Final answer:
In particle interactions, the conservation of lepton number (Le, Lu, Lτ) is a fundamental principle that ensures the total lepton numbers for electrons, muons, and taus remain constant before and after an interaction.
Step-by-step explanation:
In particle interactions, the rule about leptons is related to the conservation of lepton number. This principle is known as lepton number conservation, asserting that the lepton numbers remain the same before and after any interaction. Specifically, there are three types of lepton numbers to consider: the electron-lepton number (Le), the muon-lepton number (Lu), and the tau-lepton number (Lτ). Each of these numbers must be conserved individually in any particle interaction. For example, when an electron is created in a reaction, an antielectron neutrino must also be produced to ensure Le remains constant. Similar principles apply for muon-related and tau-related interactions where Lμ and Lτ must be conserved respectively.
Leptons, which include electrons, muons, and tau particles, as well as their corresponding neutrinos, do not interact via the strong force but are subject to weak, electromagnetic, and gravitational forces. These particles are fundamental and are considered pointlike with no discernible size beyond their wavelength. The understanding and application of lepton number conservation are crucial in predicting and verifying the outcome of particle interactions in physics.