Two-photon widths of the χcJ states of charmonium

Using a data sample of 24.5 x 10(6) psi(2S) the reactions psi(2S) -> gamma chi(cJ), chi(cJ) -> gamma gamma have been studied for the first time to determine the two-photon widths of the chi(cJ) states of charmonium in their decay into two photons. The measured quantities are B(psi(2S) - gamma chi(c0)) x B(chi(c0) -> gamma gamma) = (2.17 +/- 0.32 +/- 0.10) x 10(-5) and B(psi(2S) -> gamma chi(c2)) x B(chi(c2) -> gamma gamma ) = (2.68 +/- 0.28 +/- 0.15) x 10(-5). Using values for B(psi(2S) -> gamma chi(c0),(c2)) and Gamma(chi(c0,c2)) from the literature the two-photon widths are derived to be Gamma gamma gamma(chi(c0)) = (2.36 +/- 0.35 +/- 0.22) keV, Gamma(gamma gamma)(chi(c2)) = (0.66 +/- 0.07 +/- 0.06) keV, and R equivalent to Gamma(gamma gamma)(chi(c2))/Gamma(gamma gamma)(chi(c0)) = 0.278 +/- 0.050 +/- 0.036. The importance of the measurement of R is emphasized. For the forbidden transition, chi(c1) -> gamma gamma, an upper limit of Gamma(gamma gamma)(chi(c1)) < 0.03 keV is established.