An angular analysis of the B ⁰ → K ^*0(→ K ⁺ π ⁻)μ ⁺ μ ⁻ decay is presented. The dataset corresponds to an integrated luminosity of 3.0 fb⁻¹ of pp collision data collected at the LHCb experiment. The complete angular information from the decay is used to determine CP-averaged observables and CP asymmetries, taking account of possible contamination from decays with the K ⁺ π ⁻ system in an S-wave configuration. The angular observables and their correlations are reported in bins of q ², the invariant mass squared of the dimuon system. The observables are determined both from an unbinned maximum likelihood fit and by using the principal moments of the angular distribution. In addition, by fitting for q ²-dependent decay amplitudes in the region 1.1 < q ² < 6.0 GeV²/c ⁴, the zero-crossing points of several angular observables are computed. A global fit is performed to the complete set of CP-averaged observables obtained from the maximum likelihood fit. This fit indicates differences with predictions based on the Standard Model at the level of 3.4 standard deviations. These differences could be explained by contributions from physics beyond the Standard Model, or by an unexpectedly large hadronic effect that is not accounted for in the Standard Model predictions. Open image in new window

Resonant structures in $B^0\to {\psi }^{\prime }{\pi }^{-}{K}^{+}$ decays are analyzed by performing a four-dimensional fit of the decay amplitude, using $pp$ collision data corresponding to $3{\mathrm{fb}}^{-1}$ collected with the LHCb detector. The data cannot be described with $K^{+}{\pi }^{-}$ resonances alone, which is confirmed with a model-independent approach. A highly significant $Z(4430{)}^{-}\to {\psi }^{\prime }{\pi }^{-}$ component is required, thus confirming the existence of this state. The observed evolution of the $Z(4430{)}^{-}$ amplitude with the ${\psi }^{\prime }{\pi }^{-}$ mass establishes the resonant nature of this particle. The mass and width measurements are substantially improved. The spin parity is determined unambiguously to be $1^{+}$.

A measurement is reported of the ratio of branching fractions $\mathcal{R}(J/\psi )=\mathcal{B}(B_c^{+}\to J/\psi {\tau }^{+}{\nu }_{\tau })/\mathcal{B}(B_c^{+}\to J/\psi {\mu }^{+}{\nu }_{\mu })$, where the ${\tau }^{+}$ lepton is identified in the decay mode ${\tau }^{+}\to {\mu }^{+}{\nu }_{\mu }{\overline{\nu }}_{\tau }$. This analysis uses a sample of proton-proton collision data corresponding to $3.0{\mathrm{fb}}^{-1}$ of integrated luminosity recorded with the LHCb experiment at center-of-mass energies of 7 and 8 TeV. A signal is found for the decay $B_c^{+}\to J/\psi {\tau }^{+}{\nu }_{\tau }$ at a significance of 3 standard deviations corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be $\mathcal{R}(J/\psi )=0.71\pm 0.17(\mathrm{stat})\pm 0.18(\mathrm{syst})$. This result lies within 2 standard deviations above the range of central values currently predicted by the standard model.

Searches are performed for both promptlike and long-lived dark photons, $A^{\prime }$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{\prime }\to {\mu }^{+}{\mu }^{-}$ decays and a data sample corresponding to an integrated luminosity of $1.6{\mathrm{fb}}^{-1}$ collected with the LHCb detector. The promptlike $A^{\prime }$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{\prime }$ search is restricted to the low-mass region $214<m(A^{\prime })<350\mathrm{MeV}$. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $\gamma –A^{\prime }$ kinetic-mixing strength. The constraints placed on promptlike dark photons are the most stringent to date for the mass range $10.6<m(A^{\prime })<70\mathrm{GeV}$, and are comparable to the best existing limits for $m(A^{\prime })<0.5\mathrm{GeV}$. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.

A search for charge-parity ($CP$) violation in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is reported, using $pp$ collision data corresponding to an integrated luminosity of $5.9{\mathrm{fb}}^{-1}$ collected at a center-of-mass energy of 13 TeV with the LHCb detector. The flavor of the charm meson is inferred from the charge of the pion in $D^{*}(2010{)}^{+}\to D^0{\pi }^{+}$ decays or from the charge of the muon in $\overline{B}\to D^0{\mu }^{-}{\overline{\nu }}_{\mu }X$ decays. The difference between the $CP$ asymmetries in $D^0\to K^{-}{K}^{+}$ and $D^0\to {\pi }^{-}{\pi }^{+}$ decays is measured to be $\mathrm{\Delta }{A}_{CP}=[-18.2\pm 3.2(\mathrm{stat})\pm 0.9(\mathrm{syst})]\times {10}^{-4}$ for $\pi$-tagged and $\mathrm{\Delta }{A}_{CP}=[-9\pm 8(\mathrm{stat})\pm 5(\mathrm{syst})]\times {10}^{-4}$ for $\mu$-tagged $D^0$ mesons. Combining these with previous LHCb results leads to $\mathrm{\Delta }{A}_{CP}=(-15.4\pm 2.9)\times {10}^{-4}$, where the uncertainty includes both statistical and systematic contributions. The measured value differs from zero by more than 5 standard deviations. This is the first observation of $CP$ violation in the decay of charm hadrons.