The reaction pp->{pp}_s\gamma, where {pp}_s is a proton pair with an excitation energy E_{pp}<3 MeV, has been observed with the ANKE spectrometer at COSY-Juelich for proton beam energies of T_p=0.353, 0.500, and 0.550 GeV. This is equivalent to photodisintegration of a free 1S_0 diproton for photon energies E\gamma ~ T_p/2. The differential cross sections measured for c.m. angles 0 deg.<\theta_{pp}<20 deg. exhibit a steep increase with angle that is compatible with E1 and E2 multipole contributions. The ratio of the measured cross sections to those of np->d\gamma is on the 10^{-3}-10^{-2} level. The increase of the pp->{pp}_s\gamma cross section with T_p might reflect the influence of the Delta(1232) excitation.
@article{arxiv.0806.0648,
title = {Observation of inverse diproton photodisintegration at intermediate energies},
author = {V. Komarov and T. Azaryan and D. Chiladze and S. Dymov and M. Hartmann and A. Kacharava and I. Keshelashvili and A. Khoukaz and A. Kulikov and V. Kurbatov and G. Macharashvili and S. Merzliakov and S. Mikirtychiants and M. Papenbrock and M. Nekipelov and F. Rathmann and V. Serdyuk and H. Stroeher and D. Tsirkov and Yu. Uzikov and C. Wilkin},
journal= {arXiv preprint arXiv:0806.0648},
year = {2008}
}