THE PROTON RADIUS - ECSAC Conference Losinj Croatia

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XIX International Conference on Science, Arts and Culture

THE PROTON RADIUS

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 since more than 60 years. Unlike the proton's electric charge or its magnetic moment, which have been determined with very high precision, the charge distribution of the proton and thus its mean square charge radius is badly known and has recently been at the origin of very active research programmes pursued at various laboratories.

Traditionally, charge distributions are measured using low-energy elastic electron scattering. Measurements for the proton have partly made use of the Rosenbluth separation of the electric and magnetic form factors. The results of this method for the proton radius had been challenged about 8 years ago by high-precision muonic-hydrogen spectroscopy performed at the Paul Scherrer Institute in Switzerland.

Despite much experimental efforts over the last years, the resulting proton radius puzzle has plagued physicists ever since. At the Mainz Microtron (MAMI), an admirable experimental effort to address the proton radius using elastic ep scattering down to very small values of the transferred momentum Q2 has basically confirmed the older electron scattering results, and new and more precise experiments are on the way.

Until recently, also the spectroscopy of electronic hydrogen differed from the muonic one, though by less than ep scattering data, but a new determination of the Rydberg constant has been used to reinterprete all electronic hydrogen data, which now brings muonic and electronic hydrogen into agreement. This situation has now put more weight on the issue of lepton scattering versus spectroscopical radius measurements and quests for very low Q2 data, and there is a call to investigate the last missing experimental measurement, elastic μp scattering.

The conference will review the present experimental investigations in the four area of research, namely electronic and muonic hydrogen and ep and μp scattering. Great attention will be devoted to the corrections to the data and to the present theoretical efforts to solve the "proton radius puzzle".

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