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First observation of the decay <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msubsup><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>b</mml:mi><mml:mn>0</mml:mn></mml:msubsup><mml:mo stretchy="false">→</mml:mo><mml:msub><mml:mi>η</mml:mi><mml:mi>c</mml:mi></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mn>1</mml:mn><mml:mi>S</mml:mi><mml:mo stretchy="false">)</mml:mo><mml:mi>p</mml:mi><mml:msup><mml:mi>K</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math>
The decay $\Lambda_b^0 \to \eta_c(1S) p K^-$ is observed for the first time using a data sample of proton-proton collisions, corresponding to an integrated luminosity of 5.5 $fb^{-1}$, collected with the LHCb experiment at a center-of-mass energy of 13 TeV. The branching fraction of the decay is measured, using the …