Heavyquark

Heavy quarks are quarks whose masses lie well above the QCD scale, Λ_QCD. The characteristic heavy quarks are charm (c), bottom (b), and top (t). Their approximate masses are m_c ≈ 1.3 GeV, m_b ≈ 4.2 GeV, and m_t ≈ 173 GeV; notably, the top quark is so heavy that it typically decays via the weak interaction before forming hadrons.

In hadrons containing a single heavy quark, quantum chromodynamics (QCD) simplifies because the heavy quark acts as a static color source. Theoretical tools such as Heavy Quark Effective Theory (HQET) and heavy quark symmetry apply when m_Q ≫ Λ_QCD. These frameworks allow systematic expansions in 1/m_Q and relate properties across members of heavy quark multiplets, reducing the number of independent hadronic form factors.

Heavy quarks form mesons (for example D, B, and Bs mesons) and baryons (such as Λc and Ξb). Their spectroscopy and weak decays provide tests of QCD and CKM phenomenology. The heavy quark mass influences the dynamics of the light degrees of freedom inside the hadron, while decay rates and form factors reflect both perturbative and nonperturbative QCD effects.

Production and decay of heavy quarks occur in high-energy collisions, including proton-proton and electron-positron colliders. Charm and bottom hadrons decay via weak interactions and typically have measurable lifetimes, whereas top quarks decay too rapidly to form hadrons. Experimental measurements of their decays constrain CKM matrix elements and CP-violating phases.

Studying heavy quarks tests perturbative and nonperturbative QCD, informs lattice QCD calculations, and constrains physics beyond the Standard Model. Their masses are quoted in schemes such as MS-bar or pole mass, serving as essential inputs for particle phenomenology.