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Identification of charm jets at LHCb in Run II

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Information

Abstract

The identification of charm jets is achieved at LHCb for data collected in 2015--2018 using a method based on the properties of displaced vertices reconstructed and matched with jets. The performance of this method is determined using a dijet calibration dataset recorded by the LHCb detector and selected such that the jets are unbiased in quantities used in the tagging algorithm. The charm-tagging efficiency is reported as a function of the transverse momentum of the jet. The measured efficiencies are compared to those obtained from simulation and found to be in good agreement.

Figures and captions

Depictions of the flavour-enhanced data samples used in this analysis. The jets labelled probe are retained for further analysis. Sub-figures depict (top left) the heavy-flavour-enriched sample and (top right) the light-parton mis-tag enriched sample, as well as the further enriched (bottom left) charm and (bottom right) beauty sub-samples. Some additional requirements are applied but not included in the labeling; see text for details.

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Background-subtracted (left) invariant mass and (right) $\log\chi^2_{\text{IP}} $ projections and fit results for all (top) $ D ^0 \rightarrow K ^- \pi ^+ $ and (bottom) $ D ^+ \rightarrow K ^- \pi ^+ \pi ^+ $ candidates associated with jets reconstructed in the efficiency-reporting region. The background uncertainties, which are included in the error bars, predominantly affect the displaced components.

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Probability density functions for (left) $m_{\rm cor}({\rm DV})$ and (right) $N_{\rm trk}({\rm DV})$ used in the fits for (solid green) charm, (dashed red) beauty, and (dotted blue) light-parton jets.

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DV (left) corrected mass and (right) track multiplicity projections of fits to the flavour-enriched jet samples: (top-to-bottom) beauty-enriched sub-sample, charm-enriched sub-sample, and heavy-flavour-enriched sample fit with data-driven corrections.

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Detector-response matrices for (left) $ D ^0 $-, (right) $ D ^+ $- and (bottom) DV-tagged charm jets. The shading represents the interval-to-interval migration probabilities ranging from (white) 0 to (black) 1 such that each row sums to unity when the underflow and overflow bins are included. Jets with true (reconstructed) $ p_{\mathrm{T}} (j)$ in the 20--100 $\text{ Ge V}$ region but whose reconstructed (true) $ p_{\mathrm{T}} (j)$ is either below 15 $\text{ Ge V}$ or above 100 $\text{ Ge V}$ are included in the unfolding but not shown graphically.

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Charm-tagging efficiency in intervals of $ p_{\mathrm{T}}$ determined from (blue triangles) $ D ^0 \rightarrow K ^- \pi ^+ $ and (red squares) $ D ^+ \rightarrow K ^- \pi ^+ \pi ^+ $ decays, as well as (black circles) the weighted average. The points are offset in each $ p_{\mathrm{T}}$ interval to aid visibility.

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The (left) invariant mass and (right) $\chi^2_{\text{IP}} $ projections of example fits for (top) $ D ^0 \rightarrow K ^- \pi ^+ $ and (bottom) $ D ^+ \rightarrow K ^- \pi ^+ \pi ^+ $ candidates.

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Tables and captions

Branching and fragmentation fractions used to obtain the total charm yields from $ D ^0 \rightarrow K ^- \pi ^+ $ and $ D ^+ \rightarrow K ^- \pi ^+ \pi ^+ $ decays. The PDG [19] averages are used for both branching fractions. Charm fragmentation fractions are based on the global averages reported in Ref. [20], but have been updated as detailed in the text. The fragmentation fractions are inclusive of feed down from excited charm states.

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Relative systematic uncertainties (%) on the tagging efficiencies determined using the $ D ^0 $ and $ D ^+ $ decays as well as their weighted combination. Ranges of uncertainties are given when the value depends on the $ p_{\mathrm{T}} (j) $ interval. The total systematic uncertainty is evaluated as the sum in quadrature of the uncertainties from all sources.

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Charm-tagging efficiencies (%) determined in intervals of $ p_{\mathrm{T}} (j) $. First and second uncertainties are statistical and systematic, respectively.

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Created on 14 December 2021.