ATLAS calorimeter performance for identification of gamma, pi0 and eta particles at low transverse momentum.

In ATLAS CP violation is studied in the B meson decays. Some of the decay modes are subject to an extensive study in the liquid argon calorimeter such as $B_s$ decay to $J/\psi~\eta$ where the $\eta$ particle decays to two $\gamma$'s and $B_d \rightarrow J/\psi~K^{*0}$ where the $K^{*0}$ decays to $K^0 \pi^0$. The observability of these channels is subject to the capability of the calorimeter to detect the low $p_{\rm T}$ clusters produced from the decay of $\eta$ and $\pi^0$ particles to two $\gamma$'s . It is also important to clean up the data from the electronic noise and the pile-up effects since this latter produces mostly soft charged and neutral pions. One can achieve an $\eta$ detection efficiency of about 2.26\% with an amount of fake rate of about 24\% which makes the study of $B_s \rightarrow J/\psi~\eta$ feasible. A $\pi^0$ detection efficiency of 9\% is obtained but the fake rate amounts to 73\%. Subsequently, it is shown that $B_d \rightarrow J/\psi~K^{*0}$ mode is rather difficult to study.