First Study of the CMS Sensitivity to the Neutrinoless Decay Tau Rightarrow mu^+ mu^+ mu^-

Recent results from Super Kamiokande suggest nu_tau-nu_mu mixing and hence lepton flavor violation. Since then this subject became an important topic of investigation. An improvement of experimental sensitivity on the Branching Ratio for process involving a cross-generation change in the charged lepton sector is thus of interest, either in a discovery search or in a study for the exclusion limit. It can also bring information on the SUSY scenario potentially at work in such processes. We have undertaken a study of the possibility to observe in CMS the Standard Model forbidden decay tau rightarrow 3 mu exploiting the signal from the most important sources of tau leptons at LHC. A study of taus originating from W and Z boson decays, as well as from b-quark mesons is discussed in this note. We used the GEANT3 detector simulation program it cmsim 122 and the current C++ reconstruction progra m for CMS (ORCA version 5_3_2). We also report a comparison of the results obtained with a fast simulation based analysis for signal events from W in the barrel region. The main goal of this comparison is to check the validity of the final results and the performances foreseen for the CMS sub-detectors involved in the study of this channel. At this stage this study shows that CMS should be able to improve on the present experimental limit set by CLEO II by about a factor of 50 with 30 fb^-1 of integrated luminosity.