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Searches are performed for both promptlike and long-lived dark photons, A 0 , produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A 0 → μ þ μ − decays using a data sample corresponding to an integrated luminosity of 5.5 fb −1 collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ-A 0 kinetic mixing strength. The promptlike A 0 search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214 < mðA 0 Þ ≲ 740 MeV and 10.6 < mðA 0 Þ ≲ 30 GeV. The search for long-lived A 0 → μ þ μ − decays places world-leading constraints on low-mass dark photons with lifetimes Oð1Þ ps.

The doubly charmed baryon decay Ξ þþ cc → Ξ þ c π þ is observed for the first time, with a statistical significance of 5.9σ, confirming a recent observation of the baryon in the Λ þ c K − π þ π þ final state. The data sample used corresponds to an integrated luminosity of 1.7 fb −1 , collected by the LHCb experiment in pp collisions at a center-of-mass energy of 13 TeV. The Ξ þþ cc mass is measured to be 3620.6 AE 1.5ðstatÞ AE 0.4ðsystÞ AE 0.3ðΞ þ c Þ MeV=c 2 and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be ½BðΞ þþ cc → Ξ þ c π þ Þ × BðΞ þ c → pK − π þ Þ=½BðΞ þþ cc → Λ þ c K − π þ π þ Þ × BðΛ þ c → pK − π þ Þ ¼ 0.035 AE 0.009ðstatÞ AE 0.003ðsystÞ.

The first measurement of the lifetime of the doubly charmed baryon Ξ_{cc}^{++} is presented, with the signal reconstructed in the final state Λ_{c}^{+}K^{-}π^{+}π^{+}. The data sample used corresponds to an integrated luminosity of 1.7  fb^{-1}, collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV. The Ξ_{cc}^{++} lifetime is measured to be 0.256_{-0.022}^{+0.024}(stat)±0.014(syst)  ps.

We report the observation of a new structure in the Λ 0 b π þ π − spectrum using the full LHCb data set of pp collisions, corresponding to an integrated luminosity of 9 fb −1 , collected at ffiffi ffi s p ¼ 7, 8, and 13 TeV. A study * Full author list given at the end of the article.