall 2010. It contains a brief description of a few golden physics measurements along with accelerator and detector concepts required to achieve them, and it benefited from inputs from the users' communities of BNL and JLab. This White Paper offers the promise to propel the QCD science program in the U.S., established with the CEBAF accelerator at JLab and the RHIC collider at BNL, to the next QCD frontier., Comment: The Second Edition, 164 pages

Longitudinal, parity-violating single spin asymmetries in proton-proton collisions for RHIC kinematics are considered. The focus of this study is on the production of a single jet as well as a hadron containing a charm quark. While the asymmetry for jets is rather small, considerably larger effects can be expected for the production of charm because of the strongly reduced QCD background. The general outcome of a fixed order calculation is not much affected by the resummation of large threshold logarithms.

A review is given on single spin asymmetries in deep inelastic semi-inclusive scattering (SIDIS) and their possible theoretical understanding in the framework of QCD-induced factorization approach, wherefore predictions for transversity from the chiral quark soliton model are used. Arising difficulties to interpret most recent SIDIS data, and the possibility to access the transversity distribution in the Drell-Yan process are discussed., 20 pages, 23 eps-figures, cjpsuppl style files, presented at ASI "Spin and Symmetry", Prague, July 5-10, 2004, to be published in Czechoslovak Journal of Physics, v.55 Suppl

Predictions are made for single spin azimuthal asymmetries due to the Collins effect in pion production from semi-inclusive deeply inelastic scattering off transversely and longitudinally polarized targets for the HERMES and COMPASS experiments. The x-dependence of the asymmetries is evaluated using the parton distribution functions from the chiral quark-soliton model. The overall normalization of the predicted asymmetries is determined by the information on the Collins fragmentation function extracted from previous HERMES data on azimuthal asymmetries from a longitudinally polarized target. The single spin asymmetries from the transversely polarized proton target are found to be about 20% for positive and neutral pions both at HERMES and COMPASS. For a longitudinally polarized target we obtain for COMPASS a sin(phi) asymmetry of 1% and a sin(2phi) asymmetry of about 3%., 10 pages, 9 figures, Ref.[29] added

At leading twist the transverse momentum dependent parton distributions of the pion consist of two functions, the unpolarized ${f}_{1,\ensuremath{\pi}}(x,{\mathbit{k}}_{\ensuremath{\perp}}^{2})$ and the Boer-Mulders function ${h}_{1,\ensuremath{\pi}}^{\ensuremath{\perp}}(x,{\mathbit{k}}_{\ensuremath{\perp}}^{2})$. We study both functions within a light-front constituent model of the pion, comparing the results with different pion models and the corresponding nucleon distributions from a light-front constituent model. After evolution from the model scale to the relevant experimental scales, the results for the collinear pion valence parton distribution function ${f}_{1,\ensuremath{\pi}}(x)$ are in very good agreement with available parametrizations. Using the light-front constituent model results for the Boer-Mulders functions of the pion and nucleon, we calculate the coefficient $\ensuremath{\nu}$ in the angular distribution of Drell-Yan dileptons produced in pion-nucleus scattering, which is responsible for the violation of the Lam-Tung relation. We find a good agreement with the data, and carefully discuss the range of applicability of our approach.

We investigate the energy-momentum tensor form factors of the nucleon in a nuclear medium, based on an in-medium modified $\ensuremath{\pi}$-$\ensuremath{\rho}$-$\ensuremath{\omega}$ soliton model, with medium modifications of the mesons considered. The results allow us to establish general features of medium modifications of the structure of nucleons bound in a nuclear medium.

Dynamical breaking of chiral symmetry in QCD is caused by non-perturbative interactions on a scale ρ ∼ 0.3 fm much smaller than the hadronic size R ∼ 1 fm. This has important consequences for the nucleon structure such as the prediction that the transverse momentum distribution of sea quarks is significantly broader than the pT -distribution of valence quarks due to short-range correlations between sea quarks in the nucleon’s light-cone wave function.

