Session S12: Electromagnetic Interactions I

Live

The striking discrepancy in the proton form factor ratio, $\mu_p G_E^p/G_M^p$, measured using unpolarized and polarized techniques is still not resolved. The proposed explanation is that hard two-photon exchange (TPE) is responsible. Hard TPE is difficult to calculate without significant model dependence, and has generally not been included as a radiative correction. Furthermore, three recent experiments found only a small contribution but were limited to relatively low $Q^2$ where the discrepancy is not clear. A new test beam experiment, TPEX@DESY, would use a planned extracted beam at the DESY test beam facility together with a liquid hydrogen target and high precision lead tungstate calorimeters to measure hard TPE at higher beam energies. This would permit measurements in a $Q^2$ regime where the discrepancy in the proton form factor ratio is significant and where the expected hard TPE contribution is predicted to be large. The motivation and overview of the proposed measurements will be presented.   

Live

The two-photon exchange (TPE) contribution in elastic electron-proton scattering has been of significant interest to the nuclear physics community as a possible explanation for the proton form factor ratio discrepancy. Three experiments (VEPP-3, CLAS12, and OLYMPUS) have reported on direct measurements of TPE in recent years and show effects in agreement with phenomenological predictions, but in disagreement with theory, up to $Q^2$ = 2.5 (GeV/c)$^2$. At larger $Q^2$, for which other theoretical approaches, like those based on GPDs, become feasible the effect remains untested. The proposed Two-Photon Exchange eXperiment (TPEX) will measure hard two-photon contribution to $ep$ scattering in the momentum transfer range up to $Q^2$ = 4.7 (GeV/c)$^2$. The experiment uses electromagnetic calorimeter blocks at fixed angles to detect scattered electrons. In this talk we will discuss using a Deep Neural Net (DNN) and Multi-Layer Perceptron (MLP) to reconstruct the $ep$ hit position, and compare to the simple analytical solution. Some prelimary results from a Monte Carlo simulation of the experiment will be shown, and future work will be discussed.   

Live

In this talk we will present the results of the test measurements performed with the $3 \times 3$ PbWO$_4$ calorimeter at the DESY test beam facility. The goal of measurements was to study the performance of two data acquisition systems: trigger-based readout with a QDC and a triggerless readout based on a digitizer. Calorimeters of this type will be the key component of an experimental setup for the future Two-Photon Exchange eXperiment (TPEX), which will provide new data on the next-to-leading order term in the QED description of electron-proton scattering process up to $Q^2 =$ 4.6 (GeV/c)$^2$ (doubling the range compared to current data).   

Live

Elastic double-polarization asymmetries, made using a polarized electron beam and a polarized $^{3}\mathrm{He}$ target, are proportional to the product of $^{3}\mathrm{He}$'s electric and magnetic form factors. Unlike a Rosenbluth separation, this asymmetry measurement is sensitive to the sign of the form factors and the zero crossing of the asymmetry correspond to the location of the diffractive minima. By measuring this asymmetry as a function of $Q^{2}$, we will further constrain the location of the diffractive minima and improve our knowledge of the three-body system and help determine the source of the current discrepancies between experiment and theory. These new measurements from experiment E12-06-121A, were performed in Hall C at Jefferson Lab using the CEBAF longitudinally polarized electron beam and a custom polarized $^{3}\mathrm{He}$ gas target. Elastically scattered electrons were detected in both the High Momentum and Super High Momentum Spectrometers over a series of $Q^{2}$ values. The current analysis status will be presented.   

Live

The MUon proton Scattering Experiment (MUSE) at the PiM1 beam line of the Paul Scherrer Institute will simultaneously measure elastic scattering of muons and electrons from a liquid hydrogen target to extract the charge radius of the proton. Both beam polarities are measured over the course of the experiment. By comparing the four scattering cross sections, the experiment will provide unique muon proton scattering data with a precision sufficient to address the proton radius puzzle, and will directly measure two-photon exchange effects for both muons and electrons. This talk will present an overview introduction of the MUSE experiment, including the initial motivation, measurement capability and detector setup, as well as a brief discussion of the current status of the collaboration.   

Live

The MUSE experiment at Paul-Scherrer Institute (PSI) will measure the proton charge radius by scattering of muons and electrons. The experiment uses a telescope of three Gas Electron Multiplier (GEM) detectors to track beam particles entering the experimental area in order to accurately determine the particle scattering angles and to monitor the beam profile. The readout of the GEM telescope was recently upgraded and extensively tested with beam at PSI. This presentation will show an overview of the data acquired with the GEM telescope in the Fall 2020 test beam time.   

Live

The Muon Scattering Experiment (MUSE) at Paul Scherrer Institute (PSI) is designed to measure the proton charge radius with simultaneous elastic scattering of electrons and muons of either charge polarity. For an accurate determination of the lepton scattering angle, event­-by-­event beam particle tracking is required to reconstruct the incoming particle track. A telescope of Gas Electron Multipliers (GEM), exposed to a high flux of beam particles is used to reconstruct the incoming tracks with high spatial resolution while presenting minimal material for the beam to pass through. This presentation will lay out the procedure of the GEM alignment survey. It will detail how the alignment is inferred from the overdetermined survey data and accounted for in the data analysis. The goal of the alignment procedure is to reduce the systematic uncertainty due to misalignment on tracking and scattering angle determination to be much less than the dominating uncertainties due to multiple scattering.   

Live

A novel neutron polarimeter design has been conceived for the E12-17-004 experiment prepared for the Super-Bigbite program at JLab to measure the neutron electric-to-magnetic form factor ratio via neutron recoil polarization in quasielastic electron-deuteron scattering. Three analyzing processes are pursued: np elastic scattering with detection of small-angle neutrons and tracking of large-angle protons, as well as charge-exchange np scattering with tracking of forward-angle protons. A large set of Gas Electron Multiplier (GEM) detectors is being commissioned based on elastic and charge-exchange recoil proton detection. A detailed Geant4 simulation of the experiment combined with a digitization package to generate the detector level pseudo data similar to what is expected in the actual experiments is being developed. This talk will report on the GEM commissioning activities as well as the status of simulation digitization.