FTP, name this meson.
The decay of this type of particle is one of the few physical  processes not to exhibit time reversal invariance. Four types  exist; the two electrically charged are antiparticles of each  other, as are the two neutral. Classified as both a boson and a  hadron, their mass is between that of an electron and of a proton.  FTP, name this meson.
The decay of this type of particle is one of the few physical  processes not to exhibit time reversal invariance.
{kaon} or {K}-meson (prompt on "meson")