# Measurement of the $B^0 \to K^{*0}e^+e^-$ branching fraction at low dilepton mass

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The branching fraction of the rate decay $B^0 \rightarrow K^{*0}e^+e^-$ in the dilepton mass region from 30 to 1000 MeV$/c^2$ has been measured by the LHCb experiment, using $pp$ collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, at a centre-of-mass energy of 7 TeV. The decay mode $B^0 \rightarrow J/\psi(e^+e^-) K^{*0}$ is utilized as a normalization channel. The branching fraction $B^0 \rightarrow K^{*0}e^+e^-$ is measured to be $$B(B^0 \rightarrow K^{*0}e^+e^-)^{30-1000 MeV/c^2}= (3.1 ^{+0.9 +0.2}_{-0.8 -0.3} \pm 0.2)\times 10^{-7},$$ where the first error is statistical, the second is systematic, and the third comes from the uncertainties on the B^0 \rightarrow J/\psi K^{*0}$and$J/\psi \rightarrow e^+e^- $branching fractions. ## Figures and captions  Dominant Standard Model diagrams contributing to the decay${ B ^0 \rightarrow K ^{*0} e ^+ e ^- }$. Fig1a.pdf [9 KiB] HiDef png [154 KiB] Thumbnail [97 KiB] *.C file Fig1b.pdf [9 KiB] HiDef png [166 KiB] Thumbnail [103 KiB] *.C file Fig1c.pdf [8 KiB] HiDef png [152 KiB] Thumbnail [96 KiB] *.C file Output of the BDT for$ B ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} ( e ^+ e ^- ) K ^{*0} $data (points) and simulation (red line). Fig2.pdf [19 KiB] HiDef png [137 KiB] Thumbnail [140 KiB] *.C file Invariant mass distributions for the$ B ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} ( e ^+ e ^- ) K ^{*0} $decay mode for the (left) HWElectron and (right) HWTIS trigger categories. The dashed line is the signal PDF, the light grey area corresponds to the combinatorial background, the medium grey area is the partially reconstructed hadronic background and the dark grey area is the partially reconstructed$ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$background component. Fig3a.pdf [20 KiB] HiDef png [146 KiB] Thumbnail [76 KiB] *.C file Fig3b.pdf [21 KiB] HiDef png [156 KiB] Thumbnail [84 KiB] *.C file Invariant mass distributions for the$ B ^0 \rightarrow K ^{*0} e ^+ e ^- $decay mode for the (left) HWElectron and (right) HWTIS trigger categories. The dashed line is the signal PDF, the light grey area corresponds to the combinatorial background, the medium grey area is the partially reconstructed hadronic background and the black area is the$ B ^0 \rightarrow K ^{*0} \gamma $component. Fig4a.pdf [19 KiB] HiDef png [133 KiB] Thumbnail [71 KiB] *.C file Fig4b.pdf [19 KiB] HiDef png [146 KiB] Thumbnail [80 KiB] *.C file Animated gif made out of all figures. PAPER-2013-005.gif Thumbnail ## Tables and captions  Signal yields with their statistical uncertainties. Table_1.pdf [53 KiB] HiDef png [32 KiB] Thumbnail [15 KiB] tex code Ratios of efficiencies used for the measurement of the$ B ^0 \rightarrow K ^{*0} e ^+ e ^- $branching fraction. The ratio$r_{\rm HW}$for the HWTIS trigger category is assumed to be equal to unity. The uncertainties are the total ones and are discussed in Sec. 6. Table_2.pdf [37 KiB] HiDef png [42 KiB] Thumbnail [20 KiB] tex code Absolute systematic uncertainties on the$ B ^0 \rightarrow K ^{*0} e ^+ e ^- $branching ratio (in$10^{-7}\$) . Table_3.pdf [54 KiB] HiDef png [51 KiB] Thumbnail [22 KiB] tex code

Created on 11 July 2020.