Francis M. Pipkin Award Talk: Proton structure from laser spectroscopy of muonic hydrogen

Muonic hydrogen ($\mu$p) is the bound state of a proton and a negative muon. The large muon mass results in a small Bohr radius of the muonic hydrogen atom which in turn causes a dramatically increased sensitivity of the energy levels in $\mu$p to the finite size of the proton's charge and magnetisation distributions. The discovery of long-lived $\mu$p atoms in the metastable 2S state~[1] enabled us to perform a measurement of the 2S-2P energy splitting (Lamb shift) in muonic hydrogen for the first time~[2]. The proton radius we obtained is ten times more accurate, but $7 \sigma$ away from the current PDG value. This so-called ``proton radius puzzle'' has caused considerable activity, but no clear solution has been found yet~[3]. A second measurement in $\mu$p [4] confirms the proton radius obtained in [1], and a combination of both measurements reveals the 2S hyperfine splitting of the $\mu$p atom which is sensitive to the magnetic properties of the proton~[4].\\[4pt] [1] R.~Pohl {\it et al.}, Phys. Rev. Lett. {\bf 97}, 193402 (2006).\\[0pt] [2] R.~Pohl {\it et al.} (CREMA coll.), Nature {\bf 466}, 213 (2010).\\[0pt] [3] R.~Pohl, R.~Gilman, G.A.~Miller, K.~Pachucki, arXiv~1301.0905.\\[0pt] [4] A.~Antognini {\it et al.} (CREMA coll.), Science (2013), DOI:10.1126/science.1230016