Cross sections and polarizations in the reactionsπ−p→ΛK0andπ−p→ΛK*0(890)near the backward direction, from 3 to 6 GeV/c

Abstract

We have measured the differential cross sections and $\ensuremath{\Lambda}$ polarizations in the reactions ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{0}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{*0} (890)$ near the backward direction, at 3, 4, 5, and 6 GeV/c. Data equal to several times the world's total sample above 2 GeV/c were recorded. Both reactions are characterized by cross sections falling rapidly with beam momentum, and by large positive $\ensuremath{\Lambda}$ polarizations for ${u}^{\ensuremath{'}}$ between 0.0 and 0.6 ${\mathrm{GeV}}^{2}$. Analysis of ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{0}$ yields an effective Regge trajectory consistent with antishrinkage of the backward peak. Separation into amplitudes of definite-parity-naturality exchange shows the reaction to be dominated by unnatural-parity exchange. The energy behavior of this exchange is, however, not consistent with a single linear baryon Regge trajectory or exchange-degenerate pair of trajectories. An apparent normalization discrepancy between data on ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\Lambda}{K}^{0}$ of a CERN-ETH group and other high-statistics data including that of this experiment is discussed.