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| χ_bJ | |
|---|---|
| Name | χ_bJ |
| Other names | chi_b |
| Category | Bottomonium P-wave states |
| Constituents | b b̄ |
| Spin | 1 (triplet states J=0,1,2) |
χ_bJ The χ_bJ mesons are bottomonium P-wave triplet states observed in high-energy experiments. They play a central role in studies at facilities such as CERN, SLAC National Accelerator Laboratory, KEK, Fermilab, and DESY, and are discussed in the context of theoretical work by groups at MPI für Physik, Institute for High Energy Physics (Russia), and collaborations like ATLAS, CMS, LHCb, Belle, and BaBar. Measurements of χ_bJ relate to landmark topics including the Higgs boson era of colliders, precision tests of Quantum Chromodynamics, and inputs to global fits used by the Particle Data Group.
χ_bJ states are bottomonium states composed of a bottom quark and bottom antiquark in a P-wave configuration, forming a spin-triplet with total angular momentum J=0,1,2. Their spectroscopy complements studies of other quarkonium systems such as the J/ψ (particle), ψ′, Υ (1S), and exotic candidates like the X(3872). Experimental access to χ_bJ arises via radiative transitions from higher vector bottomonia (e.g., Υ(2S), Υ(3S)) and hadroproduction in collisions at Large Hadron Collider experiments.
The nomenclature χ_bJ follows the spectroscopic notation 1^3P_J for the lowest-triplet P-wave bottomonia, analogous to the charmonium χ_cJ family observed at SLAC and DESY. Quantum numbers include principal quantum number n, total spin S=1, orbital angular momentum L=1, and total J=0,1,2 with parity P=+ and charge conjugation C=+. Comparisons are often drawn with states such as h_b(1P), η_b(1S), and radial excitations like χ_bJ(2P), χ_bJ(3P) studied at LHCb and Belle II.
Production mechanisms include radiative transitions from Υ(nS) resonances produced at KEKB and PEP-II as well as prompt hadroproduction in proton–proton collisions at CERN machines. χ_bJ decays predominantly via electric dipole (E1) transitions to lower Υ states, emitting photons measured by electromagnetic calorimeters in detectors such as ATLAS, CMS, and CLEO. Other observed channels involve hadronic transitions related to multipole expansion calculations used by theorists at institutions like Cornell University and Princeton University. Branching fractions and photon spectra link to analyses by the BaBar Collaboration and the Belle Collaboration.
Initial evidence for χ_bJ came from experiments at Fermilab and SLAC, with precision spectroscopy advanced by CLEO and later by BaBar and Belle. At hadron colliders, the ATLAS Collaboration, CMS Collaboration, and LHCb Collaboration have reported signals in radiative Υ decays and inclusive production, often using triggers and reconstruction techniques developed with input from Brookhaven National Laboratory and Jefferson Lab instrumentation groups. Mass splittings among J states, hyperfine splittings with singlet partners like h_b(1P), and transition rates have been tabulated by the Particle Data Group and used to constrain potential models from groups at Caltech and MIT.
The χ_bJ spectrum provides testing grounds for potential models originating from work by Gordon Baym-style phenomenology, nonrelativistic QCD (NRQCD) frameworks developed by researchers at IHEP, and effective field theory approaches advanced at Harvard University and University of Chicago. Lattice QCD calculations from collaborations at Riken, FNAL and JLab have computed masses and matrix elements for χ_bJ states, confronting systematic uncertainties handled by the MILC Collaboration and the HPQCD Collaboration. Sum rules and perturbative QCD corrections from groups at CERN Theory and INSPIRE archives also contribute to predictions of radiative widths.
χ_bJ states anchor the P-wave sector of bottomonium, providing calibration points for potential models, spin-dependent interactions, and relativistic corrections. Their properties inform comparisons with charmonium P-wave states like χ_cJ studied by the BESIII Collaboration and impact interpretations of exotic candidates such as the Z_b(10610) and Z_b(10650) observed by Belle. Ongoing and planned measurements at LHC, Belle II, and future facilities such as the proposed Electron–Ion Collider will refine our understanding of χ_bJ and test predictions from lattice collaborations and phenomenology groups at Yale University and University of Oxford.