Evidence for antikaon mass modification in proton-nucleus collisions

Yields of K+ and K- mesons from proton-nucleus collisions at the subthresho ld energy range were measured. Kaons produced at 10.5 degrees with momentum 1.28 GeV/c were detected by a beam-line focusing hadron spectrometer (FHS) . The parameters of the Folding model were fixed by the best fit to the Kdata. Calculation with normal antikaon mass underestimates the experimental cross section of K- production by a factor of 2.5. In-medium antikaon mass was found to be 0.6-0.85 of its vacuum mass depending on the parametrizati on of the total cross section:for K- production in pN collisions.