Session CH: Form Factors and Spin Structure

Status of the OLYMPUS experiment

The OLYMPUS experiment at DESY has been carried out to quantify the effect of two-photon exchange in elastic lepton-proton scattering, which has been the favored explanation for the discrepancy in the form factor ratio between the Rosenbluth separation and polarization transfer methods. While the effect can not be calculated from first principles, it can be determined experimentally by comparing the positron-proton and electron-proton elastic cross sections. The OLYMPUS experiment has used intense stored positron and electron beams along with an internal unpolarized hydrogen target and a large acceptance detector to measure the ratio of elastic scattering cross sections. Particular emphasis has been put on optimal control of systematics, by redundantly monitoring luminosity, beam properties and detector efficiencies. Data taking has been completed in January 2013. An overview of the experiment will be given along with the status of the analysis.   

Status of the OLYMPUS Analysis

The OLYMPUS experiment, which completed data-taking at the DORIS $e^+/e^-$ storage ring at DESY in Hamburg, Germany in January 2013, seeks to determine definitively the two-photon contribution to lepton-proton scattering. This effect is accessible via a precision measurement of the ratio of the $e^+p$ and $e^-p$ cross-sections. To achieve the desired uncertainty in the ratio measurement ($\sim1\%$), the analysis must carefully consider any possible sources of false asymmetry between beam species such as geometric effects, shifts in beam conditions, detector inefficiencies, etc. An update on the progress of the analysis, including a discussion of progress on particle tracking and determination of systematic uncertainties, will be provided.   

Luminosity monitoring at OLYMPUS with forward-angle elastic scattering

The OLYMPUS experiment at DESY has taken data during two periods in 2012 to measure the ratio of positron-proton and electron-proton elastic scattering cross sections. The goal of OLYMPUS is to quantify the effect of two-photon exchange, which is widely considered to be responsible for the discrepancy between measurements of the proton electric to magnetic form factor ratio with the Rosenbluth separation and polarization transfer methods. In order to control the systematic uncertainties to the sub-percent level, the luminosities have been monitored redundantly and with high precision by measuring the rates for symmetric Moller and Bhabha scattering, and by measuring the ep-elastic count rates at forward angles and low momentum transfer with tracking telescopes based on GEM (Gas Electron Multiplier) and MWPC (Multi Wire Proportional Chamber) technology. Based on the data analysis of GEM and MWPC luminosity monitors, detector performances and preliminary results on the positron/electron luminosity ratio will be presented.   

Radiative corrections for the OLYMPUS experiment

A leading but untested explanation of the proton form factor discrepancy between Rosenbluth separation and polarization measurements is the unaccounted contribution of hard two-photon exchange to elastic $ep$ radiative corrections. The OLYMPUS experiment at DESY has collected $e^+p$ and $e^-p$ elastic scattering data to make a definitive measurement of the size of this effect. Additional corrections to the elastic cross sections beyond what can be eliminated using opposite-lepton-charge data must be carefully applied. The most important and challenging of these arise from bremsstrahlung processes. Radiative corrections are sensitive to detector geometries, efficiencies, resolutions, and experimental cuts, making an analytic application of them too crude for this measurement. The OLYMPUS experiment will use a custom radiative event generator with a Geant4-based Monte Carlo and simulated detector response to produce models that can be analyzed exactly like data. This new generator and its application to the experiment will be described.   

Charge form factor and sum rules of electromagnetic response functions in Carbon-12

I will present the Green's function Monte Carlo (GFMC) calculation, based on realistic nuclear potentials and electromagnetic currents, of the Carbon-12 elastic form factor and sum rules of longitudinal and transverse response functions measured in inclusive (e, e') scattering.The longitudinal elastic form factor and sum rule are found to be in satisfactory agreement with available experimental data. A direct comparison between theory and experiment is difficult for the transverse sum rule. However, it is shown that the calculated one has large contributions from two-body currents, indicating that these mechanisms lead to a significant enhancement of the quasi-elastic transverse response. This fact may have implications for the anomaly observed in recent neutrino quasi-elastic charge-changing scattering data off Carbon-12.   

