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Identification of beauty and charm quark jets at LHCb

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Abstract

Identification of jets originating from beauty and charm quarks is important for measuring Standard Model processes and for searching for new physics. The performance of algorithms developed to select $b$- and $c$-quark jets is measured using data recorded by LHCb from proton-proton collisions at $\sqrt{s}=7$ TeV in 2011 and at $\sqrt{s}=8$ TeV in 2012. The efficiency for identifying a $b(c)$ jet is about 65%(25%) with a probability for misidentifying a light-parton jet of 0.3% for jets with transverse momentum $p_{\rm T} > 20$ GeV and pseudorapidity $2.2 < \eta < 4.2$. The dependence of the performance on the $p_{\rm T}$ and $\eta$ of the jet is also measured.

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SV-tagger algorithm $ {\rm BDT}(b|c)$ versus $ {\rm BDT}(bc|udsg)$ distributions obtained from simulation for (left) $b$, (middle) $c$ and (right) $\rm light -parton$ jets.

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Efficiencies and mistag probabilities obtained from simulation for the SV-tagger and TOPO algorithms for (top) $b$, (middle) $c$ and (bottom) $\rm light -parton$ jets. The left plots show the dependence on $p_{\rm T}$ for $2.2 < \eta < 4.2$, while the right plots show the dependence on $\eta$ for $ p_{\rm T} > 20\mathrm{\,GeV} $ (see text for details). The "loose" label for the TOPO refers to the BDT requirement used in the trigger for SVs that contain muon candidates.

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Efficiencies for SV-tagging a $(b,c)$-jet versus mistag probability for a $\rm light -parton$ jet from simulation. The curves are obtained by varying the $ {\rm BDT}(bc|udsg)$ requirement.

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SV-tagger BDT fit results for the $B+$jet data sample with $10 < p_{\rm T} ({\rm jet}) < 100\mathrm{\,GeV} $: (top left) distribution in data; (top right) two-dimensional template-fit result; and (bottom) projections of the fit result with the $b$, $c$, and $\rm light -parton$ contributions shown as stacked histograms.

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Same as Fig. 4 for the $D+$jet data sample.

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Same as Fig. 4 for the $\mu(b,c)+$jet data sample.

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Two-dimensional $M_{\rm cor}$ versus SV track multiplicity fit results for (top) $B+$jet, (middle) $D+$jet and (bottom) $\mu(b,c)+$jet data samples. The left plots show the projection onto the $M_{\rm cor}$ axis, while the right plots show the projection onto the track multiplicity. The highest $M_{\rm cor}$ bin includes candidates with $M_{\rm cor} > 10\mathrm{\,GeV} $.

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Fits to the TOPO BDT distribution in (left) $B+$jet, (middle) $D+$jet and (right) $\mu(b,c)+$jet data samples with $10 < p_{\rm T} ({\rm jet}) < 100\mathrm{\,GeV} $.

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Results of $\chi^2_{\mathrm{IP}}$ calibration using $W\!+$jet data for $10 < p_{\rm T} ({\rm jet}) < 100\mathrm{\,GeV} $. The tail out to large $\chi^2_{\mathrm{IP}}$ values in the $\rm light -parton$ -jet sample is largely due to strange particle decays.

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Fits to the $\chi^2_{\mathrm{IP}}$ distribution in (top left) $B+$jet, (top right) $D+$jet and (bottom) $\mu(b,c)+$jet data samples.

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(top) SV-tagger two dimensional BDT fit results projected onto the (left) $ {\rm BDT}(bc|udsg)$ and (right) $ {\rm BDT}(b|c)$ axes and (bottom) $\chi^2_{\mathrm{IP}}$ fit results for the $B+$muon-jet subsample with $10 < p_{\rm T} ({\rm jet}) < 100\mathrm{\,GeV} $.

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Same as Fig. 11 but for the $D+$muon-jet data sample.

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Same as Fig. 11 but for the $\mu(b,c)+$muon-jet data sample.

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Efficiencies of the SV-tagger algorithm measured in data relative to those obtained from simulation for $2.2 < \eta < 4.2$: (top left) results from the (closed markers) highest- $p_{\rm T}$ track and (open markers) muon-jet samples; (top right) the combined results assuming the scale factors are the same for semileptonic and inclusive $(b,c)$-hadron decays; and (bottom left) the combined results for $(b,c)$-jet using the highest- $p_{\rm T}$ -track approach assuming the scale factors are the same for $b$ and $c$ jets. The absolute efficiencies corresponding to the combined $(b,c)$-jet results (bottom right).

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TOPO algorithm $(b,c)$-tagging efficiencies, using the "loose" BDT requirement, in data relative to those obtained in simulation.

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SV-tagger algorithm BDT distributions for backward and too-long-lived SVs in the $W\!+$jet data sample: (top left) distribution in data; (top right) two-dimensional template-fit result; and (bottom) projections of the fit result with the $b$, $c$, and $\rm light -parton$ contributions shown as stacked histograms.

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Ratio of $\rm light -parton$ -jet mistag probabilities observed in data to those in simulation for the (left) SV-tagger and (right) TOPO algorithms.

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Tables and captions

Summary of relative systematic uncertainties ($-$ denotes negligible). Systematic uncertainties that dependent on jet type and $p_{\rm T}$ are marked by a $*$ (see text for details).

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SV-tagger algorithm $(b,c)$-tagging efficiencies measured in data compared to those obtained in simulation. The $b$ and $c$ results are obtained by combining the highest- $p_{\rm T}$ track and muon-jet results under the assumption that the scale factors are the same for semileptonic and inclusive $(b,c)$-hadron decays. The $(b,c)$ results are obtained by fitting the highest- $p_{\rm T}$ -track sample under the assumption that the scale factors are the same for $b$ and $c$ jets. The absolute efficiencies observed in data are provided using the "$(b,c)$ jets" results.

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Created on 18 December 2018.Citation count from INSPIRE on 18 December 2018.