Measurement of the polarization parameter in π−p elastic scattering at 40 GeV/c

We report our first measurements of the polarization in the elastic scattering of negative pions from polarized protons at an incident pion momentum of 40 GeV/c. The momentum-transfer region covered was 0.08 < |t| < 1.3 (GeV/c)². The angular distribution of the polarization exhibits a first minimum of ～ ? 5% and the well-known zero around t ≈ ? 0.6 (GeV/c)². The energy variation of the first minimum (at around t = ? 0.2) may be expressed in a simple form, $\text{P}{}_{\mathrm{<mtext>avr</mtext>}}\text{= ?(0.48±0.06) s}{}^{\mathrm{?0.52\pm 0.05}}$.