Recently HERMES has observed an azimuthal asymmetry in electro-production of neutral pions in semi-inclusive deep-inelastic scattering of unpolarized positrons off longitudinally polarized protons. This asymmetry (like those observed in the production of charged pions) is well reproduced theoretically by using the non-perturbative calculation of the proton transversity distribution in the effective chiral quark-soliton model combined with experimental DELPHI-data on the new T-odd Collins fragmentation function. There are no free, adjustable parameters in the analysis. Using the $z$-dependence of the HERMES azimuthal asymmetry and the calculated transversity distributions the z-dependence of the Collins fragmentation function is obtained. The value obtained from HERMES data is consistent with the DELPHI result, even though these results refer to different scales.

The flavor asymmetries of the polarized antiquark distributions in the nucleon, Δ u (x) - Δ d (x) and Δ u (x) + Δ d (x) - 2Δ s , cannot be measured in inclusive DIS alone. Large flavor asymmetries have been predicted by model calculations of the polarized parton distributions at a low normalization point in the large-Nc limit (chiral quark-soliton model of the nucleon). We discuss the prospects for measuring these flavor asymmetries in two types of semi-inclusive experiments: i) Hadron production in semi-inclusive polarized eN scattering (SMC, HERMES); ii) Drell-Yan lepton pair production in polarized NN scattering at HERA or RHIC energies. In particular, the Drell-Yan spin asymmetries are shown to be remarkably sensitive to Δ u (x) - Δ d (x) .

Recent theoretical studies have provided new insight into the intrinsic transverse momentum distributions of valence and sea quarks in the nucleon at a low scale. The valence quark transverse momentum distributions (q - qbar) are governed by the nucleon's inverse hadronic size R^{-1} ~ 0.2 GeV and drop steeply at large p_T. The sea quark distributions (qbar) are in large part generated by non-perturbative chiral-symmetry-breaking interactions and extend up to the scale rho^{-1} ~ 0.6 GeV. These findings have many implications for modeling the initial conditions of perturbative QCD evolution of TMD distributions (starting scale, shape of p_T distributions, coordinate-space correlation functions). The qualitative difference between valence and sea quark intrinsic p_T distributions could be observed experimentally, by comparing the transverse momentum distributions of selected hadrons in semi-inclusive deep-inelastic scattering, or those of dileptons produced in pp and pbar-p scattering., Comment: 9 pages, 5 figures. Proceedings of QCD Evolution Workshop, Jefferson Lab, May 6-10, 2013

The dynamical breaking of chiral symmetry in QCD is caused by nonperturbative interactions on a scale rho ~ 0.3 fm, much smaller than the hadronic size R ~ 1 fm. These short-distance interactions influence the intrinsic transverse momentum distributions of partons and their correlations at a low normalization point. We study this phenomenon in an effective description of low-energy dynamics based on chiral constituent quark degrees of freedom, which refers to the large-N_c limit of QCD. The nucleon is obtained as a system of constituent quarks and antiquarks moving in a self-consistent classical chiral field (chiral quark-soliton model). The calculated distributions of constituent quarks/antiquarks are matched with QCD partons at the scale rho^{-2}. The p_T distribution of valence quarks is localized at p_T^2 ~ R^{-2} and roughly of Gaussian shape. The sea quark distribution exhibits a would-be power-like tail ~1/p_T^2 extending up to the chiral symmetry-breaking scale. Such behavior is seen in the flavor-singlet unpolarized and nonsinglet polarized sea. The high-momentum tails are the result of short-range correlations between sea quarks in the nucleon's light-cone wave function, analogous to NN correlations in nuclei. The nucleon wave function contains correlated pairs of transverse size rho << R with sigma- and pi-like quantum numbers, whose internal wave functions become identical at p_T^2 ~ rho^{-2} (restoration of chiral symmetry). These features are model-independent and represent an effect of dynamical chiral symmetry breaking on the nucleon's partonic structure. Our results have numerous implications for the P_T distributions of particles produced in hard scattering processes. The nonperturbative parton correlations predicted here could be observed in particle correlations between the current and target fragmentation regions of DIS., 46 pages, 20 figures

We derive relations between transverse momentum dependent distribution functions (TMDs) and the usual parton distribution functions (PDFs) in the 3D covariant parton model, which follow from Lorentz invariance and the assumption of a rotationally symmetric distribution of parton momenta in the nucleon rest frame. Using the known PDFs f_1(x) and g_1(x) as input we predict the x- and pT-dependence of all twist-2 T-even TMDs.

