Which particles are classified as leptons?

Leptons are classified as elementary particles that can exist independently. This definition is fundamental to understanding the nature of leptons within the Standard Model of particle physics. Leptons include, for example, the electron, muon, tau, and their corresponding neutrinos. Unlike hadrons, which are composite particles made up of quarks, leptons are not made of smaller constituents and can exist freely without needing to combine with other particles.

This characteristic distinguishes them from other particles, such as those composed of quarks, which cannot exist independently due to the confinement provided by the strong nuclear force. Additionally, while some particles might exist in pairs, this is not a defining characteristic of leptons. Leptons can be found singly, and their interactions are governed primarily by the weak nuclear force, not the strong nuclear force, which only affects particles that are hadrons. Thus, option B accurately reflects the essential definition of leptons in the context of fundamental particles in physics.