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Quantum physics

Shrinking the proton again

München, 10/06/2017

Munich Scientists, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

Vacuum chamber used in the experiments. The purple glow stems from the microwave discharge that dissociates hydrogen molecules into hydrogen atoms. The blue light is fluorescence from the ultraviolet laser that excites the atoms to the 2S state. The turquoise blue glow is stray light from the laser system used to measure the frequency of the 2S-4P transition. Source: MPQ

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.
Science 2017

Press release of the Max Planck Institute of Quantum Optics