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Search for the rare decays $W^{+}\rightarrow{D^{+}_{s}\gamma}$ and $Z\rightarrow{D^{0}\gamma}$ at LHCb

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Abstract

A search for the rare decays $W^{+}\rightarrow{D^{+}_{s}\gamma}$ and $Z\rightarrow{D^{0}\gamma}$ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 2.0 fb$^{-1}$. No significant signal is observed for either decay mode and upper limits on their branching fractions are set using $W^{+}\rightarrow{\mu^{+}\nu}$ and $Z\rightarrow{\mu^{+}\mu^{-}}$ decays as normalization channels. The upper limits are $6.5\times10^{-4}$ and $2.1\times10^{-3}$ at 95 confidence level for the $W^{+}\rightarrow{D^{+}_{s}\gamma}$ and $Z\rightarrow{D^{0}\gamma}$ decay modes, respectively. This is the first reported search for the $Z\rightarrow{D^{0}\gamma}$ decay, while the upper limit on the $W^{+}\rightarrow{D^{+}_{s}\gamma}$ branching fraction improves upon the previous best limit.

Figures and captions

Tree-level Feynman diagrams for the decay $ W ^+ \rightarrow D ^+_ s {\gamma } $ .

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Feynman diagrams contributing to the $ Z \rightarrow D ^0 {\gamma } $ decay in the SM.

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Distributions of (left) pseudomass and (right) $ p_{\mathrm{T}}$ for (upper) $ W ^+ \rightarrow D ^+_ s {\gamma } $ and (lower) $ Z \rightarrow D ^0 {\gamma } $ candidates. The blue points represent the selected data candidates, the red points represent simulated signal events, normalized to the branching fraction of $ W ^+ \rightarrow D ^+_ s {\gamma } $ ( $ Z \rightarrow D ^0 {\gamma } $ ) set to $3\times10^{-2}$ ($5\times10^{-2}$), and the green lines represent the background shape, derived from pseudodata.

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Upper limits on branching fractions of the (left) $ W ^+ \rightarrow D ^+_ s {\gamma } $ and (right) $ Z \rightarrow D ^0 {\gamma } $ decays.

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Animated gif made out of all figures.

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Tables and captions

Relative systematic uncertainties (in %) in the $\cal{R}$ measurements for the $ Z \rightarrow D ^0 {\gamma } $ and $ W ^+ \rightarrow D ^+_ s {\gamma } $ decay modes. The total systematic uncertainty is obtained from the sum in quadrature of all contributions.

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Created on 27 April 2024.