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.

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Abstract:The relative production rate of B 0 s and B 0 mesons is determined with the hadronic decays B 0 s → D − s π + and B 0 → D − K + . The measurement uses data corresponding to 1.0 fb −1 of pp collisions at a centre-of-mass energy of √ s = 7 TeV recorded in the forward region with the LHCb experiment. The ratio of production rates, f s /f d , is measured to be 0.238 ± 0.004 ± 0.015 ± 0.021, where the first uncertainty is statistical, the second systematic, and the third theoretical. This is combined with a previous LHCb measurement to obtain f s /f d = 0.256 ± 0.020. The dependence of f s /f d on the transverse momentum and pseudorapidity of the B meson is determined using the decays B 0 s → D − s π + and B 0 → D − π + . There is evidence for a decrease with increasing transverse momentum, whereas the ratio remains constant as a function of pseudorapidity. In addition, the ratio of branching fractions of the decays B 0 → D − K + and B 0 → D − π + is measured to be 0.0822 ± 0.0011 (stat) ± 0.0025 (syst). 5 Systematic uncertainties 76 Results 8 Conclusions 10The LHCb collaboration 13 IntroductionThe ratio of fragmentation fractions f s /f d quantifies the relative production rate of B 0 s mesons with respect to B 0 mesons. Knowledge of this quantity is essential when determining any B 0 s branching fraction at the LHC. The measurement of the branching fraction of the rare decay B 0 s → µ + µ − [1] is the prime example where a precise measurement of f s /f d is crucial for reaching the highest sensitivity in the search for physics beyond the Standard Model. The branching fractions of a large number of B 0 and B + decays have been measured to high precision at the B factories [2], but no B 0 s branching fraction is yet known with sufficiently high precision to be used as a normalisation channel.The relative production rates of b hadrons are determined by the fragmentation fractions f u , f d , f s , f c and f Λ , which describe the probability that a b quark will hadronize into a B q meson (where q = u, d, s, c), or a b baryon, respectively 1 . The ratio of fragmentation fractions f s /f d has been previously measured at LHCb with hadronic [3] and semileptonic decays [4], and the resulting values were combined [4].In this paper, the ratio of fragmentation fractions f s /f d is determined using B 0 s → D − s π + and B 0 → D − K + decays collected in pp collisions at a centre-of-mass energy of √ s = 7 TeV, with data corresponding to an integrated luminosity of 1.0 fb −1 recorded with the LHCb detector. Since the framework of factorization is well applicable to these decays [5], their ratio of branching fractions is theoretically well understood [6] and their 1 Charge conjugation is implied throughout this paper.-1 - JHEP04(2013)001relative decay rates can be used to determine the ratio of fragmentation fractions for B 0 s and B 0 mesons through 1) where N corresponds to a signal yield, corresponds to a total efficiency, τ B 0 s /τ B 0 = 0.984 ± 0.011 [7] corresponds to the ratio of lifetimes and B(D − → K + π ...