"The comparison between experimental values of transition frequencies in atomic hydrogen, the most simple atomic system, and the corresponding theoretical predictions provides stringent tests of bound state QED calculations. For more than a decade, this comparison has been limited by insufficient knowledge about the size of the proton, strictly speaking its r.m.s. charge radius. In 2010, a value for the proton size has been extracted from laser spectroscopy of muonic hydrogen which is ten times more accurate than any previous determination. However, this value deviates from the value found by precision spectroscopy of regular hydrogen by four combined standard deviations. An even larger inconsistency of 7σ is obtained when including electron-proton scattering data. The muonic hydrogen value has been confirmed and improved in 2013 while the source of the discrepancy, referred to as the ‘proton size puzzle’, remains unclear.In this talk, we report on a new precision spectroscopy experiment, aiming to shed light on the regular hydrogen part of the puzzle: In contrast to previous high resolution experiments probing transition frequencies between the meta-stable 2S state and a higher lying nL state (n = 3, 4, 6, 8, 12, L = S, P, D), our measurement of the 2S – 4P transition frequency is the first experiment being performed on a cryogenic beam of hydrogen atoms in the 2S state. We will discuss how this helps to efficiently suppresses leading systematic effects of previous measurements and present preliminary results obtained so far."
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