On one side of the scale lies all the matter in the Universe; on the other, all the antimatter. According to the laws of physics, at the moment of the Universe's birth, the pans should have been balanced. But we see one pan tipping: around us are stars, planets, people, and almost no antimatter. This imbalance is one of the greatest mysteries.

Physicists suspect the answer lies in the violation of a rule that forbids changing the difference between the number of heavy particles (baryons, such as protons and neutrons) and light ones (leptons, for example, electrons). This rule is abbreviated as B−L. If it is violated, the scales could have tipped in favor of matter. But how to catch such a violation? One way is to look for particle decays that are forbidden when B−L is conserved.

The authors of the study considered one such exotic decay: a neutron transforms into a kaon (a mediator particle in strong interactions) and a charged lepton. It turns out that a crucial clue lies hidden here. If B−L is conserved, such a decay is only possible under very complex, unlikely conditions, like opening a door not with a key but by assembling a mechanism of a dozen gears. But if B−L is violated, the door opens much more easily, with a half-turn. Therefore, if an experiment sees this decay and does not see other, simpler forbidden decays (for example, a proton decaying into a pion and a lepton), scientists can say with high confidence that B−L is violated. And the most interesting part is that they don't even need to know the charge of the emitted lepton, whether it's positive or negative.

Thus, the scales of the Universe might finally tell us why the matter pan turned out to be heavier.