The Muon Scattering Experiment (MUSE) at PSI and the proton radius puzzle

Michael Kohl,A Wrońska,A Magiera,H Ströher,C Guaraldo

doi:10.1051/epjconf/20148102008

Abstract

The unexplained large discrepancy of the proton charge radius measurements with muonic hydrogen Lamb shift and determinations from elastic electron scattering and Lamb shift in regular hydrogen of seven standard deviations is known as the proton radius puzzle. Suggested solutions of the puzzle range from possible errors in the experiments through unexpectedly large hadronic physics effects to new physics beyond the Standard Model. A new approach to verify the radius discrepancy in a systematic manner will be pursued with the Muon Scattering Experiment (MUSE) at PSI. The experiment aims to compare elastic cross sections, the proton elastic form factors, and the extracted proton charge radius with scattering of electrons and muons of either charge and under identical conditions. The difference in the observed radius will be probed with a high precision to verify the discrepancy. An overview of the experiment and the current status will be presented.

Highlights

The proton charge radius has been extracted from the Lamb shift of 2S-2P transitions in regular hydrogen in the framework of bound-state Quantum Electrodynamics (QED)
The CODATA2010 value [7] of rp = (0.8775 ± 0.0051) fm is based on electron scattering and Lamb shift measurements from regular hydrogen
The finite-size effect on the Lamb shift is strongly enhanced compared to regular hydrogen due to the higher mass of the muon and the tighter orbits of its atomic states

Summary

Introduction

The proton rms radius has been measured with elastic electron-proton scattering since the 1950’s by investigating how the form factor changes near Q2 = 0. The proton rms charge radius contributes to the Lamb shift of the 2S-2P atomic transitions of the hydrogen atom, due to the overlap of the atomic shells with the finite-sized nuclear center.

Results

Conclusion