# Measurement of the electron reconstruction efficiency at LHCb

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## Abstract

The single electron track-reconstruction efficiency is determined using a sample corresponding to $1.3 \mathrm{fb}^{-1}$of $pp$ collision data recorded with the LHCb detector in 2017. This measurement exploits $B^+\to J/\psi (e^+e^-)K^+$ decays, where one of the electrons is fully reconstructed and paired with the kaon, while the other electron is reconstructed using only the information of the vertex detector. Despite this partial reconstruction, kinematic and geometric constraints allow the $B^+$-meson mass to be reconstructed and the signal to be well separated from backgrounds. This in turn allows the electron reconstruction efficiency to be measured by matching the partial track segment found in the vertex detector to tracks found by LHCb's regular reconstruction algorithms. The agreement between data and simulation is evaluated, and corrections are derived for simulated electrons in bins of kinematics. The presented method allows LHCb to measure branching fractions involving single electrons with a an electron reconstruction systematic uncertainty below $1\%$.

## Figures and captions

 Schematic view of the different track types relevant for this paper, along with a schematic of the essential tracking detectors of the LHCb detector, showing the VELO detector, TT detector, magnet and the downstream tracking stations. The names of the tracking detectors are indicated in red below the figure. Fig1.pdf [11 KiB] HiDef png [84 KiB] Thumbnail [71 KiB] The simulated LHCb (left) long-track and (right) VELO-track reconstruction efficiency for electrons and muons produced in $B^{+}\rightarrow J/\psi(\rightarrow \ell^+ \ell^- ) K^+$ decays as a function of the lepton's transverse momentum. Fig2a.pdf [17 KiB] HiDef png [165 KiB] Thumbnail [157 KiB] Fig2b.pdf [17 KiB] HiDef png [157 KiB] Thumbnail [133 KiB] Distribution of the constrained invariant mass for (left) data and (right) simulated events, together with an example of a fit to this distribution. Fig3a.pdf [40 KiB] HiDef png [247 KiB] Thumbnail [209 KiB] Fig3b.pdf [40 KiB] HiDef png [200 KiB] Thumbnail [177 KiB] Measured electron detection efficiency in each $p_{\mathrm{T}}$, $\eta$ bin in data and simulation, for electrons which do not travel parallel to the RF-foil. The ratios between the results from data and simulation for are shown for each bin below. The error bars on the ratio include both the statistical and systematic uncertainties, while those for the absolute efficiencies represent only statistical uncertainties. Fig4a.pdf [17 KiB] HiDef png [201 KiB] Thumbnail [199 KiB] Fig4b.pdf [15 KiB] HiDef png [180 KiB] Thumbnail [183 KiB] Fig4c.pdf [15 KiB] HiDef png [185 KiB] Thumbnail [184 KiB] Fig4d.pdf [14 KiB] HiDef png [178 KiB] Thumbnail [185 KiB] Measured electron detection efficiency in each $p_{\mathrm{T}}$, $\eta$ bin in data and simulation, for electrons which travel parallel to the RF-foil. The ratios between the results from data and simulation for are shown for each bin below. The error bars on the ratio include both the statistical and systematic uncertainties, while those for the absolute efficiencies represent only statistical uncertainties. Fig5a.pdf [17 KiB] HiDef png [203 KiB] Thumbnail [203 KiB] Fig5b.pdf [15 KiB] HiDef png [178 KiB] Thumbnail [179 KiB] Fig5c.pdf [15 KiB] HiDef png [182 KiB] Thumbnail [186 KiB] Fig5d.pdf [15 KiB] HiDef png [175 KiB] Thumbnail [182 KiB] Efficiency in bins of number of primary vertices and the transverse momentum of the probe electron for (left) data and (right) simulation. The uncertainties shown are statistical only. Fig6a.pdf [15 KiB] HiDef png [117 KiB] Thumbnail [107 KiB] Fig6b.pdf [15 KiB] HiDef png [112 KiB] Thumbnail [109 KiB] Overview of all uncertainties on the efficiency ratio per bin of $p_{\mathrm{T}}$ and $\eta$ of the probe electron for (bottom) candidates which traverse parallel to the RF-foil and (top) the rest of the phase-space. The $x$-axis corresponds to the $p_{\mathrm{T}}$ bin number within the $\eta$ range. Fig7a.pdf [16 KiB] HiDef png [120 KiB] Thumbnail [108 KiB] Fig7b.pdf [16 KiB] HiDef png [127 KiB] Thumbnail [111 KiB] Animated gif made out of all figures. DP-2019-003.gif Thumbnail

Created on 12 October 2019.