Evidence of Spin-Orbital Angular Momentum Interactions in Relativistic Heavy-Ion Collisions

The first evidence of spin alignment of vector mesons (K*(0) and phi) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element rho(00) is measured at midrapidity (vertical bar y vertical bar < 0.5) in Pb-Pb collisions at a center-of-mass energy root(s) of 2.76 TeV with the ALICE detector. rho(00) values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum (p(T) < 2 GeV / c) for K*(0) and phi at a level of 3 sigma and 2 sigma, respectively. No significant spin alignment is observed for the K-S(0) meson (spin = 0) in Pb-Pb collisions and for the vector mesons in pp collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.