Director's Colloquium

Proton Radius

Professor Toshimi Suda
Director, Research Center for Electron-Photon Science, Tohoku University, Sendai, Japan

In the year 2010, the proton charge radius determined by a high-precision Lamb-shift measurement of muonic hydrogen was reported to disagree seriously with previous measurements by electron scattering and hydrogen spectroscopy. This was later referred to as the "proton radius puzzle”.

This proton-radius discrepancy not only concerns the fundamental properties of the proton, being the building block of visible matter in the universe, but also has a serious impact on the Rydberg constant, a fundamental physics constant. Furthermore, speculation linking this discrepancy to unknown differences in physical properties of electron and muon called public attention, as it may suggest new physics beyond the Standard Model, one of the most well-tested theories in particle physics.

In this talk, I will discuss the current status of this puzzle and introduce experimental and theoretical efforts underway worldwide, including our project in Sendai, JAPAN, to realize electron scattering from protons at the lowest energy ever achieved.

Professor Toshimi Suda is Director of the Research Center for Electron-Photon Science, Tohoku University, Sendai, JAPAN. The center operates a 1.3 GeV electron synchrotron accelerator for hadron- and nuclear-physics studies, which the largest in the university-based accelerators in Japan.

He received his Ph.D. from Tohoku University in 1988, then, moved to TU Darmstadt in Germany, as a Humboldt Fellow to carry high-energy electron scattering at Mainz University for studying nuclear physics. In 1999, he joined RIKEN as a vice chief scientist to construct the heavyion accelerator facility, Radioactive Isotope Beam Factory (RIBF), to provide the world’s most intense radioactive-isotope beams for studying the structure and reaction of exotic unstable nuclei. He constructed successfully the world’s first electron scattering facility at RIBF, the SCRIT facility, dedicated to unstable exotic nuclei. In 2010, he became a professor at Tohoku University and today serves as Director of the facility.

Join the Zoom Meeting
Meeting ID: 941 1170 1666
Password: 664 425

Date & time

Thu 20 Oct 2022, 11am–12pm


Physics Auditorium, Bldg. 160 & Via Zoom


Members of RSPE welcome