Spin Information from Vector-Meson Decay in Photoproduction

Abstract

For the photoproduction of vector mesons, physically meaningful single and double spin observables are defined in the γN center-of-mass system, which we prefer over extracting density matrix elements in the vector meson rest frame, since angle and energy evolution of spin observables can yield insights into special dynamics. Assuming negligible background amplitudes, we show that: (1) measuring the two pseudoscalar meson decay of a photoproduced ρ or φ does not determine the vector meson’s vector polarization, but only its tensor polarization and (2) vector meson decay into lepton pairs is also insensitive to the vector meson’s vector polarization, unless one measures the spin of one of the leptons. Similar conclusions hold for all double spin observables which involve observation of two-meson vector meson decay. To access the vector meson’s vector polarization, one therefore needs to either measure the spin of the decay leptons, make an analysis of background interference effects or relate the vector meson’s vector polarization to other accessible spin observables.

Talk presented by Frank Tabakin