Session F03: Electromagnetic Form Factors II

The electromagnetic radii of the proton, neutron and deuteron from spectroscopy of muonic hydrogen and deuterium with the help of chiral EFTs

The proton "charge radius puzzle" is known to affect the deuteron, so it is really a Z=1 puzzle. I will present the results of our recent calculations of the nuclear polarizability effect in muonic hydrogen (muH) and deuterium (muD) [1-4], and discuss their impact on the extraction of the charge radii of proton, neutron and deuteron. Our latest chiral EFT calculation of the polarizability effect in the hyperfine splitting (hfs) of muonic hydrogen is in condradiction with previous data-driven evaluations based on empirical spin structure functions g1 and g2 of the proton. This has an improtant implication for the first planned measurements of the ground-state hfs transition in muH by the FAMU and CREMA Collaborations.

I will highlight the prospects for improving the data-driven evaluations using the new data from the Jefferson Lab g2p experiment.

The Physics Program of MUSE

The MUon Scattering Experiment (MUSE) at the Paul Scherrer Institute (PSI) seeks primarily to address the proton radius puzzle via investigation of the proton charge radius by performing simultaneous high-precision measurements of elastic electron-proton and muon-proton scattering. Measurements of the proton charge radius will be obtained from a beam of both positively and negatively charged particles from a mixture of electrons, muons, and pions. The experiment runs at varying momenta of 115, 160, and 210 MeV/c. The unique setup of muse allows for a direct muon/electron scattering comparison, a low-Q² region for sensitivity to the proton charge radius (with Q² = 0.002 to 0.082 GeV² /c² ), and for the study of possible two-photon exchange mechanisms.   

Status of MUSE Experiment Analysis

The MUon Scattering Experiment (MUSE) is designed to address the proton radius puzzle by simultaneously measuring the proton radius through both e^±p and μ^±p elastic scattering. The experiment takes place at the Paul Scherrer Institute (PSI) and uses a mixed e, μ, π beam. The experiment utilizes beams of positive and negative polarities in the PiM1 secondary beam line. The MUSE apparatus incorporates a large-acceptance, non-magnetic detector system, enabling precise measurements of the elastic e^±p and μ^±p scattering cross sections and cross section ratios. MUSE has commissioned its apparatus to the level needed for the measurements and began data collection. This talk will cover the current status and progress of the MUSE analysis. Preliminary blinded results from a partial data set will be discussed. Additionally, we will outline the future plans and necessary steps required to achieve the final physics goal of the experiment.   

GMn Analysis Status from Q2 = 4.5 (GeV/c)2 Quasi-elastic Electron-Deuterium Scattering at ε = 0.52 and ε = 0.80

In Fall of 2021 at Jefferson Lab, the SBS Collaboration measured the quasi-elastic neutron/proton cross section ratio via d(e,e')p and d(e,e')n scattering events at several kinematic points with the Super Bigbite Spectrometer (SBS). Via the so-called “ratio” method employed to minimize systematic uncertainty, we will extract the neutron magnetic form factor GMn (experiment e12-09-019) from these data for many Q² values. The status of this extraction and analysis will be presented for two such kinematic points at the same Q², but different longitudinal polarizations of the virtual photon, comprising the nTPE  (experiment e12-20-010) data set (SBS-8(9), Q² = 4.5 (GeV/c)², ε = 0.80(0.52)). Essential to this analysis is evaluation of the proton and neutron detection efficiency for the SBS hadron arm from the SBS Hadron Calorimeter (HCal) - preliminary evaluation of this efficiency including data/monte-carlo comparisons will also be presented.   

Measuring Two-Photon Exchange Contributions to the e-n Elastic Scattering Cross-section on the SBS at JLab

The nTPE experiment ran as part of the first segment of JLab's Super BigBite Spectrometer (SBS) program. nTPE is a measurement of the two-photon exchange contribution to the electron-neutron elastic scattering cross section at a four-momentum transfer of Q² = 4.5 (GeV/c)². Comparisons of nucleon form factor measurements acquired through polarization transfer and the Rosenbluth technique show discrepancies as the four-momentum transfer, Q², increases. Two photon exchange is strongly favored as the reason for this discrepancy and this experiment assesses the contribution of two-photon exchange in e-N elastic scattering processes by elastically scattering electrons off neutrons in a deuterium target. Large-scale calibrations have completed and physics analysis to extract G_Mⁿ at each kinematic for nTPE assessment is nearing completion. This talk will discuss the nTPE experiment and associated analysis milestones.     

