The phenomenology of top quark polarisation in the decay of heavy charged Higgs bosons at LHC and beyond

It has recently been shown that through the tau v decay mode the heavy char ged Higgs bosons H (+/-) can be discovered at LHC over a vast region of the parameter space previously thought unaccessible. If the charged Higgs boso ns are discovered in this decay mode and the measurements of its mass and t an beta are made, it allows us to make greater use of the dominant but QCD background contaminated bt decay mode by cuts suppressing phase space for t he background. In this paper we study the top quark polarisation in the bt decay mode. When tan beta is either moderately large (tan beta greater than or similar to 9) or small (tan beta less than or similar to 4), the correl ation effects can be used as a signal of H+/- production. We outline the an alysis procedure for various decay signatures. As an example, we demonstrat e the application of our procedure to the dominant production process gb -- > tH(-) at LHC. The effect is clearly observable for H+/- mass between 200 GeV and 500 GeV in both leptonic and hadronic channels, but at 500 GeV the boost on the top quark starts to prevent the reconstruction of invariant ma sses, and hence the observation of the correlation effect, in the hadronic decay channel. (C) 1999 Elsevier Science B.V. All rights reserved.