Veli Lošinj, Croatia, 15th - 20th September 2019
The determination of the size of the proton, the most abundant hadron in our Universe, has been in the focus of intensive research for more than 60 years. Unlike the protons' electric charge or its magnetic moment, which have been determined with very high precision, there is much dispute about the charge distribution of the proton and thus the value of its mean square charge radius owing to conflicting measurements. This has recently spurred very active research programs pursued at various laboratories.
Traditionally, charge distributions are measured using low-energy elastic electron scattering. The determination of the electric form factor of the proton, from which the charge distribution is subsequently derived, has partly made use of the Rosenbluth method allowing to separate the electric and magnetic form factors. The determination of the proton radius from this method had been challenged about eight years ago by high-precision spectroscopy of muonic-hydrogen performed at the Paul Scherrer Institute in Switzerland leading to the so-called proton radius puzzle.
Despite much experimental efforts over the last years, this puzzle has plagued physicists ever since. At the Mainz Microtron (MAMI), an admirable experimental effort to address the proton radius using elastic e-p scattering down to very small values of the transferred momentum Q2 has basically confirmed the older electron scattering results, and new and even more precise experiments are on the way.
Until recently, also the spectroscopy of electronic hydrogen differed from the muonic one, though by less than e-p scattering data, but a new determination of the Rydberg constant has been used to reinterpret all electronic hydrogen data, which now brings muonic and electronic hydrogen into agreement. This situation has now put more weight on possible systematic peculiarities extracting the proton radius from elastic lepton scattering versus atomic laser spectroscopy and among others quests for very low Q2 data, and there is a call to investigate the last missing experimental measurement, elastic μ-p scattering.
The conference will review the present experimental investigations in the four areas of research, namely electronic and muonic hydrogen and e-p and μ-p elastic scattering. Great attention will be devoted to the necessary corrections to the data and to the present theoretical efforts to resolve the "proton radius puzzle".