A property of certain elementary particles that is expressed as a quantum number (see atom) and is used in the quark model. It was suggested to account for the unusually long lifetime of the psi particle. In this theory the three original quark–antiquark pairs were supplemented by a fourth pair – the charmed quark and its antiquark. The psi particle itself is a meson having zero charm as it consists of the charmed pair. However, charmed hadrons do exist; they are said to possess naked charm. Charm is thought to be conserved in strong and electromagnetic interactions.