Abstract:

Absolute luminosity measurements are of general interest for colliding-beam experiments at storage rings. These measurements are necessary to determine the absolute cross-sections of reaction processes and are valuable to quantify the performance of the accelerator. Using data taken in 2010, LHCb has applied two methods to determine the absolute scale of its luminosity measurements for proton-proton collisions at the LHC with a centre-of-mass energy of 7 TeV. In addition to the classic ''van der Meer scan'' method a novel technique has been developed which makes use of direct imaging of the individual beams using beam-gas and beam-beam interactions. This beam imaging method is made possible by the high resolution of the LHCb vertex detector and the close proximity of the detector to the beams, and allows beam parameters such as positions, angles and widths to be determined. The results of the two methods have comparable precision and are in good agreement. Combining the two methods, an overal precision of 3.5% in the absolute luminosity determination is reached. The techniques used to transport the absolute luminosity calibration to the full 2010 data-taking period are presented. © 2012 CERN.

Abstract:

Measurements of inclusive W and Z boson production cross-sections in pp collisions at √ s = 7TeV using final states containing muons are presented. The data sample corresponds to an integrated luminosity of 37 pb -1 collected with the LHCb detector. The W and Z bosons are reconstructed from muons with a transverse momentum above 20 GeV/c and pseudorapidity between 2:0 and 4:5, and, in the case of the Z cross-section, a dimuon invariant mass between 60 and 120 GeV/c2. The cross-sections are measured to be 831±9±27±29 pb for W+, 656±8±19±23 for W- and 76:7±1:7±3: 3±2:7 pb for Z, where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity. Differential cross-sections, W and Z cross-section ratios and the lepton charge asymmetry are also measured in the same kinematic region. The ratios are determined to be σW+→μ+ν/σW- →μ-ν = 1:27±0:02±0:01 and ( σW+→μ+ν+σW- →μ-ν)/σZ→μμ = 19:4±0:5± 0:9. The results are in general agreement with theoretical predictions, performed at nextto- next-to-leading order in QCD using recently calculated parton distribution functions. Copyright CERN.

Abstract:

Based on data corresponding to an integrated luminosity of 0.37 fb -1 collected by the LHCb experiment in 2011, the following ratios of branching fractions are measured: B(B0 → π+π -) / B(B0 → K+π-) = 0.262 ± 0.009 ± 0.017; (fs/fd) B(Bs0 → K +K-) / B(B0 → K+π -) = 0.316 ± 0.009 ± 0.019; (fs/fd) B(B s0 → π+K-) / B(B0 → K+π-) = 0.074 ± 0.006 ± 0.006; (fd/fs) ± B(B0 → K+K-) / B(B s0 → K+K-) = 0.018 -0.007 +-0.007 ± 0.009; (fs/fd) B(Bs0 → π+π-) / B(B0 → π+π-) = 0.050 -0.009+0.011 ± 0.004; BΛb0 → pπ-) / BΛb0 → pK-) = 0.86 ± 0.08 ± 0.05; where the πrst uncertainties are statistical and the second systematic. Using the current world average of B(B0 → K +π-π and the ratio of the strange to light neutral B meson production fs/fd measured by LHCb, we obtain: B(B0 → π+π-) = (5.08 ± 0.17 ± 0.37) × 10-6; B(Bs0 → K+K-) = (23.0 ± 0.7 ± 2.3) × 10-6; B(Bs0 → π+K-) = (5.4 ± 0.4 ± 0.6) × 10-6; B(B0 → K+K-) = (0.11 -0.04+0.05 ± 0.06) × 10 -6; B(Bs0 → π+π -) = (0.95 -0.17+0.21 ± 0.13) × 10-6. The measurements of B(Bs0 → K +K-), B(Bs0 → π+K-) and B(B0 → K+K -) are the most precise to date. The decay mode Bs0 → π+π- is observed for the first time with a significance of more than 5σ. Copyright CERN.