4a. What are
B mesons?
guided tour
What is a meson? The term "meson" is used to refer to any particle that is made up of one quark (any flavor) and one antiquark (any flavor).  The quark and antiquark are bound together mainly by the strong force, and they orbit each other much as the earth and moon orbit each other.  Because they must obey the laws of quantum mechanics, the quark and antiquark can only orbit each other in a few specific ways, and each orbit corresponds to a different meson with different mass.  The lightest meson containing a given quark combination is known as the "pseudoscalar" state.
    A B meson consists of a b-antiquark (called "b-bar") and either a u- or d-quark (these are the two lightest quarks) and is a pseudoscalar state.  Its antiparticle, called the B antimeson or "B-bar" meson, is made up of a b-quark and a u- or d- antiquark (u-bar or d-bar).  Usually we lump the B and B-bar mesons together and just call them "B mesons," unless the discussion requires them to be distinguished from each other.
    The B meson is a relatively heavy particle, having a mass of 5.28 GeV/c2, which is more than five times the mass of the proton.  This is because the b-quark it contains is almost that massive.