  The proton radius puzzle is known as the discrepancy of the proton radius,
obtained from muonic hydrogen spectroscopy (obtained as being roughly equal to
0.84fm), and the proton radius obtained from (ordinary) hydrogen spectroscopy
where a number of measurements involving highly excited states have
traditionally favored a value of about 0.88fm. Recently, a number of
measurements of hydrogen transitions by the Munich (Garching) groups (notably,
several hyperfine-resolved sublevels of the 2S-4P) and by the group at the
University of Toronto (2S-2P_1/2) have led to transition frequency data
consistent with the smaller proton radius of about 0.84fm. A recent measurement
of the 2S-8D transition by a group at Colorado State University leads to a
proton radius of about $0.86$\,fm, in between the two aforementioned results.
The current situation points to a possible, purely experimental, resolution of
the proton radius puzzle. However, a closer look at the situation reveals that
the situation may be somewhat less clear, raising the question of whether or
not the proton radius puzzle has been conclusively solved, and opening up
interesting experimental possiblities at TRIUMF/ARIEL.
