Shrinking the proton again
Munich Scientists, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly smaller, by four standard deviations, than previous determinations using regular hydrogen. This discrepancy and its origin have attracted much attention in the scientific community, with even extensions of the so-called standard model of physics being discussed. Now, a team of scientists from the Laser Spectroscopy Division of Professor Theodor W. Hänsch (MPQ and LMU) has made a new spectroscopic measurement of regular hydrogen. The resulting values for the Rydberg constant and the proton radius are in excellent agreement with the muonic results, but disagree by 3.3 standard deviations with the average of the previous determinations from regular hydrogen.
Press release of the Max Planck Institute of Quantum Optics