Precise measurement of spin-averaged chi_{cJ}(1P) mass using photon   conversions in psi(2S) -> gamma chi_{cJ}
  Using photon conversions to e^+e^- pairs, the energy spectrum of inclusive photons from psi(2S) radiative decays is measured by BESII at the Beijing Electron-Positron Collider. The chi_{cJ}(1P) states (J=0,1,2) are clearly observed with energy resolution between 2.3 to 3.8 MeV, and their masses and the spin-averaged chi_{cJ} mass are determined to be M_{chi_{c0}}=3414.21\pm 0.39 \pm 0.27, M_{chi_{c1}}=3510.30\pm 0.14\pm 0.16, M_{chi_{c2}}=3555.70\pm 0.59 \pm 0.39 and M(^3P_{cog})=3524.85\pm 0.32 \pm 0.30 MeV/c^2. 
M. Ablikim, J. Z. Bai, et al