# 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. Diagra[..].pdf [42 KiB] HiDef png [67 KiB] Thumbnail [35 KiB] *.C file Diagra[..].pdf [41 KiB] HiDef png [67 KiB] Thumbnail [35 KiB] *.C file 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] HiDef png [335 KiB] Thumbnail [140 KiB] *.C file 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. ACP_mumu.pdf [28 KiB] HiDef png [287 KiB] Thumbnail [234 KiB] *.C file ACP_Jpsi.pdf [28 KiB] HiDef png [233 KiB] Thumbnail [183 KiB] *.C file 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) $C_{\widehat{T}} <0,-\overline{ C}_{\widehat{T}} <0$. Todd_m[..].pdf [29 KiB] HiDef png [297 KiB] Thumbnail [250 KiB] *.C file Todd_m[..].pdf [28 KiB] HiDef png [297 KiB] Thumbnail [251 KiB] *.C file Animated gif made out of all figures. PAPER-2016-059.gif Thumbnail

## Supplementary Material [file]

 Supplementary material full pdf Supple[..].pdf [110 KiB] ﻿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 Fig1.pdf [17 KiB] HiDef png [149 KiB] Thumbnail [145 KiB] *C file Fig2.pdf [17 KiB] HiDef png [132 KiB] Thumbnail [114 KiB] *C file

Created on 11 July 2020.