  Taking advantage of large top production rates at the LHC, the leptonic final
states with J/psi are explored for a precise determination of the top quark
mass. The top is partially reconstructed by combining the isolated lepton from
the W with the J/psi from the decays of the corresponding b-quark. The method
relies, to a large extent, on the proper Monte-Carlo description of top
production and decay. The main emphasis is put on the expected systematics
uncertainties. Mass measurement accuracy is dominated by the current
understanding of theoretical uncertainties which result in a systematic error
of <~1 GeV.
