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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.

Conventional, hadronic matter consists of baryons and mesons made of three quarks and a quark–antiquark pair, respectively 1 , 2 . Here, we report the observation of a hadronic state containing four quarks in the Large Hadron Collider beauty experiment. This so-called tetraquark contains two charm quarks, a u ¯ and a d ¯ quark. This exotic state has a mass of approximately 3,875 MeV and manifests as a narrow peak in the mass spectrum of D 0 D 0 π + mesons just below the D *+ D 0 mass threshold. The near-threshold mass together with the narrow width reveals the resonance nature of the state. The LHCb Collaboration reports the observation of an exotic, narrow, tetraquark state that contains two charm quarks, an up antiquark and a down antiquark.

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 T c c + tetraquark with a quark content of 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 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 T c c + state. In addition, an unexpected dependence of the production rate on track multiplicity is observed. The existence and properties of tetraquark states with two heavy quarks and two light antiquarks have been widely debated. Here, the authors use a unitarized model to study the properties of an exotic narrow state compatible with a doubly charmed tetraquark.

A bstract The W boson mass is measured using proton-proton collision data at s = 13 TeV corresponding to an integrated luminosity of 1.7 fb − 1 recorded during 2016 by the LHCb experiment. With a simultaneous fit of the muon q/p T distribution of a sample of W → μν decays and the ϕ * distribution of a sample of Z → μμ decays the W boson mass is determined to be m w = 80354 ± 23 stat ± 10 exp ± 17 theory ± 9 PDF MeV , where uncertainties correspond to contributions from statistical, experimental systematic, theoretical and parton distribution function sources. This is an average of results based on three recent global parton distribution function sets. The measurement agrees well with the prediction of the global electroweak fit and with previous measurements.

A bstract An angular analysis of the rare decay B s 0 → ϕμ + μ − is presented, using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 8.4 fb − 1 . The observables describing the angular distributions of the decay B s 0 → ϕμ + μ − are determined in regions of q 2 , the square of the dimuon invariant mass. The results are consistent with Standard Model predictions.

This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C.

A bstract Measurements are reported of the central exclusive production of J/ψ and ψ (2 S ) 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 p b σ ψ 2 S → μ + μ − = 11.1 ± 1.1 ± 0.3 ± 0.4 p b . 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 bstract A combination of measurements sensitive to the CP violation angle γ of the Cabibbo-Kobayashi-Maskawa unitarity triangle and to the charm mixing parameters that describe oscillations between D 0 and D ¯ 0 mesons is performed. Results from the charm and beauty sectors, based on data collected with the LHCb detector at CERN’s Large Hadron Collider, are combined for the first time. This method provides an improvement on the precision of the charm mixing parameter y by a factor of two with respect to the current world average. The charm mixing parameters are determined to be x = 0.400 − 0.053 + 0.052 % and y = 0.630 − 0.030 + 0.033 % . The angle γ is found to be γ = 65.4 − 4.2 + 3.8 ° and is the most precise determination from a single experiment.

A bstract The isospin asymmetries of B → Kμ + μ − and B → K * μ + μ − decays and the partial branching fractions of the B 0 → K 0 μ + μ − , B + → K + μ + μ − and B + → K *+ μ + μ − decays are measured as functions of the dimuon mass squared, q 2 . The data used correspond to an integrated luminosity of 3 fb −1 from proton-proton collisions collected with the LHCb detector at centre-of-mass energies of 7 TeV and 8 TeV in 2011 and 2012, respectively. The isospin asymmetries are both consistent with the Standard Model expectations. The three measured branching fractions favour lower values than their respective theoretical predictions, however they are all individually consistent with the Standard Model.

The kinematic dependences of the relative production rates, $f_{\\Lambda_b^0}/f_d$, of $\\Lambda_b^0$ baryons and $\\bar{B}^0$ mesons are measured using $\\Lambda_b^0 \\rightarrow \\Lambda_c^+ \\pi^-$ and $\\bar{B}^0 \\rightarrow D^+ \\pi^-$ 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, $\\eta$, of the beauty hadron, in the studied kinematic region $1.5 < p_T < 40$ GeV/$c$ and $2 < \\eta < 5$. Using a previous LHCb measurement of $f_{\\Lambda_b^0}/f_d$ in semileptonic decays, the branching fraction $\\mathcal{B}(\\Lambda_b^0 \\rightarrow \\Lambda_c^+ \\pi^-) = \\Big( 4.30 \\pm 0.03 \\,\\, ^{+0.12}_{-0.11} \\pm 0.26 \\pm 0.21 \\Big) \\times 10^{-3}$ is obtained, where the first uncertainty is statistical, the second is systematic, the third is from the previous LHCb measurement of $f_{\\Lambda_b^0}/f_d$ and the fourth is due to the $\\bar{B}^0 \\rightarrow D^+ \\pi^-$ branching fraction. This is the most precise measurement of a $\\Lambda_b^0$ branching fraction to date.