The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described.

The calibration and performance of the opposite-side flavour tagging algorithms used for the measurements of time-dependent asymmetries at the LHCb experiment are described. The algorithms have been developed using simulated events and optimized and calibrated with B+→J/ψK+, B0→J/ψK∗0 and B0→D∗−μ+νμ decay modes with 0.37 fb−1 of data collected in pp collisions at during the 2011 physics run. The opposite-side tagging power is determined in the B+→J/ψK+ channel to be (2.10±0.08±0.24) %, where the first uncertainty is statistical and the second is systematic.

The production of J/ψ mesons in proton-proton collisions at √ s = 7 TeV is studied with the LHCb detector at the LHC. The differential cross-section for prompt J/ψ production is measured as a function of the J/ψ transverse momentum p T and rapidity y in the fiducial region p T ∈ [0; 14] GeV/c and y ∈ [2.0; 4 cross-section and fraction of J/ψ from b-hadron decays are also measured in the same p T and y ranges. The analysis is based on a data sample corresponding to an integrated luminosity of 5.2 pb −1 . The measured cross-sections integrated over the fiducial region are 10.52 ± 0.04 ± 1.40−2.20 µb for prompt J/ψ production and 1.14 ± 0.01 ± 0.16 µb for J/ψ from b-hadron decays, where the first uncertainty is statistical and the second systematic. The prompt J/ψ production cross-section is obtained assuming no J/ψ polarisation and the third error indicates the acceptance uncertainty due to this assumption.

From a sample of 1172 +/- 61 D(+)-->pi(-)pi(+)pi(+) decays, we find gamma(D(+)-->pi(-)pi(+)pi(+))/gamma(D(+)-->K-pi(+)pi(+)) = 0.0311 +/- 0.0018(+0.0016)(-0.0026). Using a coherent amplitude analysis to fit the Dalitz plot of these decays, we find strong evidence that a scalar resonance of mass 478(+24)(-23) +/- 17 MeV/c(2) and width 324(+42)(-40) +/- 21 MeV/c(2) accounts for approximately half of all decays.

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Using 34.7 pb −1 of data collected with the LHCb detector, the inclusive production of the X(3872) meson in pp collisions at √ s = 7 TeV is observed for the first time. Candidates are selected in the X(3872) → J /ψπ + π − decay mode, and used to measure σ pp → X(3872) + anything B X(3872) → J /ψπ + π − = 5.4 ± 1.3 (stat) ± 0.8 (syst) nb, where σ (pp → X(3872) + anything) is the inclusive production cross section of X(3872) mesons with rapidity in the range 2.5-4.5 and transverse momentum in the range 5-20 GeV/c. In addition the masses of both the X(3872) and ψ(2S) mesons, reconstructed in the J /ψπ + π − final state, are measured to be m X(3872) = 3871.95 ± 0.48 (stat) ± 0.12 (syst) MeV/c 2 and m ψ(2S) = 3686.12 ± 0.06 (stat) ± 0.10 (syst) MeV/c 2 .

A model-independent partial-wave analysis of the S-wave component of the K system from decays of D mesons to the three-body K ÿ final state is described. Data come from the Fermilab E791 experiment. Amplitude measurements are made independently for ranges of K ÿ invariant mass, and results are obtained below 825 MeV=c 2 , where previous measurements exist only in two mass bins. This method of parametrizing a three-body decay amplitude represents a new approach to analyzing such decays. Though no model is required for the S-wave, a parametrization of the relatively well-known reference P-and D-waves, optimized to describe the data used, is required. In this paper, a Breit-Wigner model is adopted to describe the resonances in these waves. The observed phase variation for the S-, P-, and D-waves do not match existing measurements of I 1 2 K ÿ scattering in the invariant mass range in which scattering is predominantly elastic. If the data are mostly I 1 2 , this observation indicates that the Watson theorem, which requires these phases to have the same dependence on invariant mass, may not apply to these decays without allowing for some interaction with the other pion. The production rate of K ÿ from these decays, if assumed to be predominantly I 1 2 , is also found to have a significant dependence on invariant mass in the region above 1:25 GeV=c 2. These measurements can provide a relatively model-free basis for future attempts to determine which strange scalar amplitudes contribute to the decays.

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