ATLAS Calorimeter capabilities for the observation of B^0_{s,d} -->J/psi eta

The observation potential of the decays B^0_{s,d} --> J/psi eta with the ATLAS detector at the LHC is presented. At present there exist only upper limits for the branching fractions, but at LHC, a clear signal for the decay mode B^0_{s} -->J/psi eta is expected. The branching fraction of this decay mode can thus be measured, and other parameters such as the mixing phase can be probed. The CP asymmetry predicted by the Standard Model is very small, and the observation of a sizeable effect would be a signal of physics beyond the Standard Model. The study presented here can also be regarded as an example of the physics prospects with eta-mesons at the LH