The FAMU experiment at RIKEN RAL for a precise measure of the proton radius

TL;DR

The FAMU experiment aims to precisely measure the hyperfine splitting of muonic hydrogen to determine the proton Zemach radius, potentially resolving the longstanding proton radius puzzle caused by discrepancies in previous measurements.

Contribution

This work introduces a novel experimental approach to measure the hyperfine splitting of muonic hydrogen with high precision, addressing the proton radius puzzle.

Findings

Expected to determine the proton Zemach radius with better than 0.5% accuracy

Provides a new method for comparing muonic and electronic hydrogen measurements

Aims to clarify the discrepancy in proton radius values

Abstract

The goal of the FAMU experiment at RIKEN RAL is the measure of the hyperfine splitting of the ground state of the muonic hydrogen, to allow a determination of the proton Zemach radius with a precision better than $5 \times 10^{-3}$ . The comparison of this measurement with the ones done with ordinary hydrogen may help to solve the so-called "proton radius puzzle", triggered by the $6 \sigma$ discrepancy in the proton charge radius value as extracted from muonic Lamb shift and the value based on e-p scattering and ordinary hydrogen spectroscopy.