Measurements of the mass and width of the eta_c using psi' -> gamma   eta_c
  The mass and width of the lowest lying S-wave spin singlet charmonium state, the eta_c, are measured using a data sample of 1.06x10^8 psi' decays collected with the BESIII detector at the BEPCII storage ring. We use a model that incorporates interference between the signal reaction, psi' -> gamma eta_c, and a non-resonant radiative background to successfully describe the line shape of the eta_c. We measure the eta_c mass to be 2984.3 +- 0.6 +- 0.6 MeV/c^2 and the total width to be 32.0 +- 1.2 +- 1.0 MeV, where the first errors are statistical and the second are systematic. 
BESIII Collaboration: M. Ablikim, M. N. Achasov, D. Alberto, D. J.   Ambrose, F. F. An, Q. An, Z. H. An, J. Z. Bai, R. B. Fe...
hep-ex