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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. The Large Hadron Collider beauty collaboration reports a test of lepton flavour universality in decays of bottom mesons into strange mesons and a charged lepton pair, finding evidence of a violation of this principle postulated in the standard model.

Charged-particle trajectories are usually reconstructed with the LHCb detector using combined information from the tracking devices placed upstream and downstream of the 4 T m dipole magnet. Trajectories reconstructed using only information from the tracker downstream of the dipole magnet, which are referred to as T tracks, have not been used for physics analysis to date. The challenges of the reconstruction of long-lived particles with T tracks for physics use are discussed and solutions are proposed. The feasibility and the tracking performance are studied using samples of long-lived Λ and K S 0 hadrons decaying between 6.0 and 7.6 m downstream of the proton–proton collision point, thereby traversing most of the magnetic field region and providing maximal sensitivity to magnetic and electric dipole moments. The reconstruction can be expanded upstream to about 2.5 m for use in direct searches of exotic long-lived particles. The data used in this analysis have been recorded between 2015 and 2018 and correspond to an integrated luminosity of 6  fb - 1 . The results obtained demonstrate the possibility to further extend the decay volume and the physics reach of the LHCb experiment.

A bstract The decays of the χ b1 (1P), χ b2 (1P), χ b1 (2P) and χ b2 (2P) mesons into the Υ(1S)μ + μ − final state are observed with a high significance using proton-proton collision data collected with the LHCb detector and corresponding to an integrated luminosity of 9 fb − 1 . The newly observed decays together with the Υ(2S) → Υ(1S)π + π − and Υ(3S) → Υ(2S)π + π − decay modes are used for precision measurements of the mass and mass splittings for the hidden-beauty states.

Using proton-proton collision data, corresponding to an integrated luminosity of 9 fb(-1) collected with the LHCb detector, seven decay modes of the B-C(+) meson into a J/psi or psi(2S) meson and three charged hadrons, kaons or pions, are studied. The decays B-C (+)-> (psi(2S) -> J/psi pi(+)pi(-))pi(+), B+C -> (psi(2S)pi(+)pi(-)pi(+), B+C -> J/psi K+pi(-)pi(+) and B+C -> J/K+K-K+ are observed for the first time, and evidence for the B-C (+)-> (psi(2S)K+K-pi(+), decay is found, where J/psi and psi(2S) mesons are reconstructed in their dimuon decay modes. The ratios of branching fractions between the different B-C(+) decays are reported as well as the fractions of the decays proceeding via intermediate resonances. The results largely support the factorisation approach used for a theoretical description of the studied decays.

A bstract Measurements of CP observables in B ± → D (*) K ± and B ± → D (*) π ± decays are presented, where D (∗) indicates a neutral D or D ∗ meson that is an admixture of meson and anti-meson states. Decays of the D (∗) meson to the Dπ 0 and Dγ final states are partially reconstructed without inclusion of the neutral pion or photon. Decays of the D meson are reconstructed in the K ± π ∓ , K + K − , and π + π − final states. The analysis uses a sample of charged B mesons produced in proton-proton collisions and collected with the LHCb experiment, corresponding to integrated luminosities of 2.0, 1.0, and 5.7 fb − 1 taken at centre-of-mass energies of 7, 8, and 13 TeV, respectively. The measurements of partially reconstructed B ± → D (*) K ± and B ± → D (∗) π ± with D → K ∓ π ± decays are the first of their kind, and a first observation of the B ± → D π 0 D ∗ π ± decay is made with a significance of 6.1 standard deviations. All CP observables are measured with world-best precision, and in combination with other LHCb results will provide strong constraints on the CKM angle γ .

