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Measurement of the Λb0→Λ(1520)μ+μ- Differential Branching Fraction

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Abstract

The branching fraction of the rare decay $\Lambda_{b}^{0}\to \Lambda(1520) \mu^{+}\mu^{-}$ is measured for the first time, in the squared dimuon mass intervals, $q^2$, excluding the $J/\psi$ and $\psi(2S)$ regions. The data sample analyzed was collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of $9\ \mathrm{fb}^{-1}$. The result in the highest $q^{2}$ interval, $q^{2} >15.0\ \mathrm{GeV}^2/c^4$, where theoretical predictions have the smallest model dependence, agrees with the predictions.

Figures and captions

Mass distribution for the (left) $\Lambda ^0_ b \rightarrow p K ^- J/\psi $ and (right) $\Lambda ^0_ b \rightarrow p K ^- \mu ^+\mu ^- $ , integrated over the considered $ q^2$ intervals. Fit results are overlaid.

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Distribution of the $ p K ^- $ mass, after background subtraction, for $\Lambda ^0_ b \rightarrow \Lambda(1520) \mu ^+\mu ^- $ signal candidates in the (left) $1.1< q^2 <6.0\text{ Ge V} ^2 /c^4 $ and (right) $15.0< q^2 <17.0\text{ Ge V} ^2 /c^4 $ regions. Fit results are overlaid.

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Differential branching fraction of the $\Lambda ^0_ b \rightarrow \Lambda(1520) \mu ^+\mu ^- $ decay in intervals of $ q^2$ . The error bars in black, gray and green represent the measured results with statistical, systematic and $\mathcal{B}(\Lambda ^0_ b \rightarrow p K ^- J/\psi )$ uncertainties taken into account. Also shown are the SM predictions using the form factors calculated with the nonrelativistic quark model (NRQM) [62], light-front quark model (LFQM) [63], joint lattice QCD and dispersive bound (LQCD+DB) [64] and lattice QCD (LQCD) [65]. Note that the LQCD prediction is only available for $ q^2 $ above $16\text{ Ge V} ^2 /c^4 $, and the trend instead of a rate average is shown. These theoretical predictions as well as evaluation of the corresponding uncertainties should be revisited given their significant differences.

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Mass distributions of the $\Lambda ^0_ b \rightarrow p K ^- \mu ^+\mu ^- $ signal candidates in different $ q^2$ intervals. The data are overlaid with the fit projections.

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Distribution of $ p K ^- $ mass, after background subtraction, for the $\Lambda ^0_ b \rightarrow \Lambda(1520) \mu ^+\mu ^- $ signal candidates in intervals of $ q^2$ . Distributions are overlaid with the fit projections.

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Comparison between data and SM predictions. Note that the LQCD prediction is only available for $ q^2 $ above $16\text{ Ge V} ^2 /c^4 $, and the trend instead of a rate average is shown.

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Animated gif made out of all figures.

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Tables and captions

Relative systematic uncertainties [in %] of the differential branching fraction ratio measurement. The total uncertainty is obtained as the quadratic sum of the individual contributions.

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Signal yields and the absolute differential branching fraction, in bins of $ q^2 $, for the $\Lambda ^0_ b \rightarrow \Lambda(1520) \mu ^+\mu ^- $ decay. The first uncertainty is statistical, the second systematic, and the third due to the uncertainty on the $\Lambda ^0_ b \rightarrow p K ^- J/\psi $ and $ { J \mskip -3mu/\mskip -2mu\psi } \rightarrow \mu^+\mu^-$ branching fractions.

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Created on 13 April 2024.