CPT, Majorana fermions, and particle physics beyond the Standard Model

After reviewing charge conjugation and the CPT theorem, we define Majorana fermions and clarify the relationship of Majorana, Weyl, and Dirac fields. Appearance of Majorana fermions in various scenarios of physics beyond the Standard Model is discussed, including neutrino masses, baryon asymmetry of the universe, grand unified theories, and supersymmetry.