Session LH: Nucleon Structure II

The mechanical size of the proton.

Protons are fundamental building blocks of our universe. They are composed of elementary objects, quarks and gluons. It is well established that quarks do not exist in isolation but only in the confines of protons and other hadrons. The mechanical properties of the proton including the size of the confinement volume can in principle be probed in interactions that couple directly to the quark masses through gravitation. These properties are encoded in the proton's matrix element of the energy-momentum tensor and are expressed in the gravitational form factors (GFF). Use of direct gravitational interaction in such measurements is impractical. However, recent theoretical developments have shown that the GFF may also be probed indirectly in deeply virtual Compton scattering (DCVS). This new direction of nucleon structure research has already resulted in the first determination of the pressure distribution inside the proton. Here we present the first results on the mechanical size of the proton employing the DVCS process in extracting the form factor D(t). We will compare our results to the frequently discussed charge radius of the proton and to the sizes of other hadrons.   

Partonic orbital angular momentum and contributions to the trace anomaly

Quark gluon interactions are crucial to our understanding of partonic orbital angular momentum. Despite describing a quark quark correlation function, twist three Generalized Parton Distributions (GPDs) implicitly involve quark gluon interactions. They get contributions from leading twist GPDs, the quark mass in the axial vector case and a term involving the quark gluon quark correlation function also known as the genuine twist three term. We show how each of these contributions are derived by looking at the underlying $k_T$ structure. We also highlight the role of the gauge link both in the collinear limit and the generalized transverse momentum limit. Several open questions remain about the mass decomposition of the proton. As the constituent quarks are very light, the quantum effects that make the theory non conformal and hence give the proton most of its mass are encompassed in the trace anomaly. The quark and gluon contributions to the total energy momentum tensor are parameterized using the gravitational form factors. We show how to precisely write the trace anomaly in terms of these quantities.   

Charge Symmetry Violation in Quark Distributions using Semi-Inclusive Deep Inelastic Scattering

Charge symmetry is the invariance of the strong interactions during specific isospin rotation of 180 degrees which results to the exchange of up and down quarks, while simultaneously interchanging protons and neutrons. It has generally been assumed to be valid in most parton distribution fits. The violation of this symmetry arises due to the small mass difference between up and down quarks and the electromagnetic interactions. Although charge symmetry violation (CSV) is expected to be very small, the precision of the existing data can only constrain it to be \textless \textpm 10{\%}. Jefferson Lab Hall-C experiment E12-09-002 aims to place constraints on the degree of CSV in the valence quark distributions in the nucleon via semi-inclusive deep inelastic scattering. In this experiment, a 10.6 GeV electron beam was incident on a liquid deuterium target with the scattered electrons and charged pions detected in coincidence in the HMS and SHMS spectrometers respectively. This experiment will measure the ratios of charged pion cross-sections with high precision to extract and place limits on the charge symmetry violating parton distribution. The current status of the data analysis will be discussed in this talk.   

3D Partonic Structure of Nucleons and Light Nuclei

The generalized parton distributions (GPDs) framework opens a new avenue to explore the nature of the medium modifications at the partonic level, i.e., quarks and gluons. The first step in this direction has been performed by the CLAS collaboration during the 6 GeV era, where the bound proton deeply-virtual Compton scattering (DVCS) off $^4$He is compared to the free proton. The results have indeed shown significant modification of the proton beam-spin asymmetry in $^4$He. A new groundbreaking measurement of coherent DVCS of the $^4$He nucleus is a critical step towards providing similar 3D pictures of the partonic structure of nuclei, and provides a new approach to understanding the modifications of protons and neutrons within the dense environment of a nucleus. The $^4$He nucleus is particularly important, as its partonic structure is encoded within a single chirally-even GPD. These results have proven the experimental feasibility of measuring such nuclear exclusive reactions and led the way to the approval of a next generation nuclear physics program to be carried out using the upgraded CEBAF electron beam. In my talk I will be presenting the recent results and the future planned measurements using the upgraded setup of the CLAS12 spectrometer at Jefferson Lab.   

Results on light quark fragmentation from Belle

Results on light quark fragmentation from Belle The knowledge of parton fragmentation functions are important for our understanding of semi-inclusive hard scattering processes. Fragmentation functions can be extracted from e+e- annihilation cross-section measurements independently of the knowledge of parton distribution functions. Using record breaking luminosities delivered by KEKB, the Belle experiment at KEK in Japan recorded about 1 ab$^{-1}$ between 1999 and 2010, mostly at the $\Upsilon(4S)$ resonance. This contribution will show recent results on measurements related to light quark fragmentation using Belle data and discuss prospects at Belle II.   

Pion and Kaon Form Factors with Twisted Mass Fermions

In this talk we present results on the pion and kaon form factors and generalized form factors using numerical simulations within Lattice QCD. We employ the twisted mass fermion action for an Nf=2+1+1 ensemble with pion mass of 260 MeV. Main focus is given to the electromagnetic form factors, the quark momentum fraction, and the generalized form factors of the one-derivative unpolarized operator. Systematic errors due to excited states contamination are investigated and controlled using various analysis methods.   

Global Analysis of Quark-Gluon Correlations in Hadrons

The internal, 3D structure of hadrons are often encoded in transverse momentum dependent (TMD) parton distribution functions (PDFs) and fragmentation functions (FFs). There has been intense research from both theory and experiment on observables that give access to these objects, especially those that depend on the transverse spin of particles. For example, the Sivers and Collins effects in semi-inclusive deep-inelastic scattering (SIDIS) and the Collins effect in electron-positron annihilation have been widely investigated over many years. In addition, collinear twist-3 observables involving transverse spin, like $A_{N}$ in proton-proton collisions, are of interest due to their sensitivity to quark-gluon correlations in hadrons. In this talk, I will report on the first attempt to achieve a global analysis of TMD and collinear transverse-spin data using an iterative Monte Carlo procedure. I will discuss the results of our analysis and also explore avenues for future research.