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Measurement of the $B^0_s \rightarrow J/\psi \bar{K}^{*0}$ branching fraction and angular amplitudes

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Abstract

A search for the decay $B^0_s\rightarrow J/\psi K^{*0}$ with $K^{*0} \rightarrow K^-\pi^+$ is performed with 0.37 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 7 TeV collected by the LHCb experiment, finding a $\Bs \to J\psi K^-\pi^+$ peak of $114 \pm 11$ signal events. The $K^-\pi^+$ mass spectrum of the candidates in the $B^0_s$ peak is dominated by the $K^{*0}$ contribution. Subtracting the non-resonant $K^-\pi^+$ component, the branching fraction of \BsJpsiKst is $(4.4_{-0.4}^{+0.5} \pm 0.8) \times 10^{-5}$, where the first uncertainty is statistical and the second systematic. A fit to the angular distribution of the decay products yields the \Kst polarization fractions $f_L = 0.50 \pm 0.08 \pm 0.02$ and $f_{||} = 0.19^{+0.10}_{-0.08} \pm 0.02$.

Figures and captions

Tree and penguin decay topologies contributing to the decays $ B^0_s\rightarrow J/\psi \overline{ K }{} ^{*0} $ and $ B ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \phi$. The dashed line indicates a colour singlet exchange.

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Fit to the $K\pi$ mass spectrum for (a) $ B ^0 \rightarrow J/\psi K \pi$ events, and (b) $ B^0_s\rightarrow J/\psi K \pi$ events. The $ B^0_{(s)}\rightarrow J/\psi K \pi$ yields in each bin of $K\pi$ mass are determined from a fit to the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K \pi$ mass spectrum. The pink dashed-dotted line represents the $ K ^{*0}$ , the red short-dashed line is the {S-wave} and the black dotted line is the $K^{*}_2(1430)$. The black solid line is their sum.

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Projections of the fit in $M_{J/\psi K\pi}$ and in the angular variables for the mass range indicated by the two dashed vertical lines in the mass plot. The red dashed, pink long-dashed, and blue dotted lines represent the fitted contributions from $ B ^0 \rightarrow J/\psi K^{*0}$ , $ B^0_s\rightarrow J/\psi \overline{ K }{} ^{*0} $ and background. The black solid line is their sum.

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Animated gif made out of all figures.

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Tables and captions

Summary of the measured $ B^0_s\rightarrow J/\psi \overline{ K }{} ^{*0} $ angular properties and their statistical and systematic uncertainties.

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Angular parameters of $ B ^0 \rightarrow J/\psi K^{*0}$ needed to compute $ {\cal B}(B^0_s\rightarrow J/\psi \overline{ K }{} ^{*0} )$ . The systematic uncertainties from background modelling and the mass $\rm PDF$ are found to be negligible in this case.

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Parameter values and errors for $\frac{ {\cal B}(B^0_s\rightarrow J/\psi \overline{ K }{} ^{*0} ) }{ {\cal B}( B ^0 \rightarrow J/\psi K^{*0}) }$.

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Created on 27 April 2024.