J/ψ polarisation differs in lead collisions

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, λ_θ, λ_φ and λ_θφ, where θ and φ are the polar and azimuthal emission angles. If all of them are null, no polarisation is present, whereas (λ_θ = 1, λ_φ = 0, λ_θφ = 0) and (λ_θ = –1, λ_φ = 0, λ_θφ = 0) indicate a polarisation of the spin in the transverse and longitudinal directions, respectively.

Polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma

In pp collisions, polarisation has been mainly used to investigate the J/ψ production mechanism. Reproducing the small values of polarisation parameter λ_θ observed at the LHC is a challenge for many theoretical models. Until recently, no corresponding results were available for nucleus–nucleus collisions, and in this domain polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma (QGP). The formation of this deconfined, strongly interacting medium impacts differently on the various quarkonium resonances, inducing a larger suppression on the less bound excited states ψ(2S) and χ_c, and modifying their feed-down fractions into the ground state, J/ψ. This effect may lead to a variation of the overall polarisation values since different charmonium states are expected to be produced with different polarisations. In addition, the recombination of uncorrelated heavy-quark pairs inside the QGP gives rise to an extra source of J/ψ, which can further modify the overall polarisation with respect to pp collisions.

The ALICE experiment has recently made the first measurements of the J/ψ and ϒ(1S) polarisation in Pb–Pb collisions. The data correspond to a centre-of-mass energy √(s_NN) = 5.02 TeV, and the rapidity range 2.5 < y < 4. The measurements were carried out in the dimuon decay channel, and results were obtained in two different reference frames, helicity and Collins–Soper, each of them with its own definition of the quantisation axis. In the helicity frame, the quarkonium momentum direction in the laboratory is chosen, while the bisector of the angle formed by the two colliding beams boosted in the quarkonium rest frame is used in the Collins–Soper frame. The J/ψ polarisation parameters, evaluated in three p_T bins covering the range between 2 and 10 GeV, are close to zero, but with a maximum positive deviation for λ_θ (corresponding to a transverse polarisation) of 2σ for 2 < p_T < 4 GeV in the helicity reference frame. Interestingly, the corresponding LHCb pp result for prompt J/ψ at √(s_NN) = 7 TeV instead shows a small but significant longitudinal polarisation.

The observation of a significant difference between J/ψ polarisation results in pp and Pb–Pb collisions motivates further experimental and theoretical studies, with the main goal of connecting this observable with the known suppression and regeneration mechanisms in heavy-ion collisions. For the rarer ϒ(1S), a bound state of a bottom and an antibottom quark, the inclusive polarisation parameters were found to be compatible with zero within sizeable uncertainties. A higher precision and momentum-differential measurement will be enabled by the ten-fold larger Pb–Pb luminosity expected in Run 3 of the LHC.