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| η_c | |
|---|---|
| Name | η_c |
| Composition | charm quark and charm antiquark (c c̄) |
| Mass | 2983.9 MeV/c^2 (approx.) |
| Width | 32 MeV (approx.) |
| Discovered | 1980 |
| Discovered at | SLAC National Accelerator Laboratory, DESY, Cornell University |
η_c
The η_c is the ground-state pseudoscalar meson composed of a charm quark and a charm antiquark. It occupies a central role in studies of Quantum Chromodynamics, heavy-quark bound states, and the spectroscopy of the charmonium family alongside counterparts such as the J/ψ and ψ(2S). Precision measurements of its mass, width, and decay channels provide tests of potential models, lattice Quantum Chromodynamics (QCD), and effective field theories like Non-relativistic QCD.
The η_c is the lightest spin-singlet state of the charmonium system, often denoted as the 1^1S_0 state in spectroscopic notation. It is neutral and short-lived, decaying predominantly via the strong and electromagnetic interactions into multihadron final states. The η_c sits below open-charm thresholds such as the D^0–D̄^0 threshold, making it a bound c c̄ resonance whose properties are sensitive to confinement dynamics encoded in Quantum Chromodynamics (QCD). Studies of the η_c interface with experimental programs at facilities including Large Hadron Collider, KEK, BESIII, and historical data from CLEO and BaBar.
As a 1^1S_0 pseudoscalar, the η_c has quantum numbers J^PC = 0^−+. Its measured mass around 2.98 GeV/c^2 and natural width of order tens of MeV are benchmarks for theoretical models. The level structure of charmonium places the η_c below the vector J/ψ (1^3S_1) by the hyperfine splitting, a quantity computed in potential models and lattice Quantum Chromodynamics (QCD) calculations. Radial and orbital excitations related to the η_c include the η_c(2S) and higher pseudoscalar states observed in various experiments. Hyperfine splittings connect to parameters such as the charm quark mass and spin-spin interaction terms in potential models like the Cornell potential and in effective theories such as Non-relativistic QCD. Comparisons with bottomonium states like the η_b inform heavy-quark symmetry tests and scaling relations predicted by perturbative Quantum Chromodynamics (QCD).
η_c production occurs in multiple environments: prompt production in hadron colliders like the Large Hadron Collider, two-photon fusion in e^+e^- colliders such as LEP and KEKB, radiative transitions from higher charmonium states like ψ(2S) and χ_cJ decays, and B-meson decays at experiments like Belle and BABAR. Decay channels are dominated by gluon-rich multihadron final states, for example into light mesons such as K^+, K^-, π^+, π^-, and resonant modes involving η and η' mesons. Electromagnetic decays include two-photon annihilation η_c → γγ, which provides a clean probe of the wavefunction at the origin and is measured at e^+e^- facilities. Hadronic decays probe nonperturbative Quantum Chromodynamics (QCD) dynamics and are used to study intermediate resonances like the f_0(980) or ρ(770). Branching fractions and differential cross sections measured across experiments constrain fragmentation functions, color-octet matrix elements in Non-relativistic QCD, and production mechanisms at colliders including CERN experiments.
The η_c was first reported in 1980 in radiative transitions from the J/ψ at experiments operating at SLAC National Accelerator Laboratory, DESY, and Cornell University. Subsequent confirmations and precision measurements came from collaborations such as MARK III, Crystal Ball, CLEO, BaBar, Belle, and BESIII. Modern determinations of mass and width benefit from high-statistics data samples collected at BEPCII and the Large Hadron Collider, where experiments like LHCb have measured production cross sections and differential distributions. Two-photon production studies at LEP and KEKB provided measurements of the two-photon decay width Γ(η_c → γγ). Discrepancies among early measurements of the η_c mass and width prompted global fits and reanalyses that incorporated detector effects and interference with continuum backgrounds, improving agreement with lattice Quantum Chromodynamics (QCD) predictions.
The η_c is interpreted within potential models of heavy quarkonia such as the Cornell potential and relativized quark models that include spin-dependent forces. Lattice Quantum Chromodynamics (QCD) provides ab initio computations of the η_c mass spectrum, hyperfine splitting, and decay constants, offering tests of discretization and heavy-quark actions. The framework of Non-relativistic QCD factorization describes production at high energies, partitioning short-distance coefficients calculable in perturbative Quantum Chromodynamics (QCD) from long-distance matrix elements. Studies of η_c decays confront challenges in modeling hadronization and final-state interactions, often invoking methods from chiral perturbation theory for light hadron dynamics and QCD sum rules for nonperturbative parameters. Comparisons with potential-model expectations for hyperfine splittings and with measurements of the η_c(2S) inform understanding of spin-dependent operators in the effective Hamiltonian.
The η_c serves as a laboratory for testing Quantum Chromodynamics (QCD) in the heavy-quark regime and calibrating theoretical tools used across particle physics. Precision η_c observables constrain the charm quark mass, α_s determinations, and validate lattice Quantum Chromodynamics (QCD) techniques employed for flavor-physics quantities relevant to CKM matrix studies at LHCb, Belle II, and BESIII. Measurements of two-photon widths and radiative transitions impact searches for exotic states such as tetraquarks and hybrids investigated at GlueX and PANDA. Moreover, η_c production data at hadron colliders inform background modeling for heavy-flavor and quarkonium measurements at CERN experiments and complement spectroscopy programs at international facilities like FAIR and J-PARC.