Taking advantage of large top production rates at the LHC, the leptonic fin
al states with J/psi are explored for a precise determination of the top qu
ark mass. The top is partially reconstructed by combining the isolated lept
on 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 syst
ematics uncertainties. Mass measurement accuracy is dominated by the curren
t understanding of theoretical uncertainties which result in a systematic e
rror of less than or similar to 1 GeV. (C) 2000 Published by Elsevier Scien
ce B.V. All rights reserved.