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Observation of the decays $\Lambda_b^0 \to \chi_{c1} p K^-$ and $\Lambda_b^0 \to \chi_{c2} p K^-$

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Abstract

The first observation of the decays $\Lambda_b^0 \to \chi_{c1} p K^-$ and $\Lambda_b^0 \to \chi_{c2} p K^-$ is reported using a data sample corresponding to an integrated luminosity of $3.0$ fb$^{-1}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. The following ratios of branching fractions are measured \begin{eqnarray*} \frac{{\cal B}(\Lambda_b^0 \to \chi_{c1} p K^-)}{{\cal B}(\Lambda_b^0 \to J/\psi p K^-)} = 0.242 \pm 0.014 \pm 0.013 \pm 0.009, \frac{{\cal B}(\Lambda_b^0 \to \chi_{c2} p K^-)}{{\cal B}(\Lambda_b^0 \to J/\psi p K^-)} = 0.248 \pm 0.020 \pm 0.014 \pm 0.009, \frac{{\cal B}(\Lambda_b^0 \to \chi_{c2} p K^-)}{{\cal B}(\Lambda_b^0 \to \chi_{c1} p K^-)} = 1.02 \pm 0.10 \pm 0.02 \pm 0.05, \end{eqnarray*} where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fractions of the $\chi_{c1}\to J/\psi\gamma$ and $\chi_{c2} \to J/\psi\gamma$ decays. Using both decay modes, the mass of the $\Lambda_b^0$ baryon is also measured to be $m_{\Lambda_b^0} = 5619.44 \pm 0.28 \pm 0.26$ MeV/$c^2$, where the first and second uncertainties are statistical and systematic, respectively.

Figures and captions

Feynman diagram of $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda ^*$ and $\Lambda ^0_ b \rightarrow \chi_{cJ}\Lambda ^*$ decays, where $\Lambda ^*$ refers to an excited $\Lambda $ baryon.

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Fits to the (a) $\Lambda ^0_ b \rightarrow \chi _{ c 1} p K ^- $ and (b) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ invariant mass distributions. Data points are shown in black and the results of the fits are shown as solid blue lines. The components are $\Lambda ^0_ b \rightarrow \chi _{ c 1} p K ^- $ and $\Lambda ^0_ b \rightarrow \chi _{ c 2} p K ^- $ signal and combinatorial background (Comb.).

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Distributions of (a) $m(\chi _{ c 1} p K ^- )$ versus $\Delta m + m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$, where $\Delta m$ is $m(\mu^+\mu^-\gamma) - m(\mu^+\mu^-)$ and $m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ is the known mass of the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ meson [35], and (b) $\Delta m + m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} }$ for background-subtracted $\Lambda ^0_ b \rightarrow \chi _{ c 1} p K ^- $ (green circles) and $\Lambda ^0_ b \rightarrow \chi _{ c 2} p K ^- $ (magenta triangles) candidates (cands).

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Animated gif made out of all figures.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.