Study of the hadronic production of kaon pairs below the threshold for the ϕ meson

The near threshold production of K+K− pairs in proton-proton collisions has been investigated at the cooler synchrotron COSY below and above the threshold for the ϕ meson using the COSY-11 and ANKE facilities, respectively. The excitation function determined for the pp→ppK+K− reaction revealed a statistically significant enhancement close to the threshold which may plausibly be assigned to the influence of the K−p interaction. In addition, observed consistently by both groups, a strong enhancement at low values of the ratio of the K−p to K+p invariant mass distributions shows that the proton interacts much stronger with K− than with K+. In this report we focus on the measurements performed by the COSY-11 collaboration. We explain the experimental method used and present main results of completed analyses as well as a new qualitative elaboration of the ppK+K− events population on the Goldhaber plot. We conclude with the observation that event densities increase at the region where the influence from the K+K− interaction is expected.