| Summary: | The doubly Cabibbo-suppressed decay Ξ c+ → pϕ with ϕ → 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 centre-of-mass energy of 8 TeV. The ratio of branching fractions between the decay Ξ c+ → pϕ and the singly Cabibbo-suppressed decay Ξ c+→ pK−π+ is measured to be ℬΞc+ → p ϕℬΞc+ → pK−π+ = 19.8 ± 0.7 ± 0.9 ± 0.2 × 10 −3, where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the ϕ → K+K− branching fraction.
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