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Measurement of CP violation in the decay $B^{+} \rightarrow K^{+} \pi^{0}$

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Abstract

A measurement of direct $ C P$ violation in the decay $ B ^+ \rightarrow K ^+ \pi ^0 $ is reported using a data sample corresponding to an integrated luminosity of 5.4 $\text{ fb} ^{-1}$ , collected with the LHCb experiment at a center-of-mass energy of $\sqrt{s}=13$ $\text{ Te V}$ . The $ C P$ asymmetry is measured to be $0.025 \pm 0.015 \pm 0.006 \pm 0.003$, where the uncertainties are statistical, systematic, and due to an external input, respectively. This is the most precise measurement of the $ C P$ asymmetry in the decay $ B ^+ \rightarrow K ^+ \pi ^0 $ and exceeds the precision on the current world average. This direct $ C P$ asymmetry is a key input to studies of a long-standing anomaly in $ B $ meson decays, known as the $K\pi$-puzzle. The asymmetry is consistent with the previous measurements of this quantity, confirming and significantly enhancing the observed anomalous difference between the direct $ C P$ asymmetries of the $B^{+} \rightarrow K^{+} \pi^{-}$ and $ B ^+ \rightarrow K ^+ \pi ^0 $ decays.

Figures and captions

Invariant-mass distribution of the selected candidates with fit projections overlayed. The data set is divided by the charge of the $B$ meson, with $B^+ \rightarrow K^+\pi^0$ shown on the left and $B^- \rightarrow K^-\pi^0$ on the right.

Bu_ACP[..].eps [45 KiB]
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Bu_ACP_merged.eps

Invariant mass distribution of $B^{+} \rightarrow J/\psi K^{+}$ candidates used to correct for nuisance asymmetries. The data is divided by the charge of the $B$ meson, with $B^+ \rightarrow J/\psi K^+$ shown on the left and $B^- \rightarrow J/\psi K^{-}$ on the right.

BjpsiK[..].pdf [47 KiB]
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BjpsiK_ACP_merged.pdf

Invariant mass distribution of $B^{+} \rightarrow K^{+} \pi^{0}$ candidates after the initial candidate selection. The roll-off at high mass is due to differences in the online and offline reconstruction, particularly of neutral pions.

Bu_M.pdf [17 KiB]
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Bu_M.pdf

Signal and background distributions of DOCA-$\chi^2$ after the initial candidate selection, normalized to unit area.

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DOCA.pdf

Signal and background distributions of multiplicity of tracks in a cone of $\Delta R = 1.7$ around the $ B ^+$ candidate trajectory after initial event selection, normalized to unit area.

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ConeMult.pdf

Signal and background distributions of $p_T$ asymmetry in a cone of $\Delta R = 1.7$ around the $ B ^+$ candidate trajectory after the initial candidate selection, normalized to unit area.

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ConePTASY.pdf

Signal and background distributions of the smallest $\chi^2$ of vertex formed by adding one additional track to the $ K ^+$ after the initial candidate selection, normalized to unit area.

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dChi2onelog.pdf

Signal and background distributions of the smallest $\Delta \chi^2$ of vertex formed by adding a second track to the lowest $\chi^2$ vertex formed by adding one additional track to the $ K ^+$ after the initial candidate selection, normalized to unit area.

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dChi2twolog.pdf

Signal and background distributions of the smallest change in $\chi^{2}$ of the PV when including the $ K ^+$ in the vertex fit after the initial candidate selection, normalized to unit area.

KIPC2log.pdf [18 KiB]
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KIPC2log.pdf

Signal and background distributions of $ K ^+$ $ p_{\mathrm{T}}$ after the initial candidate selection, normalized to unit area.

KPT.pdf [18 KiB]
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KPT.pdf

Signal and background distributions of multiplicity of vertices having small $\chi^2$ after the initial candidate selection, normalized to unit area.

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VtxMult.pdf

Raw asymmetries for $B^{+} \rightarrow K^{+} \pi^{0}$ separated by year and whether the magnetic field is aligned vertically upwards (MU) or downwards (MD)

consis[..].pdf [9 KiB]
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consistency_year.pdf

Diagram of the $B^{+} \rightarrow K^{+} \pi^{0}$ decay topology. The solid blue line represents the $K^{+}$ track, and the wide dashed purple line represents the reconstructed $\pi^{0}$ momentum. They are combined to form the $B^{+}$ momentum trajectory shown as a narrow red dashed line, which is assumed to originate from the primary vertex also shown in red.

MTD_wi[..].pdf [27 KiB]
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MTD_without.pdf

Animated gif made out of all figures.

PAPER-2020-040.gif
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Tables and captions

Systematic uncertainties on $A_{ C P }( B ^+ \rightarrow K ^+ \pi ^0 )$.

Table_1.pdf [63 KiB]
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tex code
Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

Supple[..].pdf [327 KiB]
Supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2020-040. The files are: Supplementary.pdf : An overview of the extra figures Justification.pdf : PRL justification *.pdf, *.png, *.eps, *.C : The figures in various formats

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Created on 27 February 2021.