The researchers focused on how often quarks turned into positively or negatively charged pions (a type of subatomic particle), depending on whether they came from protons or deuterons. This process, called fragmentation, gives physicists clues about how quarks behave when they’re released from the tight grip of the strong nuclear force.
Now here’s where the symmetry jumps in. According to a principle called charge symmetry, an up quark in a proton should behave the same way as a down quark in a neutron, once you flip the charge. That’s been a helpful assumption for decades because it simplifies calculations.
However, until now, this idea hadn’t been tested carefully in the context of fragmentation. When the researchers compared the behavior of these quarks, they found small but clear deviations, especially at lower energy levels.