A bstract The difference between the CP asymmetries in the decays Λ c +  →  pK − K + and Λ c +  →  pπ − π + is presented. Proton-proton collision data taken at centre-of-mass energies of 7 and 8 TeV collected by the LHCb detector in 2011 and 2012 are used, corresponding to an integrated luminosity of 3 fb −1 . The Λ c + candidates are reconstructed as part of the Λ b 0  →  Λ c + μ − X decay chain. In order to maximize the cancellation of production and detection asymmetries in the difference, the final-state kinematic distributions of the two samples are aligned by applying phase-space-dependent weights to the Λ c +  →  pπ − π + sample. This alters the definition of the integrated CP asymmetry to A CP wgt ( pπ − π + ). Both samples are corrected for reconstruction and selection efficiencies across the five-dimensional Λ c + decay phase space. The difference in CP asymmetries is found to be Δ A C P w g t = A C P p K − K + − A C P w g t p π − π + = 0.30 ± 0.91 ± 0.61 % , where the first uncertainty is statistical and the second is systematic.

Quantum chromodynamics, the theory of the strong force, describes interactions of coloured quarks and gluons and the formation of hadronic matter. Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. Particles with an alternative quark content are known as exotic states. Here a study is reported of an exotic narrow state in the D 0 D 0 π + mass spectrum just below the D *+ D 0 mass threshold produced in proton-proton collisions collected with the LHCb detector at the Large Hadron Collider. The state is consistent with the ground isoscalar$${{{\\rm{T}}}_{{{\\rm{c}}}{{{\\rm{c}}}^{+}$$T c c + tetraquark with a quark content of$${{{\\rm{c}}}{{{\\rm{c}}}\\overline{{{\\rm{u}}}\\overline{{{\\rm{d}}}$$c c u ¯ d ¯ and spin-parity quantum numbers J P  = 1 + . Study of the DD mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell D *+ mesons is consistent with the observed D 0 π + mass distribution. To analyse the mass of the resonance and its coupling to the D * D system, a dedicated model is developed under the assumption of an isoscalar axial-vector$${{{\\rm{T}}}_{{{\\rm{c}}}{{{\\rm{c}}}^{+}$$T c c + state decaying to the D * D channel. Using this model, resonance parameters including the pole position, scattering length, effective range and compositeness are determined to reveal important information about the nature of the$${{{\\rm{T}}}_{{{\\rm{c}}}{{{\\rm{c}}}^{+}$$T c c + state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.

A bstract The kinematic dependences of the relative production rates, f Λ b 0 / f d , of Λ b 0 baryons and B 0 mesons are measured using Λ b 0  →  Λ c + π − and B ¯ 0 → D + π − decays. The measurements use proton-proton collision data, corresponding to an integrated luminosity of 1 fb −1 at a centre-of-mass energy of 7 TeV, recorded in the forward region with the LHCb experiment. The relative production rates are observed to depend on the transverse momentum, p T , and pseudorapidity, η , of the beauty hadron, in the studied kinematic region 1 . 5 < p T < 40 GeV /c and 2 < η < 5. Using a previous LHCb measurement of f Λ b 0 / f d in semileptonic decays, the branching fraction ℬ( Λ b 0  →  Λ c + π − ) = (4.30 ± 0.03 − 0.11 + 0.12  ± 0.26 ± 0.21) × 10 − 3 is obtained, where the first uncertainty is statistical, the second is systematic, the third is from the previous LHCb measurement of f Λ b 0 / f d and the fourth is due to the b B ¯ 0 → D + π − branching fraction. This is the most precise measurement of a Λ b 0 branching fraction to date.

A bstract The first observation of the Cabibbo-suppressed decay Λ b 0 → J / ψpπ − is reported using a data sample of proton-proton collisions at 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb −1 . A prominent signal is observed and the branching fraction relative to the decay mode Λ b 0 → J / ψpK − is determined to be ℬ Λ b 0 → J / ψp π − ℬ Λ b 0 → J / ψp K − = 0.0824 ± 0.0025 stat ± 0.0042 syst . A search for direct CP violation is performed. The difference in the CP asymmetries between these two decays is found to be A CP Λ b 0 → J / ψp π − − A CP Λ b 0 → J / ψp K − = + 5.7 ± 2.4 stat ± 1.2 syst % , which is compatible with CP symmetry at the 2.2 σ level.