Flavour-changing neutral current (FCNC) refers to a type of elementary particle interaction that involves the transformation of one type of quark into another by the exchange of a neutral force carrier, such as a photon or a Z boson. These interactions are not allowed in the Standard Model of particle physics at tree level, as they violate conservation of flavor. However, FCNC processes can occur at loop level, providing an important testing ground for physics beyond the Standard Model. Researchers study FCNC processes to better understand the fundamental forces and particles that make up the universe and to search for new physics phenomena.