# A measurement of the $CP$ asymmetry difference in $\varLambda_{c}^{+} \to pK^{-}K^{+}$ and $p\pi^{-}\pi^{+}$ decays

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## Abstract

The difference between the $CP$ asymmetries in the decays $\varLambda_{c}^{+} \to pK^{-}K^{+}$ and $\varLambda_{c}^{+} \to p\pi^{-}\pi^{+}$ is presented. Proton-proton collision data taken at centre-of-mass energies of $7$ and $8 \mathrm{TeV}$ collected by the LHCb detector in 2011 and 2012 are used, corresponding to an integrated luminosity of $3 \mathrm{fb}^{-1}$. The $\varLambda_{c}^{+}$ candidates are reconstructed as part of the $\varLambda_{b}^{0} \to \varLambda_{c}^{+}\mu^{-}X$ decay chain. In order to maximize the cancellation of production and detection asymmetries in the difference, the final-state kinematic distributions of the two samples are aligned by applying phase-space-dependent weights to the $\varLambda_{c}^{+} \to p\pi^{-}\pi^{+}$ sample. This alters the definition of the integrated $CP$ asymmetry to $A_{CP}^{\text{wgt}}(p\pi^{-}\pi^{+})$. Both samples are corrected for reconstruction and selection efficiencies across the five-dimensional $\varLambda_{c}^{+}$ decay phase space. The difference in $CP$ asymmetries is found to be \begin{align*} \Delta{A^{\text{wgt}}_{CP}} &= A_{CP}(pK^{-}K^{+}) - A_{CP}^{\text{wgt}}(p\pi^{-}\pi^{+})\\ &= (0.30 \pm 0.91 \pm 0.61) \%, \end{align*} where the first uncertainty is statistical and the second is systematic.

## Figures and captions

 The $p h^{-} h^{+}$ invariant mass spectra from the fully selected $\Lambda _{ c }^{+} \rightarrow p K ^- K ^+$ (left) and $\Lambda _{ c }^{+} \rightarrow p\pi ^- \pi ^+$ (right) datasets summed over all data-taking conditions. The results of the fit to each dataset are shown for illustration. The widths of the signal distributions differ due to the different $Q$-value between the two decays, where the larger value for the $\Lambda _{ c }^{+} \rightarrow p\pi ^- \pi ^+$ mode results in a broader shape. Fig1-left.pdf [28 KiB] HiDef png [424 KiB] Thumbnail [342 KiB] *.C file Fig1-right.pdf [28 KiB] HiDef png [391 KiB] Thumbnail [318 KiB] *.C file Background-subtracted distributions of the $\Lambda ^0_ b$ candidate transverse momentum (top row), the muon candidate transverse momentum (middle row), and the proton candidate pseudorapidity (bottom row) both before (left column) and after (right column) weighting the $p\pi ^- \pi ^+$ sample (blue points) to match the $p K ^- K ^+$ sample (black points). The data are summed across all data-taking conditions. Fig2-t[..].pdf [25 KiB] HiDef png [282 KiB] Thumbnail [238 KiB] *.C file Fig2-t[..].pdf [25 KiB] HiDef png [280 KiB] Thumbnail [228 KiB] *.C file Fig2-m[..].pdf [26 KiB] HiDef png [290 KiB] Thumbnail [245 KiB] *.C file Fig2-m[..].pdf [26 KiB] HiDef png [288 KiB] Thumbnail [242 KiB] *.C file Fig2-b[..].pdf [24 KiB] HiDef png [276 KiB] Thumbnail [235 KiB] *.C file Fig2-b[..].pdf [24 KiB] HiDef png [273 KiB] Thumbnail [225 KiB] *.C file Background-subtracted and efficiency-corrected $\Lambda _{ c }^{+} \rightarrow p K ^- K ^+$ (left) and $\Lambda _{ c }^{+} \rightarrow p\pi ^- \pi ^+$ (right) data in the $m^2 ( p h^{-} )$ -- $m^2 ( h^{-} h^{+} )$ plane, integrated across all data-taking subsamples. The $p K ^- K ^+$ data feature a prominent $\phi \rightarrow K ^- K ^+$ component, whilst the $p\pi ^- \pi ^+$ data exhibit $\rho(770)/\omega(782) \rightarrow \pi ^- \pi ^+$ and $f_{0}(980) \rightarrow \pi ^- \pi ^+$ components. Fig3-left.pdf [35 KiB] HiDef png [733 KiB] Thumbnail [577 KiB] *.C file Fig3-right.pdf [44 KiB] HiDef png [979 KiB] Thumbnail [629 KiB] *.C file Values of and statistical uncertainties on the asymmetries $A_{\text{raw}} ( p K ^- K ^+ )$ (top left), $A_{\text{raw}}^{\text{wgt}} ( p\pi ^- \pi ^+ )$ (top right), and $\Delta A_{ C P }^{\text{wgt}}$ (bottom centre), for the four data subsamples (two centre-of-mass energies, 7 and \SI{8}{\TeV}, and two polarities of the dipole magnet, up and down). For each asymmetry, the average of the four data points, as described in the text, is also shown, where the band indicates the uncertainty. Fig4-t[..].pdf [20 KiB] HiDef png [181 KiB] Thumbnail [192 KiB] *.C file Fig4-t[..].pdf [20 KiB] HiDef png [198 KiB] Thumbnail [176 KiB] *.C file Fig4-b[..].pdf [20 KiB] HiDef png [207 KiB] Thumbnail [206 KiB] *.C file Animated gif made out of all figures. PAPER-2017-044.gif Thumbnail

## Tables and captions

 Signal yields measured in the fit for each of the four subsets of the $p K ^- K ^+$ and $p\pi ^- \pi ^+$ data (two centre-of-mass energies, 7 and \SI{8}{\TeV}, and two polarities of the dipole magnet, up and down). The corresponding integrated luminosity of each subset is also given. Table_1.pdf [46 KiB] HiDef png [63 KiB] Thumbnail [30 KiB] tex code Systematic uncertainties on $\Delta A_{ C P }^{\text{wgt}}$ and their magnitudes. The dash indicates that the uncertainty is assessed to be negligible. Table_2.pdf [38 KiB] HiDef png [41 KiB] Thumbnail [19 KiB] tex code

Created on 24 August 2019.Citation count from INSPIRE on 24 August 2019.