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The LHCb experiment has been taking data at the Large Hadron Collider (LHC) at CERN since the end of 2009. One of its key detector components is the Ring-Imaging Cherenkov (RICH) system. This provides charged particle identification over a wide momentum range, from 2–100 GeV/ c . The operation and control, software, and online monitoring of the RICH system are described. The particle identification performance is presented, as measured using data from the LHC. Excellent separation of hadronic particle types ( π , K, p) is achieved.

In the Standard Model of particle physics, the strength of the couplings of the b quark to the u and c quarks, | V ub | and | V cb |, are governed by the coupling of the quarks to the Higgs boson. Using data from the LHCb experiment at the Large Hadron Collider, the probability for the Λ b 0 baryon to decay into the p final state relative to the final state is measured. Combined with theoretical calculations of the strong interaction and a previously measured value of | V cb |, the first | V ub | measurement to use a baryonic decay is performed. This measurement is consistent with previous determinations of | V ub | using B meson decays to specific final states and confirms the existing incompatibility with those using an inclusive sample of final states. The accurate determination of quark mixing parameters is essential for the understanding of the Standard Model. The LHCb collaboration now reports the coupling strength of the b quark to the u quark through the measurement of a baryonic decay mode.

The production of the η c ( 1 S ) state in proton-proton collisions is probed via its decay to the p p ¯ final state with the LHCb detector, in the rapidity range 2.0 < y < 4.5 and in the meson transverse-momentum range p T > 6.5 GeV / c . The cross-section for prompt production of η c ( 1 S ) mesons relative to the prompt J / ψ cross-section is measured, for the first time, to be σ η c ( 1 S ) / σ J / ψ = 1.74 ± 0.29 ± 0.28 ± 0 . 18 B at a centre-of-mass energy s = 7 TeV using data corresponding to an integrated luminosity of 0.7 fb - 1 , and σ η c ( 1 S ) / σ J / ψ = 1.60 ± 0.29 ± 0.25 ± 0 . 17 B at s = 8 TeV using 2.0 fb - 1 . The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the η c ( 1 S ) and J / ψ decays to the p p ¯ final state. In addition, the inclusive branching fraction of b -hadron decays into η c ( 1 S ) mesons is measured, for the first time, to be B ( b → η c X ) = ( 4.88 ± 0.64 ± 0.29 ± 0 . 67 B ) × 10 - 3 , where the third uncertainty includes also the uncertainty on the J / ψ inclusive branching fraction from b -hadron decays. The difference between the J / ψ and η c ( 1 S ) meson masses is determined to be 114.7 ± 1.5 ± 0.1 MeV / c 2 .

The high data rates at the LHC necessitate the use of biasing selections already at the trigger level. Consequently, the correction of the biases induced by these selections becomes one of the main challenges for analyses. This paper presents the LHCb implementation of a data driven method for extracting such biases which entirely avoids uncertainties associated with detector simulation. Its novelty lies in the LHCb trigger which is implemented entirely in software, allowing its decisions to be reproduced in an exact manner offline. It is demonstrated that this method allows the control of selection biases to better than 0.1%, and that it greatly enhances the range of physics which can be performed by the LHCb experiment. The implications of trigger and software architectures on the long term viability of this method, in particular with respect to the reproducibility of trigger decisions when running the same code on different underlying hardware or compilers, is discussed.

A bstract The differential branching fraction of the rare decay Λ b 0  →  Λμ + μ − is measured as a function of q 2 , the square of the dimuon invariant mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of 3 . 0 fb −1 , collected by the LHCb experiment. Evidence of signal is observed in the q 2 region below the square of the J/ψ mass. Integrating over 15 < q 2 < 20 GeV 2 /c 4 the differential branching fraction is measured as d ℬ Λ b 0 → Λ μ + μ − / d q 2 = 1.18 − 0.08 + 0.09 ± 0.03 ± 0.27 × 10 − 7 GeV 2 / c 4 − 1 , where the uncertainties are statistical, systematic and due to the normalisation mode, Λ b 0  →  J / ψΛ , respectively. In the q 2 intervals where the signal is observed, angular distributions are studied and the forward-backward asymmetries in the dimuon ( A FB ℓ ) and hadron ( A FB h ) systems are measured for the first time. In the range 15 < q 2 < 20 GeV 2 /c 4 they are found to be A F B ℓ = − 0.05 ± 0.09 stat ± 0.03 syst and A F B h = − 0.29 ± 0.07 stat ± 0.03 syst .

