The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.

The production of J/ψ mesons in proton-proton collisions at √ s = 7 TeV is studied with the LHCb detector at the LHC. The differential cross-section for prompt J/ψ production is measured as a function of the J/ψ transverse momentum p T and rapidity y in the fiducial region p T ∈ [0; 14] GeV/c and y ∈ [2.0; 4 cross-section and fraction of J/ψ from b-hadron decays are also measured in the same p T and y ranges. The analysis is based on a data sample corresponding to an integrated luminosity of 5.2 pb −1 . The measured cross-sections integrated over the fiducial region are 10.52 ± 0.04 ± 1.40−2.20 µb for prompt J/ψ production and 1.14 ± 0.01 ± 0.16 µb for J/ψ from b-hadron decays, where the first uncertainty is statistical and the second systematic. The prompt J/ψ production cross-section is obtained assuming no J/ψ polarisation and the third error indicates the acceptance uncertainty due to this assumption.

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 standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.

An angular analysis of the B 0 → K Ã0 ð→ K þ π − Þμ þ μ − decay is presented using a dataset corresponding to an integrated luminosity of 4.7 fb −1 of pp collision data collected with the LHCb experiment. The full set of CP-averaged observables are determined in bins of the invariant mass squared of the dimuon system. Contamination from decays with the K þ π − system in an S-wave configuration is taken into account. The tension seen between the previous LHCb results and the standard model predictions persists with the new data. The precise value of the significance of this tension depends on the choice of theory nuisance parameters.