$Q^2$-Evolution of the Spin Structure Function $g_1 (x, Q^2)$ of the Proton and the Deuteron

Inelastic scattering using polarized nucleon targets and polarized charged lepton beams allows the extraction of spin structure functions that provide information about the helicity structure of the nucleon. A program designed to study such processes at low and intermediate $Q^2$ for the proton and deuteron has been pursued by the CLAS Collaboration at Jefferson Lab since 1998. The latest inclusive data with high statistical precision were measured using polarized 6 GeV electrons incident on a 2.5\% r.l. polarized ammonia target. In the framework of pQCD, these results can be used to better constrain the polarization of quarks and gluons in the nucleon, as well as high-twist contributions. Some preliminary results will be shown, along with the expected impact on the NLO global fits and phenomenological models of valent spin structure.   

Deuteron Spin Structure function $g_1$ at low $Q^2$

The spin structure function $g_1(x,Q^2)$ and its moments provide crucial information on the internal structure of the nucleon. At low momentum transfer $Q^2$, one can study the transition from partonic (quark-gluon) to hadronic (nucleonic) degrees of freedom and test effective theories based on QCD, such as Chiral Perturbation Theory ($\chi$PT). As $Q^2$ goes to zero, the first moment of $g_1$ is constrained by the GDH sum rule and its $\chi$PT extensions, which makes measurements of $g_1$ in this region uniquely interesting. As part of a large program of spin structure function measurements with CLAS at Jefferson Lab, the EG4 experiment measured the polarized cross section difference (between the cases of longitudinally polarized electron beam and proton/deuteron target having parallel and antiparallel spins) down to about 7 degrees in the scattering angles. From these differences, $g_1$ can be extracted, with minimal model uncertainties, down to $Q^2$ as low as 0.01 GeV$^2$. We will discuss the experiment and the status of its analysis and present preliminary results.   

The Study of the D(e,e'p)n Reaction at High Four- Momentum Transfer

D(e,e'p)n reaction mechanism studies at high four -momentum transfer $Q^2$ are very important to understand the dynamics of the two nucleon system at very short space time separation. The E01-020 experiment, carried out in HallA at Jefferson Lab, determined D(e,e'p)n cross sections for several values of constant $Q^2$ over wide range of kinematic settings. Cross sections for several fixed values of missing momentum as a function of the angle of the recoiling neutron with respect to the momentum transfer will be presented for $Q^2$ of 0.8 and 2.1 $(GeV/c)^2$. Experimental momentum distributions for several fixed recoiling angles will be shown as well.   

Single and Double Spin Asymmetries from Deeply Virtual Exclusive $\pi^0$ Production on a Longitudinally Polarized Proton with CLAS

Deeply Virtual $\pi^0$ production offers a unique opportunity to study the internal structure of the nucleon at the parton level. This reaction was identified as especially sensitive to the chiral-odd transversity GPDs. They encode correlations of parton distributions in transverse impact parameter space that are accesible through the measurements of the $x_B$ and $t$ dependence of $\pi^0$ production. Longitudinally polarized electron beam at Jefferson Lab and dynamically polarized $NH_3$ target with the spins of free protons aligned along the beam axis in Hall B allow the measurements of three polarization observables, greatly improving the extraction of polarized structure function ratios. Preliminary results for single and double spin asymmetries will be presented and compared to recent theoretical calculations using the handbag mechanism.   

Dihadron Electroproduction in DIS with Transversely Polarized $^{3}$He Target at 12 GeV Jefferson Lab

The transversity distribution function is one of the important and least known parton distribution functions (PDFs) of the nucleon. It can be studied via both single-hadron and double-hadron electro-production from a transversely polarized target in the deep inelastic scattering (DIS) region. Due to the low cross section, the data for the transversity distribution functions are very scarce. After 12 GeV upgrade, the high intensity 12 GeV electron accelerator at Jefferson Lab (JLab), togather with tThe large acceptance of the proposed Solenoidal Large Intensity Device (SoLID) in Hall A, will provide very good opportunities to study the transversity distribution functions in high precision. In this talk, we will present the dihadron program with SoLID. We plan to measure the single target spin asymmetries (SSA) of dihadron production in DIS region using 11 and 8.8 GeV electron beam on a transversely polarized $^{3}$He target. We will map the SSA in a 4-D space of $x$, $Q^{2}$, $z_h$ and $M_h$. Assuming leading twist dominance, the transversity distribution, $h_{1}$, can be extracted by combine with the world data on dihadron fragmentation functions (DiFF). These data will provide crucial inputs to the flavor separation of the transversity, especially the d quark distribution.