We present a summary of a recent workshop held at Duke University on Partonic Transverse Momentum in Hadrons: Quark Spin-Orbit Correlations and Quark-Gluon Interactions. The transverse momentum dependent parton distribution functions (TMDs), parton-to-hadron fragmentation functions, and multi-parton correlation functions, were discussed extensively at the Duke workshop. In this paper, we summarize first the theoretical issues concerning the study of partonic structure of hadrons at a future electron-ion collider (EIC) with emphasis on the TMDs. We then present simulation results on experimental studies of TMDs through measurements of single spin asymmetries (SSA) from semi-inclusive deep-inelastic scattering (SIDIS) processes with an EIC, and discuss the requirement of the detector for SIDIS measurements. The dynamics of parton correlations in the nucleon is further explored via a study of SSA in D (`D) production at large transverse momenta with the aim of accessing the unexplored tri-gluon correlation functions. The workshop participants identified the SSA measurements in SIDIS as a golden program to study TMDs in both the sea and valence quark regions and to study the role of gluons, with the Sivers asymmetry measurements as examples. Such measurements will lead to major advancement in our understanding of TMDs in the valence quark region, and more importantly also allow for the investigation of TMDs in the sea quark region along with a study of their evolution., Comment: 44 pages 23 figures, summary of Duke EIC workshop on TMDs accepted by EPJA

We present results for leading-twist azimuthal asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to naively time-reversal odd transverse-momentum dependent parton distribution functions from the light-cone constituent quark model. We carefully discuss the range of applicability of the model, especially with regard to positivity constraints and evolution effects. We find good agreement with available experimental data from COMPASS and HERMES, and present predictions to be tested in forthcoming experiments at Jefferson Lab., Comment: 10 pages, 7 figures, discussion of evolution effects extended, to appear in Phys.Rev.D

Intrinsic transverse parton momenta pT play an important role in the understanding of azimuthal/spin asymmetries in semi-inclusive deep-inelastic scattering (SIDIS) and the Drell-Yan process (DY). We review and update what is presently known about pT from these processes. In particular, we address the question to which extent data support the popular Gauss model for the pT-distributions. We find that the Gauss model works very well, and observe that the intrinsic transverse momenta in SIDIS and DY are compatible, which is a support for the factorization approach. As a byproduct we recover a simple but practical way of taking into account the energy dependence of pT-distributions., 19 pages, 11 figures

The 'pretzelosity' distribution is discussed. Theoretical properties, model results, and perspectives to access experimental information on this leading twist, transverse momentum dependent parton distribution function are reviewed. Its relation to helicity and transversity distributions is highlighted.

Transverse parton momentum dependent distribution functions (TMDs) of the nucleon are studied in a covariant model, which describes the intrinsic motion of partons in terms of a covariant momentum distribution. The consistency of the approach is demonstrated, and model relations among TMDs are studied. As a byproduct it is shown how the approach allows to formulate the non-relativistic limit., 16 pages

We present results for all leading-twist azimuthal spin asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to T-even transverse-momentum dependent parton distribution functions on the basis of a light-cone constituent quark model. Attention is paid to discuss the range of applicability of the model, especially with regard to the scale dependence of the observables and the transverse-momentum dependence of the distributions. We find good agreement with available experimental data and present predictions to be further tested by future CLAS, COMPASS and HERMES data., Comment: 23 pages, 14 figures, 1 table

