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Search for the lepton-flavour violating decays $B^0 \to K^{*0} \mu^\pm e^\mp$ and $B_s^0 \to \phi \mu^\pm e^\mp$

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Abstract

A search for the lepton-flavour violating decays $B^0 \to K^{*0} \mu^\pm e^\mp$ and $B_s^0 \to \phi \mu^\pm e^\mp$ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of $9 \text{fb}^{-1}$. No significant signals are observed and upper limits of \begin{align} {\cal B}( B^0 \to K^{*0} \mu^+ e^- ) &< \phantom{1}5.7\times 10^{-9} (6.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} \mu^- e^+ ) &< \phantom{1}6.8\times 10^{-9} (7.9\times 10^{-9}),\newline {\cal B}( B^0 \to K^{*0} \mu^\pm e^\mp ) &< 10.1\times 10^{-9} (11.7\times 10^{-9}),\newline {\cal B}( B_s^0 \to \phi \mu^\pm e^\mp ) &< 16.0\times 10^{-9} (19.8\times 10^{-9}) \end{align} are set at $90\% (95\%)$ confidence level. These results constitute the world's most stringent limits to date, with the limit on the decay $B_s^0 \to \phi \mu^\pm e^\mp$ the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.

Figures and captions

Mass distributions for the normalisation channels (left) $ B ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow \mu ^+\mu ^- ) K ^{*0} $ and (right) $ B ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow \mu ^+\mu ^- )\phi $ combining the different data taking periods, overlaid with the fit results.

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Mass distributions for (top left) $ B ^0 \rightarrow K ^{*0} \mu^+ e^- $, (top right) $ B ^0 \rightarrow K ^{*0} \mu^- e^+ $, (bottom left) $ B ^0 \rightarrow K ^{*0} \mu^\pm e^\mp $, and (bottom right) $ B ^0_ s \rightarrow \phi\mu^\pm e^\mp $ candidates. The data are overlaid with the fit results. For illustration, the signal shape, scaled to a branching fraction of $5\times 10^{-8}$ for the $ B ^0 \rightarrow K ^{*0} \mu^\pm e^\mp $ decays and $1\times 10^{-7}$ for $ B ^0_ s \rightarrow \phi\mu^\pm e^\mp $, is drawn as red dashed line.

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Observed and expected (background-only hypothesis) limits for (top left) $ B ^0 \rightarrow K ^{*0} \mu^+ e^- $, (top right) $ B ^0 \rightarrow K ^{*0} \mu^- e^+ $, (bottom left) $ B ^0 \rightarrow K ^{*0} \mu^\pm e^\mp $, and (bottom right) $ B ^0_ s \rightarrow \phi\mu^\pm e^\mp $.

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Differential decay rate as a function of the four-momentum transfer $ q^2$ and the three decay angles in a left-handed ($C_9^{\mu e}=-C_{10}^{\mu e}\neq 0$) NP model for (top) the signal decay $ B ^0 \rightarrow K ^{*0} \mu^\pm e^\mp $ and (bottom) $ B ^0_ s \rightarrow \phi\mu^\pm e^\mp $. The left-handed NP scenario is compared with the nominal phase space and a scalar ($C_S^{\mu e}\neq 0$) model, normalised to the same area.

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Reconstruction and selection efficiency, combined for the 2011 - 2018 samples, for (top) $ B ^0 \rightarrow K ^{*0} \mu^\pm e^\mp $ and (bottom) $ B ^0_ s \rightarrow \phi\mu^\pm e^\mp $ signal decays depending on $ q^2$ and the three decay angles $\cos{\theta_\ell} $, $\cos{\theta_K} $, and $\phi$. The selection efficiency drops in the $ q^2$ region around the $ { J \mskip -3mu/\mskip -2mu\psi } $ and $\psi {(2S)} $ masses as a result of the veto against misidentified backgrounds.

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Animated gif made out of all figures.

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Tables and captions

Normalisation mode yields $[10^3]$ for different periods of data taking.

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Normalisation constant $\alpha$ $[10^{-9}]$ with associated statistical and systematic uncertainties, added in quadrature, for different periods of data taking. The total uncertainty is dominated by systematic effects, which are discussed in Sec. 6. The year-to-year $B^0/B_s^0$ ratio variation is due to different BDT criteria against combinatorial background, tuned individually for each data taking period and mode.

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Sources of relative systematic uncertainties $[\%]$ on the normalisation constant $\alpha$ defined in Eq. 1. Where the uncertainty depends on the year of data taking, a range is provided.

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Expected (background-only hypothesis) and observed limits $[10^{-9}]$ at $90\%$ ($95\%$) CL.

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Exclusion limits $[10^{-9}]$ on the $B_{(s)}^0$ branching fractions for a scalar ($C_s^{\mu e}\neq 0$) and left-handed ($C_9^{\mu e}=-C_{10}^{\mu e}\neq 0$) NP model at $90\%$ ($95\%$) CL.

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Created on 20 April 2024.