Recent results on pp → ppϕ/K+K− production and the momentum dependence of ϕ-meson nuclear transparency

Kaon pair production by 2.83 GeV protons has been investigated in two experiments using the ANKE magnetic spectrometer at COSY-Jülich: (1) Differential and total cross sections for the pp → ppK+ K− reaction are separated into kaon pairs arising from the decay of the ϕ-meson and the remainder. The measurements show that higher partial waves represent the majority of the pp → ppϕ cross section at an excess energy of 76 MeV. Taken together with existing low energy ANKE data, the energy dependence of the results seem to suggest some S-wave ϕp enhancement near threshold. The non-ϕ K+K− data can be described in terms of the combined effects of two-body final state interactions, using the same effective scattering parameters already determined from lower energy ANKE data. (2) The inclusive production of ϕ mesons on C, Cu, Ag, and Au targets has also been studied using the K+K− decay to detect the ϕ. The momentum dependence of the nuclear transparency ratio, the in-medium ϕ width, and the differential cross section for ϕ meson production at forward angles have been determined for these nuclei in the momentum range of 0.6 – 1.6 GeV/c. There are indications of a significant momentum dependence in the values extracted for the ϕ width, which on average exceed the free value by about an order of magnitude. The results are compared with data from other experiments at slightly higher momenta.