Study of $B^{0}\to K^{*0}\gamma $ with conversions and $B^{0}\to K^{*0}e+e-$ at very low $q^2$

Lepton Flavour Universality is a property of the Standard Model. In recent years, several anomalies have been observed by LHCb in rare flavour changing neutral current decays of $B$ hadrons characterized by $b\rightarrow s\ell\ell$ transition, where $\ell$ stands for electrons or muons final states. In order to validate analysis and correction to simulation methods used to measure the $\mathcal{R}_X$ ratios, \textit{i.e.} the ratio of branching fraction of $b\rightarrow s \mu\mu$ with respect to $b\rightarrow s ee$ processes, the branching fraction $\mathcal{B}(B^0 \rightarrow K^{*0} e^+ e^-)$ is measured at very low invariant dielectron mass ($q^2 = m^2_{ee}$) range. Fits to data collected by the LHCb experiment between 2011 and 2018 are performed in two regions of $q^{2}$: the gamma and very low $q^2$ bins, corresponding to $q^{2}\in[0,0.00098]$ and $q^{2}\in[0.00098, 0.10160]$ GeV$^2$/c$^4$, respectively. The same fits to data are also used to check and verify the stability of the observed yields over efficiency ratio under different particle identification and multivariate selection criteria.\\ The branching fraction in the very low $q^2$ $\mathcal{B}(B^0 \rightarrow K^{*0} e^+ e^-, vl)$ is measured to be between (1.53$\pm$0.12)$\cdot10^{-7}$ and (1.57$\pm$0.12)$\cdot10^{-7}$ in four different setups while the value obtained from theoretical prediction is either (1.58$\pm$0.10)$\cdot10^{-7}$ or (1.50$\pm$0.08)$\cdot10^{-7}$ depending on the assumptions made. The measured result is compatible with the prediction, which demonstrates the validity of the analysis and correction to simulation in the full $q^2$ spectrum of $b \rightarrow s ee$.