The proposed measurements of nucleon FFs and the accompanied theoretical studies are of crucial importance to understand the glaring discrepancy between electron scattering measurements of the proton radius and Lamb shift measurements in μH.
The main theoretical plans in the second funding period are:
- Extension of the dispersive analysis of TPE in e-p and μp scattering by including inelastic intermediate states; quantifying full TPE effect on the extraction of GMp
- Study of the beam normal spin asymmetry for the e-p → e-Δ process
- Theoretical study of the photoproduction of e-e+ versus μ-μ+ pairs on a proton with the aim to perform a lepton universality test in the proton charge form factor
- Ab initio calculations of isovector FFs based on 2+1 flavor lattice QCD at lattice spacings of 0.086 fm, 0.064 fm, and 0.05 fm with a pion mass down to 200 MeV, and with one ensemble directly at the physical pion mass
- Inclusion of disconnected diagrams in lattice QCD calculations for the nucleon FFs
- Further implementation and refinements of two-loop radiative corrections in electron scattering, jointly with project P2
For the experimental program at MAMI and MESA, the plans in the second funding period include:
- Finalizing the data analysis of the proton FF at larger Q2
- Installation of a gas-jet target for initial state radiation
- Installation of a neutron polarimeter for neutron electric FF measurements and first data taking
- Feasability studies to measure the magnetic FF of the proton at low Q2 at MESA
For the experimental program at BESIII, the milestones in the second funding period are:
- Full Analysis and publication of the ISR tagged analysis for neutron EM time like FFs.
- Full analysis and publication of the high luminosity energy scan data at center of mass energies between 2.0 GeV and 3.08 GeV in order to determine for the first time the ratio R= GE/GM for the neutron in this energy range.