Analysis Update for the Neutron Two-Photon Exchange Experiment in Hall A at Jefferson Lab

The neutron Two-Photon Exchange (nTPE) experiment in Hall A, which uses the 12 GeV electron accelerator at Jefferson Lab and is the part of the Super BigBite Spectrometer (SBS) program, will be the first measurement of the two-photon exchange contribution in elastic electron-neutron scattering from a deuterium target at Q² = 4.5 (GeV/c)². The two-photon exchange contribution to e-N elastic scattering will be extracted by measuring the ratio(neutron/proton) of quasi-elastic yields at a single Q², but at two different beam energies (and electron scattering angles). This will allow an extraction of the Rosenbluth slope for the neutron which is sensitive to two-photon exchange amplitudes. The experiment was performed using the BigBite Spectrometer which detected the scattered electrons, and the Super BigBite Spectrometer which detected the scattered nucleons using a large aperture dipole magnet and hadron calorimeter. The scope of this talk will be an overview of the physics motivation for the nTPE experiment and an update about the ongoing data analysis for my thesis.   

A Direct Measurement of Hard Two-Photon Exchange with Electrons and Positrons at CLAS12

One of the most surprising discoveries made at Jefferson Lab is the discrepancy in determinations of the proton's form factor ratio μ_pG^p_E/G^p_M between unpolarized and polarized measurements.  The leading hypothesis for this discrepancy is a non-negligible contribution from hard two-photon exchange (TPE).  Theoretical calculations of TPE are highly model-dependent, and existing experimental data are not able to conclusively confirm or refute this hypothesis.  A better understanding of multi-photon exchange over a wide phase space is needed.  We propose a comprehensive measurement of TPE at JLab using the CLAS12 detector.  Specifically, the measurement would use the proposed positron injector upgrade at JLab to measure the ratio of positron-proton to electron-proton elastic scattering cross sections over a wide range of momentum transfer and scattering angle.  This measurement would be a definitive test of whether or not hard TPE is the root cause of the proton form factor discrepancy, and provide critical benchmarks for theoretical calculations of TPE.  This talk will summarize the technique, kinematic coverage, and impact of the proposed measurement.   

Nucleon axial-vector radius and form factor from lattice QCD, MINERvA antineutrino-proton data, and future neutrino experiments

We compare a new MINERvA measurement of the nucleon axial-vector form factor with lattice-QCD calculations and deuterium bubble-chamber data, provide uncertainty projections for future extractions, and present recent calculations of radiative corrections to charged-current (anti)neutrino-nucleon scattering.   

Nucleon mass, isovector charges and couplings, and isovector form factors calculated in domain-wall lattice QCD at physical mass

Current status of lattice-QCD numerical calculations by joint LHP and RBC collaborations of nucleon mass, isovector vector and axial vector charges and tensor and scalar couplings, and isovector vector- and axialvector-current form factors using a 2+1-flavor dynamical domain-wall fermions lattice QCD ensemble generated jointly by RBC and UKQCD collaborations will be presented. The lattice spacing is set at about 0.1141(3) fm, and the lattice spatial extent is 48 spacings or about 5.4750(14) fm. The dynamical strange and degenerate up and down quark mass values are set at their essentially physical values to provide the physical Ω mass and a degenerate pion mass of 0.1392(2) GeV. Our nucleon mass estimate is about 0.947(6) GeV. We see a possible indication of contamination from excited states in our calculated isovector vector charge. Compared with the experiment, we observe a small but statistically significant deficit in the calculated isovector axialvector charge. We obtain good signals for isovector tensor coupling but not for scalar coupling. Possible excited-state contaminations in the calculated vector- and axialvector-current form factors are hidden below larger statistical noises. The numerical details of the shape parameters, such as the mean squared radii, the anomalous magnetic moment, or the pseudoscalar coupling, extracted from the form factors, will be given along with their possible impacts on the present and future experiments.   

Two-Photon Exchange Measurement on Neutron with Super BigBite Spectrometer

The nucleon form factors are one of the most fundamental aspects of the nucleon structure. At high Q2, they are linked to the generalized parton distributions which provide information on the nucleon spin. The Super Bigbite Spectrometer (SBS) in Hall A at Jefferson Lab is dedicated to measure the form factors at high Q2, and has started taking data during Fall of 2021.

Among those experiments, nTPE measures the two-photon exchange contribution in elastic electron-neutron scattering at a four-momentum transfer Q2= 4.0 GeV2. The two-photon exchange is suspected to be the root cause of the discrepancies between the form factor Rosenbluth measurements and polarization transfer measurements on the proton, but it has never been measured on the neutron. Specifically, nTPE measures the ratio of quasi-elastic en over ep cross sections off deuterium at the aforementioned value of Q2 and two beam energies, 4.03 and 6.00 GeV. The comparison between the obtained experimental Rosenbluth slope and the current estimation of GEn will provide an experimental estimation of the two-photon exchange contribution.

In this presentation we provide a very brief overview of the SBS neutron form factor measurements with an emphasis on nTPE, its experimental analysis method, and the progress of the data analysis.