[ExI] proton measurement upset

[Damien Broderick]

Shrunken proton leaves scientists stunned
Thursday, 8 July 2010
Agence France-Presse


Measurements with lasers have revealed that the proton may be a touch 
smaller than predicted by current theories



PARIS: ... revised measurements shave 4% off the [proton]'s radius, 
according to a study in Nature.

...if borne out in further experiments, the findings could challenge 
fundamental precepts of quantum electrodynamics, the theory of how 
quantum light and matter interact, said its authors.

A team of 32 international scientists led by Randolf Pohl of the Max 
Planck Institute in Garching, Germany, had initially set out only to 
confirm what was already known rather than overturn time-honoured 
assumptions.

For decades, particle physicists have used the hydrogen atom as a 
benchmark for measuring the size of protons, which are part of the core 
of atoms. The advantage of hydrogen is its unrivalled simplicity: one 
electron circles a single proton.

But this unit of measure turns out to have been wrong by a small but 
critical margin, if the paper is right.

"We didn't imagine that there would be a gap between the known measures 
of the proton and our own," admitted co-author Paul Indelicato, director 
of the Kastler Brossel Laboratory at the Pierre and Marie Curie 
University in Paris.

The new experiment - at least 10 times more accurate than any performed 
to date - was envisioned by physicists 40 years ago, but only recent 
developments in technology made it feasible.

The trick was to replace the electron in the hydrogen atom with a 
negative muon, a particle with the same electric charge but more than 
200 times heavier and unstable to boot.

The muon's larger mass gives muonic hydrogen a smaller atomic size and 
allows a much larger interaction with the proton. As a result, the 
proton's structure can be probed more accurately than by using hydrogen.

Jeff Flowers, a researcher at Britain's National Physical Laboratory in 
Teddington, near London, said the work could take the theories of 
particle physics into new territory.

If confirmed, it would do even more than the multi-billion-dollar giant 
particle smasher at CERN in Switzerland to test the so-called Standard 
Model, which sets down the notional list of sub-atomic particles, he 
said in a commentary published in the same journal.

Either the previously accepted measures upon which hundreds of 
calculations have been based are wrong, or there is a problem with the 
theory of quantum electrodynamics itself.

Either way, physicists still have some serious explaining to do.

"Now the theoreticians are going to redo their equations, and more 
experiments will be done to confirm or overturn it," said Indelicato.

"In two years we will do another experiment with the same equipment, but 
this time with muonic helium," he added.