# S1: Baryon form factors

### Research areas

• Lattice gauge theory

### Goals

The project S1 studies elastic baryon electromagnetic form factors (FFs). The precise know-ledge of these fundamental structure properties is required in order to interpret precision experiments searching for new physics using electromagnetic probes. Besides electron scattering experiments, information on baryon structure can also be obtained from atomic spectroscopy. It came as a big surprise that the recent extractions of the proton charge radius from muonic hydrogen Lamb shift measurements are in strong contradiction, by around 7 standard deviations, with the values obtained from energy level shifts in electronic hydrogen or from electron-proton scattering. This so-called "proton radius puzzle" has triggered a large activity and is the subject of intense debate. Project S1 has as main aim to improve upon the baryon FF measurements using a combination of experimental and theoretical techniques.

In the second funding period, project S1 will focus in detail on:

• Dispersive analyses of two-photon exchange processes in $$e^-p$$ and $$\mu p$$ scattering by including inelastic intermediate states; Study of the beam normal spin asymmetry for the $$e^-p\to e^-\Delta$$ process
• Theoretical study of the dilepton photoproduction ($$e^-e^+$$ versus $$\mu^-\mu^+$$) on a proton with the aim to perform a lepton universality test in the proton charge form factor
• Ab initio calculations of isovector form factors based on 2+1 flavor lattice QCDwith 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
• Spacelike nucleon FF program at MAMI: Finalizing the data analysis of the proton FF at larger Q2; Development and 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
• Timelike neutron FF measurements at BESIII: Full analysis and publication of the ISR tagged analysis for neutron timelike FFs; Full analysis and publication of the high luminosity energy scan data to determine for the first time the ratio R= GE/GM for the neutron in the energy range between 2.0 GeV and 3.08 GeV.