Measurement of polarization observables of the associated strangeness production in proton proton interactions

The $ \Lambda$ polarization, the analyzing power, and the $ \Lambda$ spin transfer coefficient of the reaction pp $ \rightarrow$ pK + $ \Lambda$ were measured at beam momenta of 2.70GeV/c and 2.95GeV/c corresponding to excess energies of 122MeV and 204MeV. While the analyzing power and the spin transfer coefficient do not change significantly with the excess energy, the $ \Lambda$ polarization varies strongly and changes its sign. As this is the first measurement of polarization observables below an excess energy of 200MeV, the change of the sign of the $ \Lambda$ polarization was not observed before. The high statistics of the data ( $ \approx$ 200 k events for each momentum) enables detailed studies of the dependence of the $ \Lambda$ polarization and the analyzing power on the center-of-mass momentum of the particles. The results of the spin transfer coefficient are in qualitative agreement with the DISTO experiment. The $ \Lambda$ polarization data of 2.95GeV/c are only conform with the DISTO experiment, while both the 2.70GeV/c and 2.95GeV/c data differ strongly from all previous measurements, whether exclusive or inclusive