Spin alignment of vector mesons in heavy ion and proton-proton collisions

The spin alignment matrix element rho(00) for the vector mesons K*(0) and phi(1020) has been measured in RHIC at central rapidities. These measurements are consistent with the absence of polarization with respect to the reaction plane in mid-central Au + Au collisions whereas, when measured with respect to the production plane in the same reactions and in p + p collisions, a non-vanishing and p(perpendicular to)-dependent rho(00) is found. We show that this behavior can be understood in a simple model of vector meson production where the spin of their constituent quarks is oriented during hadronization as the result of Thomas precession. (C) 2009 Elsevier B.V. All rights reserved.