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New algorithms for identifying the flavour of ${\mathrm {B}} ^0$ mesons using pions and protons

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Abstract

Two new algorithms for use in the analysis of $pp$ collision are developed to identify the flavour of $B^0$ mesons at production using pions and protons from the hadronization process. The algorithms are optimized and calibrated on data, using $B^0 \rightarrow D^- \pi^+$ decays from $pp$ collision data collected by LHCb at centre-of-mass energies of 7 and 8 TeV. The tagging power of the new pion algorithm is 60 % greater than the previously available one; the algorithm using protons to identify the flavour of a $B^0$ meson is the first of its kind.

Figures and captions

Mass distribution of $ B ^0 \rightarrow D^-\pi^+$ candidates with fit projections overlaid. Data points (black dots) correspond to the $ B ^0$ candidates selected in the 2 $ fb^{-1}$ data sample collected at $\sqrt{s} =8 \mathrm{ Te V} $. The solid blue curve represents the total fit function which is the sum of signal (red dashed) and combinatorial background (green dash-dotted).

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Distribution of the BDT output of signal (correct-tag decision) and background (wrong-tag decision) tagging particles, for (left) $\rm SS$ $\pi $ and (right) $\rm SS$ $ p $ taggers. In case of multiple tagging candidates per $ B ^0$ candidate, only the candidate with the highest BDT output value is shown.

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Measured average mistag fraction in bins of (left) $\rm SS$ $\pi $ and (right) $\rm SS$ $ p $ BDT output. The plots are obtained with the test sample of background-subtracted $ B ^0 \rightarrow D^-\pi^+$ candidates. The green-shaded area shows the confidence range within $\pm 1\sigma$.

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(top left) Distribution of the mistag probability $\eta^\rm SS\pi $ and (top right) measured mistag fraction $\omega$ as a function of $\eta^\rm SS\pi $. (bottom left) Distribution of the mistag probability $\eta^\rm SS p $ and (bottom right) measured mistag fraction $\omega$ as a function of $\eta^\rm SS p $. The green-shaded area shows the 68% confidence range.

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Mixing asymmetry in bins of mistag probability using the $\rm SS$ $\pi $ tagger.

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Mixing asymmetry in bins of mistag probability using the $\rm SS$ $ p $ tagger.%

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Animated gif made out of all figures.

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Tables and captions

Expected correlation between the flavour of a $ B $ meson and the hadronization products.

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Loose selection requirements for the $\rm SS$ $\pi $ and $\rm SS$ $ p $ algorithms. The variables used as input for the BDT classifiers are indicated by \checkmark.

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Calibration parameters for the $\rm SS$ $\pi $ , $\rm SS$ $ p $ and $\rm SScomb$ taggers where the first uncertainties are statistical and the second are systematic.

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Tagging efficiencies and tagging power of the $\rm SS$ $\pi $ , $\rm SS$ $ p $ and $\rm SScomb$ algorithms. The $\rm SScomb$ efficiencies are shown splitting the sample in candidates tagged exclusively by $\rm SS$ $\pi $ or $\rm SS$ $ p $ , or by both. As explained in the text, there is a large overlap between the $\rm SS$ $\pi $ and $\rm SS$ $ p $ taggers.

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Calibration parameters for the $ B ^0 \rightarrow K ^+ \pi ^- $ decay sample. Uncertainties are statistical only.

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Systematic uncertainties on the calibration parameters of $\rm SS$ $\pi $ , $\rm SS$ $ p $ and $\rm SScomb$ taggers. The total systematic uncertainty is the squared sum of all contributions. A dash indicates a value negligible with respect to the quoted precision.

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Systematic uncertainties related to the decay-time model. A dash indicates a value negligible with respect to the quoted precision.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-039. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in variuous formats

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Created on 19 October 2019.