Evidence of spin-orbital angular momentum interactions in relativistic heavy-ion collisions

Abstract

The first evidence of spin alignment of vector mesons ( K * 0 and ϕ ) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ρ 00 is measured at midrapidity ( | y | < 0.5 ) in Pb-Pb collisions at a center-of-mass energy ( √ s NN ) of 2.76 TeV with the ALICE detector. ρ 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 ϕ at a level of 3 σ and 2 σ , respectively. No significant spin alignment is observed for the K 0 S meson ( spin = 0 ) in Pb-Pb collisions and for the vector mesons in p p 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.