A change of perspective in quarkonium production: All data are equal, but some are more equal than others

Abstract

Abstract Quarkonium polarization data, usually considered a difficult challenge for the QCD description of quarkonium production and relegated to an a posteriori test of predictions exclusively driven by cross-section measurements, with puzzling results, provide, in reality, the most fundamental, direct and model-independent connection to the production mechanisms. We have simultaneously fitted the ψ ( 2 S ) and ϒ ( 3 S ) differential cross sections and polarizations, reliably measured at the LHC up to higher transverse momentum p T values than ever before, as a superposition of colour-singlet and colour-octet contributions perturbatively calculated up to next-to-leading order. We show that, except for the lowest p T cross-section data, where factorization between short-distance and long-distance QCD effects is not expected to be applicable, all the measurements are very well reproduced. Besides providing a straightforward solution to the “quarkonium polarization puzzle”, our study shows that quarkonium production is dominated by the unpolarized S 0 [ 8 ] 1 octet term, an observation that opens new paths towards the understanding of bound-state formation in QCD.