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

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 ρ00 is measured at midrapidity (|y|<0.5) in Pb-Pb collisions at a center-of-mass energy (sNN) of 2.76 TeV with the ALICE detector. ρ00 values are found to be less than 1/3 (1/3 implies no spin alignment) at low transverse momentum (pT<2 GeV/c) for K*0 and ϕ at a level of 3σ and 2σ, respectively. No significant spin alignment is observed for the KS0 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.

Note:

19 pages, 3 captioned figures + 1 figure and 1 table in appendix, authors from page 11, published version, figures at http://alice-publications.web.cern.ch/node/5657

Nuclear Physics
spin: alignment
heavy ion: scattering
spin: density matrix
polarization: measured
decay: angular distribution
quark: polarization
vector meson
angular momentum
CERN LHC Coll

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See the Supplemental Material at for complete angular distribution, consistency checks, analytical relation between EP and PP, toy model simulation to show the relation between EP, PP and RNDEP, and centrality classes