Session K15: Electro-Magnetic Interactions

Two-photon exchange contribution to elastic electron-proton scattering

Two experimental techniques, Rosenbluth separation and recoil polarization transfer, used to extract proton's electromagnetic form factors ratio $\frac{G_E}{G_M}$ yield markedly different results. Modern theoretical calculations suggest that two-photon exchange might be responsible for the observed discrepancy and that it is epsilon dependent. Jefferson Lab Experiment E05-017 was designed to measure the two-photon exchange contribution over a wide range of $\varepsilon$ and $Q^2$. In contrast with the conventional Rosenbluth method, E05-017 detected the elastically scattered proton rather than the electron. This approach returns a much more precise extraction of the form factor ratios. After a brief description of the experimental goals and techniques, the current status of the analysis will be presented.   

Radiative Corrections for Lepton-Proton Scattering: When the Mass Matters

Radiative corrections procedures for electron-proton and muon-proton scattering are well established under the assumption that the leptons are considered in an ultra-relativistic approximation. MUSE experiment at PSI and COMPASS experiment at CERN entered the regions of kinematics where explicit dependence of radiative corrections on the lepton mass becomes important. MUSE will consider the scattering of muons with momenta of the order ~100 MeV/c, therefore lepton mass corrections become important for the entire kinematic domain. COMPASS experiment uses scattering of ~100 GeV/c muons, and the muon mass effects are especially relevant in the quasi-real photo production limit, $Q^2\to$0. A dedicated Monte Carlo generator of radiative events is being developed for MUSE, which also includes effects of interference between the lepton and proton bremsstrahlung. Parts of the radiative corrections are expected to be suppressed for muons due to the larger muon mass. Two-photon exchange corrections are generally expected to be small, and should be similar for electrons and muons. We classify the radiative corrections into two categories, C-even and C-odd under the lepton charge reversal, and discuss their role separately for the above experiments.   

Proton Magnetic Form Factor from Existing Elastic e-p Cross Section Data

The proton magnetic form factor GMp, in addition to being an important benchmark for all cross section measurements in hadron physics, provides critical information on proton structure. Extraction of GMp from e-p cross section data is complicated by two-photon exchange (TPE) effects, where available calculations still have large theoretical uncertainties. Studies of TPE contributions to e-p scattering have observed no nonlinear effects in Rosenbluth separations. Recent theoretical investigations show that the TPE correction goes to 0 when $\varepsilon $ approaches 1, where $\varepsilon $ is the virtual photon polarization parameter. In this talk, existing e-p elastic cross section data are reanalyzed by extrapolating the reduced cross section for $\varepsilon $ approaching 1. Existing polarization transfer data, which is supposed to be relatively immune to TPE effects, are used to produce a ratio of electric and magnetic form factors. The extrapolated reduced cross section and polarization transfer ratio are then used to calculate GEp and GMp at different $Q^2$ values.   

Charge Radius Measurements of Some Light Nuclei

The ability of variational Monte Carlo models to calculate the properties of light nuclei has inspired a new generation of elastic scattering measurements. Hall A at Jefferson Lab has now determined the differential cross-section for several light nuclei (H, D,$^6Li$ , $^{10}B$ ,$^{12}C$ ) using beam energies down to 360 MeV beam and spectrometer angles down to $12.5 ^{o}$.The results have been parametrized using a model independent Fourier-Bessel parametrization. For those nuclei where world data exists, we found good agreement. For the boron data, we made use of the NIKHEF suggestion to use $B_4C$ instead of the problematic pure boron. After subtracting the carbon yield from the $B_4C$ data, we obtain the boron cross section from 0.39$fm^{-1}$ to 2$fm^{-1}$ and we will present our preliminary charge radius result for this nucleus.   