Transverse momentum dependent parton distribution functions are a key ingredient in the description of spin and azimuthal asymmetries in deep-inelastic scattering processes. Recent results from non-perturbative calculations in effective approaches are reviewed, with focus on relations among different parton distribution functions in QCD and models., Comment: Based on talks at "Transverse Partonic Structure of Hadrons", Yerevan, Armenia, 21-26 June 2009; "Recent Advances in Perturbative QCD and Hadronic Physics", ETC, Trento, Italy, 21-25 July 2009; "3D parton structure of the nucleon"," INT Seattle, WA, U.S.A. 14-18 September 2009. Dedicated to 75th birthday of A.V.Efremov by co-authors

We review the present understanding of the nucleon transversity distribution and Collins fragmentation function, based on Ref.\cite{Efremov:2006qm}, and discuss how Drell-Yan experiments will improve it., Comment: Talk at Moscow ICHEP 26.7-2.8.2006 and Kyoto Symposium on Spin Physics 2-7.10.2006, 5 pages, 5 figures, AIP style files included. Some pictures chenged, one reference added, typos and some errors in formulas are corrected

The nucleon form factors of the energy-momentum tensor are studied in the large-Nc limit in the framework of the chiral quark-soliton model for model parameters that simulate physical situations in which pions are heavy. This allows for a direct comparison to lattice QCD results., Comment: 17 pages, 12 figures

Theoretical advances in studies of the nucleon structure have been spurred by recent measurements of spin and/or azimuthal asymmetries worldwide. One of the main challenges still remaining is the extraction of the parton distribution functions, generalized to describe transverse momentum and spatial distributions of partons from these observables with no or minimal model dependence. In this review we present the latest developments in the field with emphasis on requirements for Monte Carlo event generators, indispensable for studies of the complex 3D nucleon structure, and discuss examples of possible applications.

On the basis of a fit to the Sivers effect in deep-inelastic scattering, we make predictions for single-spin asymmetries in the Drell-Yan process at BNL RHIC.

The Collins fragmentation function is extracted from HERMES data on azimuthal single spin asymmetries in semi-inclusive deeply inelastic scattering, and BELLE data on azimuthal asymmetries in electron positron annihilations. A Gaussian model is assumed for the distribution of transverse parton momenta and predictions are used from the chiral quark-soliton model for the transversity distribution function. We find that the HERMES and BELLE data yield a consistent picture of the Collins fragmentation function which is compatible with COMPASS data and the information previously obtained from an analysis of DELPHI data. Estimates for future experiments are made., 13 pages, 11 figures. v2: Clarifications added, matches published version

Estimates are given for the double spin asymmetry in lepton-pair production from collisions of transversely polarized protons and antiprotons for the kinematics of the recently proposed PAX experiment at GSI on the basis of predictions for the transversity distribution from the chiral quark soliton model., 4 pages, 2 eps-figures, style files. To be published in Proceedings of 16th Intern. spin physics symposium, October 10-16, Trieste, Italy

Estimates are given for the double spin asymmetry in lepton pair production from collisions of transversely polarized protons and antiprotons for the kinematics of the recently proposed PAX experiment at GSI on the basis of predictions for the transversity distribution from the chiral quark soliton model., Comment: 5 pages, 5 figures, references added, to appear in EPJC

The first information on the chirally odd twist-3 proton distribution function ${e}^{a}(x)$ is extracted from the azimuthal asymmetry ${A}_{\mathrm{LU}}$ in the electroproduction of pions from deeply inelastic scattering of longitudinally polarized electrons off unpolarized protons, which has been recently measured by the CLAS Collaboration. Furthermore parameter-free predictions are made for the azimuthal asymmetries ${A}_{\mathrm{UL}}$ from scattering of an unpolarized beam on a polarized proton target for CLAS kinematics.

The pion nucleon sigma-term is extracted on the basis of the soliton picture of the nucleon from the mass spectrum of usual and the recently observed exotic baryons, assuming that they have positive parity. The value found is consistent with that inferred by means of conventional methods from pion nucleon scattering data. The study can also be considered as a phenomenological consistency check of the soliton picture of baryons., Comment: 8 pages, 2 figures, references added, discussion extended, to appear in Eur.Phys.J.A

Spin azimuthal asymmetries in pion electro‐production in deep inelastic scattering off longitudinally polarized protons, measured by HERMES, are well reproduced theoretically with no adjustable parameters. Predictions for azimuthal asymmetries for a longitudinally polarized deuteron target are given. Using this the z‐dependence of the Collins fragmentation function is extracted for the first time. The first information on e(x) is extracted from CLAS ALU asymmetry.

