In particle physics, B mesons are mesons composed of a bottom antiquark and either an up ( B+ ), down ( B0 ), strange ( B0 s) or charm quark ( B+ c). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime. The combination of a bottom antiquark and a bottom quark is not a B meson, but rather bottomonium which is something else entirely.