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Title: Why two 1S-3S hydrogen measurements give different proton radii
Plain: Two labs measured the same hydrogen line and extracted proton sizes that disagree by more than two standard errors. Finding why would tell which systematic effects affect such experiments.
Precise: The LKB Paris 1S-3S measurement (Fleurbaey et al 2018) gives $r_p = 0.877(13)\,\mathrm{fm}$, while the MPQ Garching 1S-3S measurement (Grinin et al 2020) gives $r_p = 0.8482(38)\,\mathrm{fm}$. Identify the systematic shift (second-order Doppler, quantum interference of neighboring lines, light shift, pressure shift) responsible for the difference of about 6 kHz (2.1 combined sigma) in the 1S-3S transition frequency.
Settled by: A remeasurement of 1S-3S in the LKB apparatus with a revised line-shape model that agrees with the Garching value, or a third 1S-3S measurement.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 2020