qbar

Qbar, denoted with a bar over the letter (Q̄), is the standard symbol in particle physics for the antiquark of a quark Q. The bar indicates antiparticle status, with opposite flavor quantum numbers and color other than the quark’s. In quantum chromodynamics (QCD) quarks are color triplets and antiquarks are color antitriplets, and Q̄ participates in strong interactions as the antiparticle counterpart of Q.

In hadronic systems, Q̄ combines with a quark Q to form mesons. Quark–antiquark bound states of the same flavor, such as charmonium (c c̄) and bottomonium (b b̄), are called quarkonium and are flavor-neutral. When Q̄ binds with a light quark q (u, d, or s), it forms heavy-light mesons, including the B mesons (for example, B+ = u b̄, B0 = d b̄, and Bs = s b̄). These states are central to studies of flavor physics, CP violation, and the parameters of the Cabibbo–Kobayashi–Maskawa (CKM) matrix.

Top quarks present a special case: they decay before hadronizing, so a top antiquark (t̄) is not observed as a bound state in nature. Nevertheless, the Q̄ concept remains fundamental for describing the full spectrum of heavy-flavor phenomena and for theoretical treatments of quarkonia and heavy-light mesons.

Theoretical frameworks such as nonrelativistic QCD (NRQCD) and heavy quark effective theory (HQET) leverage the properties of heavy quarks and their antiquarks to simplify calculations and interpret experimental results. Lattice QCD and other nonperturbative methods often include Q̄ in simulations of hadron masses and decays.