  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.
