Precise measurement of spin-averaged ${\ensuremath{\chi}}_{cJ}(1P)$ mass using photon conversions in $\ensuremath{\psi}(2S)\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\chi}}_{cJ}$

Precise measurement of spin-averaged $χ_{c J} (1 P)$ mass using photon conversions in $ψ (2 S) \to γ χ_{c J}$

M. Ablikim et al. (BES Collaboration)

Phys. Rev. D 71, 092002 – Published 11 May 2005

Abstract

Using photon conversions to $e^{+} e^{-}$ pairs, the energy spectrum of inclusive photons from $ψ (2 S)$ radiative decays is measured with photon energy resolution ( $σ_{E_{γ}}$ ) in the range from 2.3 to 3.8 MeV by BESII at the Beijing Electron-Positron Collider. The $χ_{c J} (1 P)$ states ( $J = 0, 1, 2$ ) are clearly observed, and their masses and the spin-averaged $χ_{c J}$ mass are determined to be $M_{χ_{c 0}} = 3414.21 \pm 0.39 \pm 0.27$ , $M_{χ_{c 1}} = 3510.30 \pm 0.14 \pm 0.16$ , $M_{χ_{c 2}} = 3555.70 \pm 0.59 \pm 0.39$ , and $M ({}^{3}P_{cog}) = 3524.85 \pm 0.32 \pm 0.30 MeV / c^{2}$ , respectively.