Alongside the proton? From ScienceDaily:
The deuteron — one of the simplest atomic nuclei, consisting of just one proton and one neutron — is considerably smaller than previously thought, say researchers who measured the proton and found a significantly smaller value than previous research did, using new experimental methods.
…
The new measurement of the deuteron’s size has now given rise to an analogous mystery. It is possible that this will lead to an adjustment of the Rydberg constant, a fundamental quantity in physics. Another possible explanation is that a physical force as yet unknown is at work. More.
See also: What no new particles means for physics?
Follow UD News at Twitter!