Spin azimuthal asymmetries in pion electro-production in deep inelastic scattering off longitudinally polarized protons, measured by HERMES, are well reproduced theoretically with no adjustable parameters. Predictions for azimuthal asymmetries for a longitudinally polarized deuteron target are given. The z-dependence of the Collins fragmentation function is extracted. The first information on e(x) is extracted from CLAS A_LU asymmetry., Comment: 5 pages, 7 figures. Talk presented by A.V. Efremov at DIS2002, Cracow, Poland, 30 April to 4 May 2002. To appear in Acta Physica Polonica B

We compute the quark and antiquark transversity distributions in the nucleon at a low normalization point of 600 MeV in the large-$N_c$ limit, where the nucleon can be described as a soliton of an effective chiral theory (chiral quark-soliton model). The flavor-nonsinglet distributions, $\delta u(x) - \delta d(x)$ and $\delta\bar u(x) - \delta\bar d(x)$, appear in leading order of the $1/N_c$-expansion, while the flavor-singlet distributions, $\delta u(x) + \delta d(x)$ and $\delta\bar u(x) + \delta\bar d(x)$, are non-zero only in next-to-leading order. The transversity quark and antiquark distributions are found to be significantly different from the longitudinally polarized distributions $\Delta u (x) \pm \Delta d (x)$ and $\Delta\bar u (x) \pm \Delta\bar d (x)$, respectively, in contrast to the prediction of the naive non-relativistic quark model. We show that this affects the predictions for the spin asymmetries in Drell-Yan pair production in transversely polarized pp and ppbar collisions.

Predictions are made for azimuthal asymmetries in pion and kaon production from SIDIS off a longitudinally polarized deuterium target for HERMES kinematics, based on information on the 'Collins fragmentation function' from DELPHI data and on predictions for the transversity distribution function from non-perturbative calculations in the chiral quark-soliton model. There are no free parameters in the approach, which has been already successfully applied to explain the azimuthal asymmetries from SIDIS off polarized proton targets observed by HERMES and SMC., Comment: 10 pages, 4 figures. Sign mistake in expressions for azimuthal asymmetries corrected

Measurements of the flavor asymmetry of the polarized antiquark distributions in the nucleon, Δū(x)−Δd(x), would provide a unique opportunity to discriminate between different models of the role of the chiral structure of the nucleon in deep-inelastic scattering. In the pion cloud model of the nucleon’s sea quark distributions the polarized flavor asymmetry is zero, while the large-Nc picture of the nucleon as a chiral soliton predicts large polarized flavor asymmetries. We discuss the prospects for measuring the polarized antiquark flavor asymmetries in two types of semi-inclusive scattering experiments: i) Hadron production in semi-inclusive polarized eN scattering (SMC, HERMES); ii) Drell-Yan lepton pair production in polarized NN scattering at HERA or RHIC energies. In particular, the Drell-Yan spin asymmetries are shown to be remarkably sensitive to Δū(x)−Δd(x).

We investigate the role of the flavor asymmetry of the nucleon's polarized antiquark distributions in Drell-Yan lepton pair production in polarized nucleon-nucleon collisions at HERA (fixed-target) and RHIC energies. It is shown that the large polarized antiquark flavor asymmetry predicted by model calculations in the large-N_c limit (chiral quark-soliton model) has a dramatic effect on the double spin asymmetries in high mass lepton pair production, as well as on the single spin asymmetries in lepton pair production through $W^\pm$-bosons at $M^2 = M_W^2$., Comment: 10 pages, LaTeX2e, 9 eps figures included

We summarize recent progress in understanding the effects of nonperturbative short-range interactions in QCD on the nucleon's partonic structure at a low scale: (a) Sea quarks have intrinsic transverse momenta up to the chiral symmetry-breaking scale rho^{-1} ~ 0.6 GeV, much larger than those of valence quarks. (b) Sea quarks in the nucleon light-cone wave function exist partly in correlated in pairs of transverse size rho with sigma and pi-like quantum numbers and a distinctive spin structure (L = 1 components). The effects are demonstrated in an effective model of the low-energy dynamics resulting from chiral symmetry breaking in QCD. They have numerous implications for the P_T distribution of hadrons in semi-inclusive DIS and multiparton processes in high-energy pp collisions.