The $^3$He-$^3$H Charge Radius Difference

The upcoming E12-14-009 experiment at Jefferson Lab will determine the charge radius difference between $^3$He and $^3$H from elastic electron scattering. This measurement will utilize a low-activity tritium target that is available for a limited time at Jefferson Lab. We will measure the ratio of the $^3$He and $^3$H electric form factors at 0.05--0.09 GeV$^2$ using a high-resolution spectrometer in Hall A. The relative charge radii will be extracted from the data with a statistical(systematic) uncertainty \textless 0.5(2.5)\%. These data will reduce the uncertainty in the charge radius difference from $\sim$0.1 fm to $\sim$0.03 fm. The results will provide a direct comparison to recent calculations of the charge radii. Additionally, the individual proton and neutron radii can be determined and compared to ab initio calculations of the three-body nuclei.   

Quasielastic Transverse and Longitudinal Response Functions in the range 0.55 GeV/c$\leq \mid \overrightarrow{q}\mid\leq$1.0 GeV/c

In order to determine the Coulomb sum in nuclei, a precision measurement of inclusive electron scattering cross sections in the quasi-elastic region was performed at Jefferson Lab. Incident electrons with energies ranging from 0.4 GeV to 4 GeV scattered from $^{4}He$,$^{12}C$,$^{56}Fe$ and $^{208}Pb$ nuclei at four scattering angles ($15^{\circ},60^{\circ},90^{\circ},120^{\circ}$) and scattered energies ranging from 0.1 GeV to 4 GeV. The Rosenbluth separation method is used to extract the transverse and longitudinal response functions at three-momentum transfers in the range 0.55 GeV/c$\leq \mid \overrightarrow{q}\mid\leq$1.0 GeV/c. The Coulomb Sum is obtained for $^{56}Fe$ and $^{12}C$ ,and compared to predictions. We will discuss the impact of our results on short range nucleon-nucleon correlations and the possible modification of the nucleon electromagnetic properties in the nuclear medium.   

Pion Electroproduction and VCS in the $\Delta $ Resonance Region

The study of the N to $\Delta $ transition has been a subject of intense scientific interest for more than two decades. The pion electroproduction and VCS channels of the transition allow, through the measurement of the transition quadrupole amplitudes, the exploration for non-spherical angular momentum amplitudes in hadrons while the VCS channel also provides access to the nucleon generalized polarizabilities. Results from the recent JLab/Hall-A and MAMI experiments will be presented and future prospects will be discussed.   

The Spring 2015 JLab Hall A Deeply Virtual Compton Scattering Run

The Jefferson Lab Hall A Deeply Virtual Compton Scattering (DVCS) experiment E12-06-114 will take data at 8 and 10 GeV in Spring 2015. This experiment will measure absolute cross sections of the H$(\vec{e},e'\gamma)p$ and H$(\vec{e},e'\pi^0)p$ reactions. The Spring run will complete $Q^2$ scans at $x_{Bj} = 0.36$ and $0.50$. The $Q^2$-dependent cross sections allow the separation of the leading-twist Generalized Parton Distribution (GPD) amplitude from the higher-twist scaling-violating terms. The GPDs encode the transverse spatial distribution of partons as a function of longitudinal momentum. We will present preliminary results from the Spring run and projections for the complete experiment.   

Monte Carlo Study of Electromagnetic Calorimeter Performance for Proton Form Factor Measurements at JLab

Jefferson Lab's continuous electron beam, capable of luminosities approaching 10$^{39}$ cm$^{-2}$ s$^{-1}$, has recently been upgraded to a maximum energy of 12 GeV, allowing the proton electromagnetic form factor ratio $G_E^p/G_M^p$ to be measured at momentum transfer $Q^2$ of up to 12 GeV$^2$ via the polarization transfer method. The measurement will use the Super BigBite Spectrometer (SBS), a new magnetic spectrometer capable of operating at forward scattering angles under high luminosity conditions, to detect and measure the polarization of elastically scattered protons. The measurement of $G_E^p/G_M^p$ also requires detection of the elastically scattered electron in coincidence using a lead-glass electromagnetic calorimeter (ECal). ECal, the subject of this talk, is based on the ``BigCal'' detector used in a predecessor experiment in Hall C, re-configured to match the acceptance of SBS for elastically scattered protons. This talk will present an overview of ECal, its role in the measurement of GEp at large $Q^2$, and the technical challenges involved in its operation in the high-radiation environment of Hall A, followed by the results of of detailed GEANT4-based Monte Carlo simulations characterizing its expected performance under experimental conditions.