text: The semileptonic decay of this particle was studied by the CPLEAR experiment and is currently being studied by the NA62 experiment at CERN.
full_question: The semileptonic decay of this particle was studied by the CPLEAR experiment and is currently being studied by the NA62 experiment at CERN. This particle is a member of both an octet of pseudoscalar particles and an octet of vector particles. The branching ratio for the FCNC-mediated decay of this particle was explained by the GIM mechanism, which predicted the existence of the charm quark. This particle can undergo particle-antiparticle oscillations with a frequency proportional to the difference between the masses of its "short" and "long" lifetime variants. This particle was originally thought to be two particles termed the tau and the theta due to this particle's ability to decay into either two or three pions. That decay was studied experimentally by Cronin and Fitch to confirm the existence of CP violation. This particle consists of a either an up or a down quark or antiquark and a strange quark or antiquark. For 10 points, name this meson which is lighter than the D and B mesons but heavier than the pion.
first_sentence: The semileptonic decay of this particle was studied by the CPLEAR experiment and is currently being studied by the NA62 experiment at CERN.
raw_answer: kaons [or K mesons]