The LHC will be a top factory; current Next to Leading Order (NLO) calculations of  σ( pp -> tt) indicate that once the LHC reaches its design energy of 14  TeV, more than 8  10 ^6   top quark pairs will be produced every year at the low luminosity phase. The sizable tt samples expected at the LHC will permit a precise measurements of the top sector, revealing, perhaps, Physics Beyond the Standard Model.

In the SM the coupling structure between fermions and weak gauge bosons is universal. A direct consequence of universality is the near equality in branching ratios of the W to the different lepton families.Hence, the SM predicts that the ratio between BR(t → mu) and BR(t → tau) close to 1. So, A measurement of this ratio inconsistent with 1 will be a strong indication for physics beyond the standard model.

We present a method of measuring this ratio, and its expected performance in the ATLAS expermient.