Physicists at Colorado State University have measured the radius of a proton with record-breaking precision. The new result puts an end to a decade-long debate over one of nature’s fundamental constants. The experiment has confirmed that the proton is smaller than previously thought, and no new physics is needed to explain the discrepancies.
What is the essence of the puzzle?
For many years, scientists obtained different values for the size of the proton depending on the measurement method. Classical experiments with electrons yielded a single result of approximately 0.876 femtometers. However, experiments with muons—heavier relatives of electrons—consistently indicated a smaller radius.
This discrepancy gave rise to the so-called “proton radius puzzle” and led some physicists to speculate that an unknown force or particle beyond the Standard Model might be at play. If this were confirmed, it would require a complete rewriting of the fundamentals of modern physics.
A new measurement method
A team led by Professor Dylan Yost has developed a technique that uses two laser fields simultaneously. This has made it possible to significantly improve accuracy. The researchers created a beam of atomic hydrogen in a vacuum chamber and used ultraviolet lasers to excite the electrons, causing them to transition between different energy levels.
Since the size of the proton has a barely perceptible effect on the behavior of electrons around the nucleus, precise measurements of these energy transitions made it possible to calculate the radius. The result was approximately 0.84 femtometers, which is less than one quadrillionth of a meter. It matches later measurements, rather than the old standard value.
Accuracy that settles the matter
The measured value agrees with the predictions of the Standard Model to within one part in a trillion. The probability that the discrepancy was caused by a new force or an unknown particle is virtually ruled out. “Our test shows a perfect match with the theory, ruling out the possibility of new physics in this case,” Yost noted.
The challenge lay in the fact that fast hydrogen atoms interact with the laser for only a very brief moment. As a result, the signals lose their clarity and are difficult to measure. Graduate student Ryan Bullis, the study’s lead author, explained that using two laser fields simultaneously made it possible to overcome this limitation. The result was independently confirmed by a team from the Max Planck Institute using a different measurement approach.
The researchers plan to apply the developed laser technique to more complex forms of hydrogen, particularly deuterium. This will help explore other aspects of atomic physics. According to Yost, the work demonstrates how precision bench-top experiments can complement large accelerators in the search for deviations from known theories. The study was published in the journal Physical Review Letters.
According to interestingengineering.com
