J/psi production in Indium-Indium collisions at 158 GeV/nucleon

TL;DR

This paper reports a precise measurement of J/psi production in Indium-Indium collisions at 158 GeV/nucleon, revealing additional suppression beyond cold nuclear matter effects in central collisions, consistent with prior Pb-Pb findings.

Contribution

It provides new experimental data on J/psi suppression in In-In collisions and compares it with existing models and Pb-Pb results, highlighting unexplained additional suppression mechanisms.

Findings

Extra suppression observed in collisions with >80 participant nucleons

Qualitative agreement with previous Pb-Pb suppression results

Current theories cannot fully explain the observed suppression patterns

Abstract

The NA60 experiment studies muon pair production at the CERN SPS. In this letter we report on a precision measurement of J/psi in In-In collisions. We have studied the J/psi centrality distribution, and we have compared it with the one expected if absorption in cold nuclear matter were the only active suppression mechanism. For collisions involving more than ~80 participant nucleons, we find that an extra suppression is present. This result is in qualitative agreement with previous Pb-Pb measurements by the NA50 experiment, but no theoretical explanation is presently able to coherently describe both results.