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Measurement of the ratio of branching fractions of the decays $\Lambda_b^0 \to \psi(2S) \Lambda$ and $\Lambda_b^0 \to J / \psi \Lambda$

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Abstract

Using $pp$ collisions corresponding to 3$ \text{fb}^{-1}$ integrated luminosity, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8$ \text{TeV}$, the ratio of branching fractions \begin{align*} \mathcal{B}(\Lambda^0_b \to\psi(2S) \Lambda )/\mathcal{B}(\Lambda^0_b \to J/\psi \Lambda)= 0.513 \pm 0.023 (\text{stat}) \pm 0.016 (\text{syst}) \pm 0.011 (\mathcal{B}) \end{align*} is determined. The first uncertainty is statistical, the second is systematic and the third is due to the external branching fractions used.

Figures and captions

Interpolated efficiency function for long-track candidates for (a) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (b) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ and for downstream-track candidates for (c) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (d) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates. The distribution of the candidates on data is shown with black dots (each dot refers to one candidate). Statistical fluctuations of the simulated sample are contributing to the efficiency function at the phase-space boundaries, where data candidates are not affected.

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Fits to the (unweighted) invariant-mass distributions of long-track candidates for (a) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (b) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ and for downstream-track candidates for (c) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (d) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates. The signal (blue, dashed), the combinatorial background (green, dotted), the $ B ^0 \rightarrow \psi K ^0_{\mathrm{ \scriptscriptstyle S}} $ background (cyan, long-dash-dotted) and the $\Xi ^-_ b \rightarrow \psi \Xi ^- $ background (violet, dash-triple-dotted) are indicated.

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Fits to the weighted invariant-mass distributions of long-track candidates for (a) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (b) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ and for downstream-track candidates for (c) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and (d) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ candidates. The signal (blue, dashed), the combinatorial background (green, dotted), the $ B ^0 \rightarrow \psi K ^0_{\mathrm{ \scriptscriptstyle S}} $ background (cyan, long-dash-dotted) and the $\Xi ^-_ b \rightarrow \psi \Xi ^- $ background (violet, dash-triple-dotted) are indicated.

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Animated gif made out of all figures.

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Tables and captions

Yields from the invariant-mass fits in the range 5350 to 5750 $ {\mathrm{ Me V /}c^2}$ of (top) $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ decays and (bottom) $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ decays for each component.

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Efficiency-corrected yields of $\Lambda ^0_ b \rightarrow \psi {(2S)} \Lambda $ and $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \Lambda $ signal decays from the fit to the weighted invariant mass for both track types.

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Relative systematic uncertainties on the ratio of branching fractions.

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Created on 30 May 2020.