We derive relations between polarized transverse momentum dependent distribution functions (TMDs) and the usual parton distribution functions (PDFs) in the 3D covariant parton model, which follow from Lorentz invariance and the assumption of a rotationally symmetric distribution of parton momenta in the nucleon rest frame. Using the known PDF $g_{1}^{q}(x)$ as input we predict the $x$- and $\mathbf{p}_{T}$-dependence of all polarized twist-2 naively time-reversal even (T-even) TMDs., Comment: 6 pages, 2 figures, Contribution to the Proceedings of the 19th International Spin Physics Symposium (SPIN2010), J\"{u}lich, Germany, September 27 - October 2, 2010

The quark spin densities related to generalized parton distributions in impact-parameter space and to transverse-momentum dependent parton distributions are reviewed within a light-cone quark model, with focus on the role of the different spin-spin and spin-orbit correlations of quarks.Results for azimuthal spin asymmetries in semi-inclusive deep-inelastic scattering due to T-even transverse-momentum dependent parton distributions are also discussed., proceedings of the workshop "Recent Advances in Perturbative QCD and Hadronic Physics", 20-25 July 2009, ECT*, Trento (Italy), in Honor of Prof. Anatoly Efremov's 75th Birthday Celebration; also based on the talk given at the workshop "Transverse Partonic Structure of Hadrons", 21-26 June 2009, Yerevan (Armenia); to appear in Mod. Phys. Lett. A

The Sivers function is extracted from HERMES data on single spin asymmetries in semi-inclusive deeply inelastic scattering. The result is used for making predictions for the Sivers effect in the Drell-Yan process., presented at "The International Workshop on Transverse Polarisation Phenomena in Hard Processes" (Transversity 2005), Villa Olmo (Como), 7-10th. September 2005

A complete list of the so-called Lorentz invariance relations between parton distribution functions is given and some of their consequences are discussed, such as the Burkhardt-Cottingham sum rule. The violation of these relations is considered in a model independent way. It is shown that several Lorentz invariance relations are not violated in a generalized Wandzura-Wilczek approximation, indicating that numerically their violation may be small., Comment: 10 pages; Proceedings of the workshop "Recent Advances in Perturbative QCD and Hadronic Physics", July 20-24, 2009, at ECT*, Trento (Italy), in honor of Anatoli V. Efremov on the occasion of his 75th birthday; to appear in Mod. Phys. Lett. A

Higher-twist transverse momentum dependent parton distribution functions (TMDs) are a valuable probe of the quark-gluon dynamics in the nucleon, and play a vital role for the explanation of sizable azimuthal asymmetries in hadron production from unpolarized and polarized deep-inelastic lepton-nucleon scattering observed in experiments at CERN, DESY and Jefferson Lab. The associated observables are challenging to interpret, and still await a complete theoretical explanation, which makes guidance from models valuable. In this work we establish the formalism to describe unpolarized higher-twist TMDs in the light-front framework based on a Fock-space expansion of the nucleon state in terms of free on-shell parton states. We derive general expressions and present numerical results in a practical realization of this picture provided by the light-front constituent quark model. We review several other popular quark model approaches including free quark ensemble, bag, spectator and chiral quark-soliton model. We discuss how higher-twist TMDs are described in these models, and obtain results for several TMDs not discussed previously in literature. This study contributes to the understanding of non-perturbative properties of subleading twist TMDs. The results from the light-front constituent quark model are also compared to available phenomenological information, showing a satisfactory agreement.