Evidence for higher twist mechanisms in high p_⊥π^-p events with prompt ρ⁰ production at 38 GeV/c

The π^-p interactions with at least one charged secondary produced at polar angle ∼90° in c.m.s. and having the transverse momentum above 1 GeV/c were investigated. The data were obtained using streamer chamber magnetic spectrometer RISK at 38 GeV/c π beam from Serpukhov accelerator. The analyzis of associated production in reconstructed events suggests, that if the transverse momentum of a pair of oppositely charged secondaries compensates the trigger particle p_⊥ practically completely, this pair is the product of the ρ⁰ decay in marked fraction of such events. We observed a large spin-alignment for the ρ⁰-mesons selected as described above: the probability of zero spin projection onto the normal to the ρ⁰ production plane is equal to ρ₀₀^T=0.86±0.23. The enhancement of the number of events, in which the ρ⁰ picks up practically full momentum transfer carrying by the exchange, and also the enlarged tensor polarization for the ρ⁰-mesons in these events could be qualitatively explained as manifestation of direct ρ⁰-production via the QCD higher twist processes in the high p_⊥π^-p collisions. At the same time, the observed effects are markedly larger than the values predicted with QCD model in which the higher twist corrections were included.