Particle physics is the study of the qualities of atoms and molecules and the way they behave and react. The branch of physics that deals with subatomic particles. the study of the way that parts of atoms develop and behave. or high-energy physics Study of the fundamental subatomic particles, including both matter (and antimatter) and the carrier particles of the fundamental interactions as described by quantum field theory. Particle physics is concerned with structure and forces at this level of existence and below. Fundamental particles possess properties such as electric charge, spin, mass, magnetism, and other complex characteristics, but are regarded as pointlike. All theories in particle physics involve quantum mechanics, in which symmetry is of primary importance. See also electroweak theory, lepton, meson, quantum chromodynamics, quark