B meson

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.

Each B meson has an antiparticle that is composed of a bottom quark and an up (
B−
), down (
B0
), strange (
B0
s) or charm antiquark (
B−
c) respectively.

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Wikipedia