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?
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