Psi2S

Psi2S, also written as psi(2S) and commonly called psi prime, is a charmonium state formed by a charm quark and an anti-charm quark bound together in a radial excitation of the J/psi. It is a vector meson with quantum numbers J^PC = 1^−−. As the first radial excitation of the J/psi, psi2S is an important element in the study of heavy-quark bound systems and tests of quantum chromodynamics (QCD) in the nonperturbative regime.

The mass of psi2S is about 3.686 GeV/c^2, placing it below the threshold for decays into open-charm meson pairs. Because of this, its strong decays to pairs of D mesons are kinematically forbidden, and its decays proceed mainly through radiative transitions to lower charmonium states or via hadronic transitions mediated by gluons. Typical decay channels include radiative decays to chi_cJ states (via the emission of a photon) and hadronic transitions to J/psi accompanied by light hadrons, such as pi+ pi- J/psi. Additionally, psi2S can decay to light hadrons through annihilation into gluons, though these modes are comparatively suppressed relative to transitions to lower charmonium.

Psi2S has been extensively studied in e+e− annihilation and in B-meson decays, with measurements conducted by a range of experiments at facilities such as CLEO, BESIII, BaBar, Belle, and others. Its properties and decays provide important tests of quarkonium potential models, lattice QCD calculations, and the understanding of strong interaction dynamics in heavy-quark systems. The psi(2S) state, together with the J/psi, helps map the charmonium spectrum and helps calibrate techniques used in heavy-quark spectroscopy.