No abstract

We use the world's data on elastic electron-proton scattering and calculations of two-photon exchange effects to extract corrected values of the proton's electric and magnetic form factors over the full Q 2 range of the existing data. Our analysis combines the corrected Rosenbluth cross section and polarization transfer data, and is the first extraction of GE and GM including explicit two-photon exchange corrections and their associated uncertainties. In addition, we examine the angular dependence of the corrected cross sections to look for possible nonlinearities of the cross section as a function of ε.

A detailed study of two-photon exchange in unpolarized and polarized elastic electron-nucleon scattering is presented, taking particular account of nucleon finite size effects. Contributions from nucleon elastic intermediate states are found to have a strong angular dependence, which leads to a partial resolution of the discrepancy between the Rosenbluth and polarization transfer measurements of the proton electric to magnetic form factor ratio, G E /G M . The two-photon exchange contribution to the longitudinal polarization transfer P L is small, whereas the contribution to the transverse polarization transfer P T is enhanced at backward angles by several percent, increasing with Q 2 . This gives rise to a small, < ∼ 3% suppression of G E /G M obtained from the polarization transfer ratio P T /P L at large Q 2 . We also compare the two-photon exchange effects with data on the ratio of e + p to e − p cross sections, which is predicted to be enhanced at backward angles.Finally, we evaluate the corrections to the form factors of the neutron, and estimate the elastic intermediate state contribution to the 3 He form factors.

We present a new set of leading twist parton distribution functions, referred to as "CJ15", which take advantage of developments in the theoretical treatment of nuclear corrections as well as new data. The analysis includes for the first time data on the free neutron structure function from Jefferson Lab, and new high-precision charged lepton and W -boson asymmetry data from Fermilab. These significantly reduce the uncertainty on the d/u ratio at large values of x and provide new insights into the partonic structure of bound nucleons.

We perform the first global QCD analysis of parton distribution functions (PDFs) in the pion, combining πA Drell-Yan data with leading neutron electroproduction from HERA within a Monte Carlo approach based on nested sampling. Inclusion of the HERA data allows the pion PDFs to be determined down to much lower values of x, with relatively weak model dependence from uncertainties in the chiral splitting function. The combined analysis reveals that gluons carry a significantly higher pion momentum fraction, ∼ 30%, than that inferred from Drell-Yan data alone, with sea quarks carrying a somewhat smaller fraction, ∼ 15%, at the input scale. Within the same effective theory framework, the chiral splitting function and pion PDFs can be used to describe thē d −ū asymmetry in the proton.

We present three new sets of next-to-leading order parton distribution functions (PDFs) determined by global fits to a wide variety of data for hard scattering processes. The analysis includes target mass and higher twist corrections needed for the description of deep inelastic scattering data at large x and low Q 2 , and nuclear corrections for deuterium targets. The PDF sets correspond to three different models for the nuclear effects, and provide a more realistic uncertainty range for the d quark PDF, in particular, compared with previous fits. We describe the PDF error sets for each choice of the nuclear corrections, and provide a user interface for utilizing the distributions.

An extensive theoretical analysis of off-forward parton distributions ͑OFPDs͒ is presented. The OFPDs and the form factors of the quark energy-momentum tensor are estimated at a low energy scale using a bag model. Relations among the second moments of OFPDs, the form factors, and the fraction of the nucleon spin carried by quarks are discussed.

We perform the first global QCD analysis of polarized inclusive and semi-inclusive deep-inelastic scattering and single-inclusive e + e − annihilation data, fitting simultaneously the parton distribution and fragmentation functions using the iterative Monte Carlo method. Without imposing SU(3) symmetry relations, we find the strange polarization to be very small, consistent with zero for both inclusive and semi-inclusive data, which provides a resolution to the strange quark polarization puzzle. The combined analysis also allows the direct extraction from data of the isovector and octet axial charges, and is consistent with a small SU(2) flavor asymmetry in the polarized sea.