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Observation of $ \Lambda_b^0 \to \psi(2S)pK^-$ and $ \Lambda_b^0 \to J/\psi \pi^+ \pi^- pK^-$ decays and a measurement of the $\Lambda_b^0$ baryon mass

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Abstract

The decays $ \Lambda_b^0 \to \psi(2S)pK^-$ and $ \Lambda_b^0 \to J/\psi \pi^+ \pi^- pK^-$ are observed in a data sample corresponding to an integrated luminosity of 3fb$^{-1}$, collected in proton-proton collisions at 7 and 8TeV centre-of-mass energies by the LHCb detector. The $\psi(2S)$ mesons are reconstructed through the decay modes $\psi(2S)\to\mu^+\mu^-$ and $\psi(2S)\to J/\psi\pi^+\pi^-$. The branching fractions relative to that of $ \Lambda_b^0 \to J/\psi pK^-$ are measured to be \begin{eqnarray*} \frac{\mathcal{B}(\Lambda_b^0 \to \psi(2S) pK^-)} {\mathcal{B}(\Lambda_b^0 \to J/\psi pK^-)} & = & (20.70\pm 0.76\pm 0.46\pm 0.37)\times10^{-2} , \frac{\mathcal{B}(\Lambda_b^0 \to J/\psi \pi^+ \pi^- pK^-)} {\mathcal{B}(\Lambda_b^0 \to J/\psi pK^-)} & = & (20.86\pm 0.96\pm 1.34)\times10^{-2} , \end{eqnarray*} where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of $J/\psi$ and $\psi(2S)$ branching fractions. The mass of the $\Lambda_b^0$ baryon is measured to be $$ M(\Lambda_b^0) = 5619.65 \pm 0.17 \pm 0.17\mathrm{MeV}/c^2,$$ where the uncertainties are statistical and systematic.

Figures and captions

Mass distributions of selected (left) $\Lambda ^0_ b \rightarrow \psi {(2 S )} [\rightarrow \mu ^+\mu ^- ] p K ^- $ and (right) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $ candidates. The total fit function (solid red), the $\Lambda ^0_ b $ signal contribution (dotted magenta) and the combinatorial background (dashed blue) are shown. The error bars show 68% Poissonian confidence intervals.

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(left) Mass distribution of selected $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- p K ^- $ candidates. (right) Background-subtracted ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } $ mass distribution for that mode. The total fit function and the signal contributions are shown by solid red and dotted magenta lines, respectively. The combinatorial background in the left plot and nonresonant contribution in the right plot are shown by dashed blue lines.

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(left) Mass distribution of selected $\Lambda ^0_ b \rightarrow \psi {(2 S )} [\rightarrow { { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } ] p K ^- $ candidates with an additional constraint for the $\psi {(2 S )}$ mass [36]. (right) Mass distribution of selected $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- p K ^- $ candidates with a requirement of the ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } $ combination mass to be outside the range $3670<m({ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } )<3700 {\mathrm{ Me V /}c^2} $. The total fit function (solid red), the $\Lambda ^0_ b $ signal contribution (dotted magenta) and the combinatorial background (dashed blue) are shown.

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Animated gif made out of all figures.

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Tables and captions

Signal yields of $\Lambda ^0_ b $ decay channels. Uncertainties are statistical only.

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Ratios of efficiencies. The uncertainties reflect the limited size of the simulation sample.

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Systematic uncertainties (in %) on the ratios of branching fractions $R^{\psi {(2 S )} }$ and $R^{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- }$.

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Systematic uncertainties (in $ {\mathrm{ Me V /}c^2}$ ) on the $\Lambda ^0_ b $ mass using the decay modes $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p K ^- $, $\Lambda ^0_ b \rightarrow \psi {(2 S )} [\rightarrow \mu ^+\mu ^- ] p K ^- $, $\Lambda ^0_ b \rightarrow \psi {(2 S )} [\rightarrow { { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } ] p K ^- $ and $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- p K ^- $ with the ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- } $ mass outside the $\psi {(2 S )}$ region.

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Measured $\Lambda ^0_ b $ mass in different decay channels. The first uncertainty is statistical and the second is systematic.

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Supplementary Material [file]

Supplementary material full pdf

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Created on 26 April 2024.