The doubly Cabibbosuppressed decay $\Xi_{c}^{+}\to p\phi$ with $\phi\to K^{+}K^{}$ is observed for the first time, with a statistical significance of more than fifteen standard deviations. The data sample used in this analysis corresponds to an integrated luminosity of 2 fb$^{1}$ recorded with the LHCb detector in $pp$ collisions at a centreofmass energy of 8 TeV. The ratio of branching fractions between the decay $\Xi_{c}^{+}\to p\phi$ and the singly Cabibbosuppressed decay $\Xi_{c}^{+}\to pK^{}\pi^{+}$ is measured to be \begin{equation*} \frac{\mathcal{B}(\Xi_{c}^{+}\to p\phi)}{\mathcal{B}(\Xi_{c}^{+}\to pK^{}\pi^{+})} = (19.8 \pm 0.7 \pm 0.9 \pm 0.2) \times 10^{3}, \end{equation*} where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the $\phi\to K^{+}K^{}$ branching fraction.
Tree quark diagram for the $\Xi ^+_ c \rightarrow p \phi $ decay. 
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(Left) Fit results for the $\Xi ^+_ c \rightarrow p K ^ K ^+ $ decay. The candidates are selected in the $\phi $ meson region, i.e. with the requirement of $ M_{ K ^ K ^+ } <1.07 {\mathrm{ Ge V /}c^2} $. The red dotted line corresponds to the signal component, the black dashed line reflects the background distribution, and the blue solid line is their sum. (Right) Background subtracted $ K ^ K ^+ $ mass distribution for the $\Xi ^+_ c \rightarrow p K ^ K ^+ $ decay. The red dotted line shows the $\Xi ^+_ c \rightarrow p \phi $ contribution, the black dashed line represents the non $\phi $ contribution, and the solid blue line is the total fit function. 
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Fit results for the $\Xi ^+_ c \rightarrow p K ^ \pi ^+ $ decay. The red dotted line corresponds to the signal component, the black dashed line reflects the background distribution and the blue solid line is their sum. 
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Animated gif made out of all figures. 
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Systematic uncertainties relative to the central value of the ratio $R_{p\phi}$. 
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Created on 11 November 2019.