A bstract A study is presented of central exclusive production of Υ( nS ) states, where the Υ( nS ) resonances decay to the μ + μ − final state, using pp collision data recorded by the LHCb experiment. The cross-section is measured in the rapidity range 2 < y (Υ) < 4 . 5 where the muons are reconstructed in the pseudorapidity range 2 < η ( μ ± ) < 4 . 5. The data sample corresponds to an integrated luminosity of 2.9 fb −1 and was collected at centre-of-mass energies of 7 TeV and 8 TeV. The measured Υ(1 S ) and Υ(2 S ) production cross-sections are σ pp → p Υ 1 S p = 9.0 ± 2.1 ± 1.7 p b and σ pp → p Υ 2 S p = 1.3 ± 0.8 ± 0.3 p b , where the first uncertainties are statistical and the second are systematic. The Υ(1 S ) crosssection is also measured as a function of rapidity and is found to be in good agreement with Standard Model predictions. An upper limit is set at 3.4 pb at the 95% confidence level for the exclusive Υ(3 S ) production cross-section, including possible contamination from χ b (3 P ) → Υ(3 S )γ decays.

A bstract The production of J/ψ mesons in proton-proton collisions at a centre-of-mass energy of s = 13 TeV is studied with the LHCb detector. Cross-section measurements are performed as a function of the transverse momentum p T and the rapidity y of the J/ψ meson in the region p T < 14 GeV/ c and 2 . 0 < y < 4 . 5, for both prompt J/ψ mesons and J/ψ mesons from b -hadron decays. The production cross-sections integrated over the kinematic coverage are 15 . 30 ± 0 . 03 ± 0 . 86 μb for prompt J/ψ and 2 . 34 ± 0 . 01 ± 0 . 13 μb for J/ψ from b -hadron decays, assuming zero polarization of the J/ ψ meson. The first uncertainties are statistical and the second systematic. The cross-section reported for J/ψ mesons from b -hadron decays is used to extrapolate to a total b b ¯ cross-section. The ratios of the cross-sections with respect to s = 8 TeV are also determined.

A bstract A measurement of the production cross-section for Z bosons that decay to muons is presented. The data were recorded by the LHCb detector during pp collisions at a centre-of-mass energy of 7 TeV, and correspond to an integrated luminosity of 1.0 fb −1 . The cross-section is measured for muons in the pseudorapidity range 2 . 0 < η < 4 . 5 with transverse momenta p T > 20 GeV /c . The dimuon mass is restricted to 60 < M μ + μ − < 120 GeV /c 2 . The measured cross-section is σ Z → μ + μ − = 76.0 ± 0.3 ± 0.5 ± 1.0 ± 1.3 p b where the uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity. This result is in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. The cross-section is also measured differentially as a function of kinematic variables of the Z boson. Ratios of the production cross-sections of electroweak bosons are presented using updated LHCb measurements of W boson production. A precise test of the Standard Model is provided by the measurement of the ratio σ W + → μ + ν μ + σ W − → μ − ν ¯ μ σ Z → μ + μ − = 20.63 ± 0.09 ± 0.12 ± 0.05 , where the uncertainty due to luminosity cancels.

The production ofJ/ψmesons accompanied by open charm, and of pairs of open charm hadrons are observed in pp collisions at a centre-of-mass energy of 7 TeV using an integrated luminosity of355pb⁻¹collected with the LHCb detector. Model independent measurements of absolute cross-sections are given together with ratios to the measuredJ/ψand open charm cross-sections. The properties of these events are studied and compared to theoretical predictions.