Following the tantalizing suggestion of a discrepancy in the masses of certain neutrinos and their anti-partners, another commonly used particle has thrown up a bit of a surprise. The proton, that seemingly fundamental building block of matter, might be a little smaller than previously assumed.
Measuring the radius of the proton is pretty difficult. Firing particles, even photons at it just has the effect of measuring the strength of the electromagnetic field at close quarters. Attention normally focuses on how the radius of the proton – itself related to the spacing of the grouping of three quarks (two ups and a down) that make up the particle – affects the spacing between allowed energy levels of the electron. The spacing means that outside the radius of the proton, the electromagnetic fields fall off in a rather simple manner, but inside they’re the product of the three quark fields interacting.
Now a series of experiments have been carried out that replace electrons in a hydrogen atom with Muons, which are another type of lepton. The difference is muons have a greater mass than electrons and so are more sensitive than electrons to these changes. Muons decay after 2.2 seconds, meaning the experiments had to be carried out rather quickly, with a laser being fired to get muons that had been fired at hydrogen and replaced the electron, to jump to a higher energy state within the atom and fall down again, releasing light that could be measured.
Previous measurements of the proton’s radius gave a value of 8.768 x 10^(-16) m. This measurement had held roughly right to within a percent or so for the past fifty years. This measurement, however, suggests that it is four percent out – such a serious discrepancy that some aspects of theoretical physics – such as Quantum ElectroDynamics – that relied on the previous measurement being correct to within its error bars may have a little bit of correcting to do. That in turn, and along with the neutrino mass difference suggestion, could well mean there’s an impact on the Standard Model of particle physics.
So long as it can all be confirmed.