Measurement of the Cross-Section for the Process GammaGamma -> Ppbar at   sqrt(s)_ee=183-189 GeV at LEP
  The exclusive production of proton-antiproton pairs in the collisions of two quasi-real photons had been studied using data taken at sqrt(s)_ee=183 GeV and 189 GeV with the OPAL detector at LEP. Results are presented for Ppbar invariant masses, W, in the range 2.15 W< <3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on the quark-diquark model. 
The OPAL collaboration, G. Abbiendi, et al