Study of the Eta(c)(1 S-1(o)) State of Charmonium Formed In Pp Annihilations and A Search For the Eta'(c)(2 S-1(o))

The E760 Collaboration performed an experiment in the Antiproton Accumulator at Fermilab to study the two photon decay of the eta(c)(1(1)S(0)) charmonium state formed in ($) over bar pp annihilations. This resulted in a new measurement of the mass M(eta c) = 2988.3(-3.1)(+3.3) MeV/c(2) and of the product B(eta(c) --> ($) over bar pp) x Gamma(eta(c) --> gamma gamma) = (8.1(-2.0)(+22.9)) eV. We performed a search for the process ($) over bar pp --> eta(c)'(2(1)S(0)) --> gamma gamma over a limited range of center-of-mass energies. Since no signal was observed, we derived upper limits on the product of branching ratios B(eta(c)' --> ($) over bar pp) x B(eta(c)' --> gamma gamma) in the center-of-mass energy range 3584 less than or equal to root s less than or equal to 3624 MeV. We observed no signal for the nonresonant process ($) over bar p + p --> gamma + gamma and obtain upper limits.