# The $\pi^0$ mass and the first experimental verification of Coulomb de-excitation in pionic hydrogen

### Submission summary

 As Contributors: Manfred Daum Preprint link: scipost_202105_00042v2 Date accepted: 2021-07-06 Date submitted: 2021-07-05 10:00 Submitted by: Daum, Manfred Submitted to: SciPost Physics Proceedings Proceedings issue: Review of Particle Physics at PSI Academic field: Physics Specialties: High-Energy Physics - Experiment Approach: Experimental

### Abstract

The most precise value for the $\pi^0$ mass was obtained from the measurement of the mass difference $m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)$\,MeV/c$^2$ in the charge exchange reaction $\pi^-$p $\rightarrow \pi^0$n at PSI. With the most precise charged pion mass value, $m_{\pi^+} = 139.570\,21(14)$\,MeV/c$^2$ and the validity of the CPT theorem ($m_{\pi^-} = m_{\pi^+}$), a value $m_{\pi^0} = 134.976\,57(50)$\,MeV/c$^2$ is obtained. The measurements also revealed, for the first time, evidence of an unexpectedly large contribution from Coulomb de-excitation states during the pionic atom cascade.

Published as SciPost Phys. Proc. 5, 012 (2021)