correspond to decays of $\pi^0$ and $\eta$. At lower the peaks sit on top of a large combinatorial background. The shape of the background is estimated by event mixing, i.e. from the $m_{\rm inv}$ distribution obtained by combining photons from different events that nevertheless have similar collision vertex and centrality. The mixed event distributions are normalized and subtracted from the real event distributions. The mixed events are normalized outside of the meson peaks from $0.080<m_{\rm inv}<0.085$ and $0.3<m_{\rm inv}<0.4$ GeV/$c^2$ for the and $0.7<m_{\rm inv}<0.8$for the $\eta$. The combinatorial background decreases rapidly with increasing , therefore, mixed event subtraction is carried out only for $p_T$ below 7–10depending on the collision centrality. Above that the background under the peaks is estimated from the average counts in real events outside, but close to the peaks (sideband).

The resulting, combinatorial-subtracted invariant mass distributions are fit to a combination of a Gaussian to describe signal and a polynomial to describe the residual background. First- and second-order polynomials were used in $\pi^0$ and $\eta$ measurements, respectively. Meson raw yields were obtained as the difference between the integral of the bin content in the mass peak regions and the integral of the polynomial fits to the 