Total cross section of thepp→ppηreaction near threshold

The $\mathrm{pp}\to \mathrm{pp}\eta$ reaction is studied at energies near the $\eta$ production threshold. The total cross sections at nominal machine energies of 1260, 1265, and 1300 MeV are 90±15, 790±120, and 3460±690 nb, respectively. None of the existing perturbative model calculations reproduces the energy dependence, which deviates strongly from phase space. This suggests that the cross section is enhanced in the near vicinity of the production threshold by a large $\eta -pp$ scattering length.