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Observation of the decay $\Lambda^0_b \to p K^- \mu^+ \mu^-$ and a search for $C P$ violation

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Abstract

A search for $C P$ violation in the decay $\Lambda^0_b \to p K^- \mu^+ \mu^-$ is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of $C P$ violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of $3$ $\mathrm{fb}^{-1}$. The $\Lambda^0_b \to p K^- \mu^+ \mu^-$ decay is observed for the first time, and two observables that are sensitive to different manifestations of $C P$ violation are measured, $\Delta\mathcal{A}_{C P} \equiv \mathcal{A}_{C P}(\Lambda^0_b \to p K^- \mu^+ \mu^-)-\mathcal{A}_{C P}(\Lambda^0_b\to pK^- J/\psi)$ and $a_{C P}^{\widehat{T}-odd}$, where the latter is based on asymmetries in the angle between the $\mu^+\mu^-$ and $p K^-$ decay planes. These are measured to be $\Delta\mathcal{A}_{C P} = (-3.5 \pm 5.0(stat) \pm 0.2(syst))\times 10^{-2}$, $a_{C P}^{\widehat{T}-odd} = (1.2 \pm 5.0(stat) \pm 0.7(syst))\times 10^{-2}$, and no evidence for $C P$ violation is found.

Figures and captions

Diagrams for the decay $\Lambda ^0_ b \rightarrow p K ^- \mu ^+ \mu ^- $ , in which $V_{bq}$ and $V_{qs}$ are CKM matrix elements and $q$ represents one of the three up-type quarks $u$, $c$ or $t$, the $t$-quark contribution being dominant. The $u\overline{u}$ pairs originate from the hadronization process.

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Diagram_box.pdf

Definition of the angle $\chi$ for $\Lambda ^0_ b \rightarrow p K ^- \mu ^+ \mu ^- $ decays, in the $\Lambda ^0_ b $ rest frame.

 DecayP[..].pdf [830 KiB]
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DecayPlanes_pKMuMu_v1.pdf

Invariant mass distributions of (top) $\Lambda ^0_ b \rightarrow p K ^- \mu ^+ \mu ^- $ and (bottom) $\Lambda ^0_ b \rightarrow p K ^- { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $ candidates, with fit results superimposed. Plots refer to the (left) $\Lambda ^0_ b $ and (right) $\overline{\Lambda} {}^0_ b $ subsamples.

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ACP_Jpsi.pdf [28 KiB]
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ACP_Jpsi.pdf

Invariant mass distributions of the $\Lambda ^0_ b \rightarrow p K ^- \mu ^+ \mu ^- $ subsamples used for the $ A_{\widehat{T}}$ and $\overline{ A}_{\widehat{T}}$ measurements. Plots refer to (top) $\Lambda ^0_ b $ and (bottom) $\overline{\Lambda} {}^0_ b $ decays divided into the subsamples (left) $ C_{\widehat{T}} >0,-\overline{ C}_{\widehat{T}} >0$ and (right) $ C_{\widehat{T}} <0,-\overline{ C}_{\widehat{T}} <0$.

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Todd_mumu_Lb.pdf
Todd_m[..].pdf [28 KiB]
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Todd_mumu_Lbbar.pdf

Animated gif made out of all figures.

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Supplementary Material [file]

Supplementary material full pdf

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 Supplementary.pdf

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-059. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in variuous formats

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Created on 27 April 2024.