Searches are performed for a low-mass dimuon resonance, X, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using a data sample corresponding to an integrated luminosity of 5.1 fb−1 and collected with the LHCb detector. The X bosons can either decay promptly or displaced from the proton-proton collision, where in both cases the requirements placed on the event and the assumptions made about the production mechanisms are kept as minimal as possible. The searches for promptly decaying X bosons explore the mass range from near the dimuon threshold up to 60 GeV, with nonnegligible X widths considered above 20 GeV. The searches for displaced X → μ+μ− decays consider masses up to 3 GeV. None of the searches finds evidence for a signal and 90% confidence-level exclusion limits are placed on the X → μ+μ− cross sections, each with minimal model dependence. In addition, these results are used to place world-leading constraints on GeV-scale bosons in the two-Higgs-doublet and hidden-valley scenarios.

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Study of the ψ 2 (3823) and χ c1 (3872) states in B + → (J/ψπ + π −) K + decays The LHCb collaboration

Measurements are reported of the central exclusive production of J/ψ and ψ(2S) mesons in pp collisions at a centre-of-mass energy of 13 TeV. Backgrounds are significantly reduced compared to previous measurements made at lower energies through the use of new forward shower counters. The products of the cross-sections and the branching fractions for the decays to dimuons, where both muons are within the pseudorapidity range 2.0 < η < 4.5, are measured to be σ J/ψ→µ + µ − = 435 ± 18 ± 11 ± 17 pb σ ψ(2S)→µ + µ − = 11.1 ± 1.1 ± 0.3 ± 0.4 pb. The first uncertainties are statistical, the second are systematic, and the third are due to the luminosity determination. The cross-sections are also measured differentially for meson rapidities between 2.0 and 4.5. Good agreement is observed with theoretical predictions. Photoproduction cross-sections are derived and compared to previous experiments, and a deviation from a pure power-law extrapolation of lower energy data is observed.

A Dalitz plot analysis of decays is performed using data samples of pp collisions collected with the detector at centre-of-mass energies of and , corresponding to a total integrated luminosity of . A satisfactory description of the data is obtained when including a contribution representing an exotic resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are and , respectively. The spin-parity assignments and are both consistent with the data. In addition, the first measurement of the branching fraction is performed and gives where the first uncertainty is statistical, the second systematic, and the third is due to limited knowledge of external branching fractions.