By: Joaquim dos Santos
From: Univ. Coimbra
At: Instituto de Investigação Interdisciplinar, Anfiteatro
The measurement of the Lamb-shift in muonic hydrogen was achieved successfully, at last. We have measured the energy splitting in μp with an experimental accuracy of 15 ppm, twice better than our proposed goal. Using todayâs QED calculations for the fine-, hyperfine, QED and finite size contributions we obtain a root-mean square proton charge radius of rp= 0.84184 (67) fm. This value is ten times more precise, but 5 standard deviations smaller, than the value of 2006 CODATA. The cause of this discrepancy is not known and the scientific community is at present concentrating efforts to solve âthe proton radius puzzleâ, finding out the reasons for such difference.
The importance of this discovery justified its publication in âNatureâ, being even chosen for the cover of the respective volume. The experiment was developed throughout one decade in the âHollywood movieâ type, âeverything goes wrong until the very last five minutes before the ENDâ and it is an example that, by pushing the limits of technology, one may reach results well beyond the initially established goals.