Quantify direct versus feed-down contributions to ground-state quarkonium production in pp collisions

Determine, in proton–proton collisions, the fractions of the observed \JPsi and \Upsilon(1S) yields that arise from direct production as opposed to feed-down decays of heavier quarkonium states, across the relevant rapidity and transverse-momentum ranges, to enable reliable interpretation of sequential suppression patterns in nucleus–nucleus collisions.

Background

The sequential suppression conjecture predicts that quarkonium states with lower binding energies are more strongly suppressed in a deconfined quark–gluon plasma. Confronting this prediction with data requires precise knowledge of baseline production in proton–proton collisions, including how much of the observed ground-state yields come directly versus via feed-down from heavier S- and P-wave states.

The review emphasizes that even for the most copiously produced ground states (\JPsi and \Upsilon(1S)), the direct and feed-down components in pp collisions are not sufficiently well known. This uncertainty limits quantitative interpretation of the nuclear modification factors measured in PbPb collisions and complicates theory–data comparisons on sequential suppression.