Observation of the spin alignment of the ϱ0-meson produced in ¯pp interactions at 22.4 and 5.7 GeV/c

Studying the ϱ0-meson production in ¯pp interactions at 22.4 GeV/c and in 4-prong annihilation channels of ¯pp interactions at 5.7 GeV/c, we have observed an essential ϱ0-meson spin alignment. The values of the ϱ00 element of the ϱ0-meson spin density matrix (thez-axis is directed along the normal to the production plane) are equal to 0.08 ± 0.07 and 0.55 ± 0.03, respectively, i.e. the ϱ0-meson spin lies preferably in the production plane. The absence of such an effect in pp interactions at 12 and 24 GeV/c and also the essentially larger ϱ0 production cross section in ¯pp interactions at these energies make it possible to connect the observed ϱ0-meson spin alignment with the annihilation processes. The character of the observed spin alignment is unexpected from the point of view of usual models, e.g. multiperipheral models. This effect could be described by the spontaneous polarization of quarks and antiquarks during the state preceding their recombination into mesons.