Why is the J/ψ suppression enhanced at large transverse energy?

We study the ratio of differential cross sections dσ/dET of J/ψ to minimum bias event samples in Pb-Pb collisions at SPS energy. The NA50 data exhibit a sharp turnover at ET∼100 GeV (close to the knee of the ET distribution) followed by a steady, steep decrease at larger ET. We show that this behavior can be explained by the combined effects of a small decrease of the hadronic ET in the J/ψ event sample (due to the ET taken by the J/ψ trigger), together with a sharp decrease of the ET distributions in this ET region (tail). This phenomenon does not affect the (true) ratio of J/ψ to DY differential cross sections (obtained by the NA50 standard analysis), but it does affect the one obtained by the so-called minimum-bias analysis. A good agreement is obtained with the data coming from both analysis--as well as with those on the ratios of J/ψ and DY to minimum-bias differential cross sections - in the whole ET region