A new measurement of J/psi suppression in Pb-Pb collisions at 158 GeV per nucleon

TL;DR

This paper presents a new measurement of J/$\psi$ suppression in Pb-Pb collisions at 158 GeV per nucleon using NA50 data from 2000 collected with a vacuum-target, enabling accurate peripheral analyses. The analysis relies on the ratio $B_{\mu\mu}\sigma({\rm J}/\psi)/\sigma({\rm DY})$ extracted from a multi-step fit to the dimuon mass spectra, with DY normalization mitigating luminosity and efficiency uncertainties and GRV 94 LO PDFs used for consistency. The results show peripheral Pb-Pb are consistent with the normal nuclear absorption baseline derived from p–A data, while semi-central and central collisions exhibit a persistent, centrality-dependent suppression of J/$\psi$, indicating anomalous absorption beyond the baseline. The study uses three independent centrality estimators and finds consistent suppression patterns, strengthening the case for deconfinement-related effects and providing a robust baseline for future heavy-ion studies. The findings have significant implications for understanding quark-gluon plasma signatures in heavy-ion collisions and the centrality dependence of quarkonium suppression.

Abstract

We present a new measurement of J/psi production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the (J/psi)/Drell-Yan cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the (J/psi)/Drell-Yan cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions.

Figures (8)

• Figure 1: The NA50 target region (beam entering from bottom right), including the target vacuum tank, the Multiplicity Detector and the upstream face of the carbon absorber; the Electromagnetic Calorimeter (not shown) with the BeO preabsorber in its center is inserted between the MD and the carbon absorber during data taking. The inset shows part of the target vacuum tank and the upstream face of the Multiplicity Detector's first plane (beam entering from left). During data taking conditions the target system is moved down-stream, almost touching the MD.
• Figure 2: Correlation between transverse and forward energy, showing $\pm 2 \sigma$ and $\pm 3 \sigma$ diagonal bands. Contours connect pixels of equal event density, ranging from 20 (external one) to 5024 (closest to the highest density), scaled with a constant factor.
• Figure 3: Opposite-sign dimuon invariant mass spectrum for the centrality class $35 < E_{\rm T} < 45$ GeV. The results from the fit are also shown.
• Figure 4: The (J/$\psi$) / Drell-Yan cross-sections ratio as a function of transverse energy for the Pb-Pb 2000 data sample (left). The normal absorption curve is presented together with the combined error from the Glauber fit and the rescaling procedure (dashed curves). The inset shows the ratio Measured/Expected, i.e. data over normal nuclear absorption. The right panel presents the same data compared to the absorption curve computed using also S-U NA38 data for $\sigma_{\rm abs}$ determination and curve normalization.
• Figure 6: The (J/$\psi$) / Drell-Yan cross-sections ratio as a function of charged mul-ti-pli-ci-ty for the Pb-Pb 2000 data sample. The inset shows the ratio Measured/Expected, i.e. data over normal nuclear absorption.