Protonium annihilation into KSKL at three different target densities

The frequency of the protonium annihilation channel <(p)over bar p> --> KSKL has been measured at three different target densities: liquid hydrogen (LH), gaseous hydrogen at NTP conditions and gaseous hydrogen at low pressure (5 mbar). The obtained results are: f(<(p)over bar p> --> KSKL, LH) = (7.8 +/- 0.7(stat) +/- 0.3(sys)) x 10(-4), f(<(p)over bar p> --> KSKL, NTP) = (3.5 +/- 0.5(stat) +/- 0.2(sys)) x 10(-4) and f(<(p)over bar p> --> KSKL, 5 mbar) = (1.0 +/- 0.3(stat) +/- 0.1(sys)) x 10(-4). Since the KSKL final state can be originated only from the S-3(1) initial state, these values give direct information on the scaling of the protonium spin-triplet S-wave annihilation probability with the density.