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Measurement of the electron reconstruction efficiency at LHCb

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Abstract

The single electron track-reconstruction efficiency is calibrated 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. These correction factors allow LHCb to measure branching fractions involving single electrons with a 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.

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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.

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Distribution of the constrained invariant mass for (left) data and (right) simulated events, together with an example of a fit to this distribution. The simulated events contain at least one signal decay, resulting in a higher signal purity than is observed in data.

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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 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.

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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 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.

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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.

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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.

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Created on 30 May 2020.