Quarkonia, the bound states of charm and anti-charm or bottom and anti-bottom quarks, are an important tool to test our knowledge of quantum chromodynamics (QCD). At the LHC, the study of quarkonia polarisations offers a valuable new window onto how heavy quarks bind together in such states. Understanding quarkonium polarisation has already proven to be difficult at lower energies, however, and measurements at the LHC pose significant further challenges.
ALICE measures quarkonia spin orientations with respect to a chosen axis via a measurement of the anisotropy in the angular distribution of the decay products. The angular distribution is parametrised in terms of the polarisation parameters, λθ, λφ and λθφ, where θ and φ are the polar and azimuthal emission angles. If all of them are null, no polarisation is present, whereas (λθ = 1, λφ = 0, λθφ = 0) and (λθ = –1, λφ = 0, λθφ = 0) indicate a polarisation of the spin in the transverse and longitudinal directions, respectively.
More details in the Courier website ....