X3872

X(3872) is a charmonium-like hadron first observed in 2003 by the Belle collaboration in B meson decays, with the decay X(3872) → J/ψ π+ π−. It has since been confirmed by several experiments, including BaBar, CDF, D0, LHCb, CMS, and ATLAS. The state has a mass of about 3871.6 MeV, very close to the threshold for D0 and D*0 meson pair production, and it is anomalously narrow, with a width less than a few MeV. The quantum numbers have been measured as JPC = 1++, making it consistent with a vector-like state with positive charge parity.

The X(3872) does not fit neatly into the conventional charmonium spectrum and is often described as an exotic hadron. Its decays to J/ψ π+ π− (via a ρ0) and to J/ψ γ indicate a cc̄ component with positive C-parity, while decays to D0 D̄*0 (near threshold) and isospin-violating transitions to J/ψ ρ and J/ψ ω suggest a more complex structure. The leading interpretations include a loosely bound D0 D̄*0 molecular state, a tetraquark configuration, or a charmonium–molecule admixture.

X(3872) has become a benchmark in the study of XYZ states, highlighting the possibility of hadrons beyond the conventional quark-antiquark picture. Its production in both B decays and high-energy collisions further informs models of hadron formation and the interactions of charm quarks with light quarks.