A proton slams into a deuteron — a tiny bundle of a proton and a neutron. In the past, such collisions were described by equations where time flows the same for everyone, as in everyday life. But when particles are accelerated to near-light speeds, time slows down for them and lengths contract. It's like two runners on a track measuring the distance with different rulers.
Physicists took two sets of rules: the old Newtonian one and a new relativistic one that accounts for time distortion. They calculated how a proton scatters off a deuteron at various energies. It turned out that at 135 MeV (roughly the energy of a proton accelerated to 15% of the speed of light), the difference is visible when the proton bounces backward. At energies above 300 MeV, differences also appear in how the particles spin: their polarization.
The result is like a map with two different projections: over a small area they nearly coincide, but over a large one they diverge. Same here: up to 300 MeV the old calculations still work, but above that they don't. So for future accelerator experiments, we need to switch to relativistic equations to avoid missing the mark in predictions.
The deuteron is the only stable nucleus made of two different particles: a proton and a neutron. It is so loosely bound that its size is three times that of an ordinary hydrogen nucleus.