No, the weak interaction does not always involve leptons. While the weak interaction is most commonly associated with interactions involving leptons, such as beta decay where a neutron decays into a proton, an electron, and an electron antineutrino, it can also involve quarks.
The weak interaction is one of the four fundamental forces of nature, along with gravity, electromagnetism, and the strong interaction. It is responsible for certain types of radioactive decays and interactions that involve particles with flavor-changing processes.
In addition to leptons, the weak interaction can also involve quarks in a process known as weak flavor-changing neutral currents (FCNCs). These interactions involve the exchange of W and Z bosons, which are the carriers of the weak force. Quarks can change their flavor during weak FCNCs, resulting in processes such as quark flavor oscillations.
Examples of weak FCNCs involving quarks include processes like neutral meson oscillations (such as the neutral kaon or B meson oscillations) and rare decays of particles containing quarks, where a quark changes its flavor during the decay process.
So, while the weak interaction is often associated with leptons, it can also involve quarks through weak flavor-changing neutral currents, allowing for flavor-changing interactions and processes in the realm of quark physics.