Physics Letters B 694 (2010) 209–216 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Measurement of σ ( pp → b bX ) at √ s = 7 TeV in the forward region ✩ .LHCb Collaboration article info abstract Article history: Received 14 September 2010 Received in revised form 7 October 2010 Accepted 10 October 2010 Available online 12 October 2010 Editor: W.-D. Schlatter Keywords: LHC b-quark Cross-section Decays of b hadrons into final states containing a D 0 meson and a muon are used to measure the b b production cross-section in proton–proton collisions at a centre-of-mass energy of 7 TeV at the LHC. In the pseudorapidity interval 2 < η < 6 and integrated over all transverse momenta we find that the average cross-section to produce b-flavoured or b-flavoured hadrons is (75.3 ± 5.4 ± 13.0) μb.  2010 CERN. Published by Elsevier B.V. All rights reserved. 1. Introduction Quantum chromodynamics predicts the cross-section for the production of b-flavoured hadrons in proton–proton collisions, for which higher order calculations are available [1]. The first data taken with the LHCb experiment at 7 TeV centre-of-mass energy allows this cross-section to be measured and compared to pre- dictions. Knowledge of the b yield is also critical in ascertaining the sensitivity of experiments that aim to measure fundamen- tal parameters of interest involving, for example, CP violation. It is also useful for normalising backgrounds for measurements of higher mass objects that decay into b b, such as the Higgs boson. In this Letter we present a measurement of the production cross- section for the average of b-flavoured and b-flavoured hadrons in proton–proton collisions at a centre-of-mass energy of 7 TeV in the pseudorapidity interval 2 < η < 6, where η =− ln[tan(θ/2)], and θ is the angle of the weakly decaying b or b hadron with respect to the proton direction. We extrapolate this measurement to the en- tire rapidity interval. Our sensitivity extends over the entire range of transverse momentum of the b-flavoured hadron. The LHCb detector [2] was constructed as a forward spectrom- eter primarily to measure CP violating and rare decays of hadrons containing b and c quarks. The detector elements are placed along the beam line of the LHC starting with the Vertex Locator (VELO), a silicon strip device that surrounds the proton–proton interaction region and is positioned 8 mm from the beam during collisions. It provides precise locations for primary pp interaction vertices, the locations of decays of long lived hadrons, and contributes ✩ © CERN, for the benefit of the LHCb Collaboration. to the measurement of track momenta. Other detectors used to measure track momenta comprise a large area silicon strip de- tector located before a 3.7 Tm dipole magnet, and a combination of silicon strip detectors and straw drift chambers placed after- ward. Two Ring Imaging Cherenkov (RICH) detectors are used to identify charged hadrons. Further downstream an Electromagnetic Calorimeter (ECAL) is used for photon detection and electron iden- tification, followed by a Hadron Calorimeter (HCAL), and a system consisting of alternating layers of iron and chambers (MWPC and triple-GEM) that distinguishes muons from hadrons (MUON). The ECAL, MUON, and HCAL provide the capability of first-level hard- ware triggering. Two independent data samples, recorded at different times, are examined. For the earliest period of data taking the number of colliding bunches was sufficiently low that the high-level trigger could process all crossings and accept events when at least one track was reconstructed in either the VELO or the tracking stations. This data set, called “microbias”, has an integrated luminosity, L, of 2.9 nb −1 . The second sample, referred to as “triggered”, uses trig- gers designed to select a single muon. Here L equals 12.2 nb −1 . These samples are analysed independently and the results subse- quently combined. Most D 0 mesons are produced directly via pp → c cX interac- tions, where X indicates any combination of final state particles. These particular D 0 mesons are denoted as “Prompt”. D 0 mesons produced in pp → b bX collisions where the b-flavoured hadron decays into a final state containing a D 0 meson are called “Dfb”. We use the decay channel b → D 0 X μ − ν , as it has a large branch- ing fraction of (6.84 ± 0.35)% [3], and is advantageous from the point of view of signal to background. Throughout this Letter men- tion of a particular mode implies the inclusion of the charge con- jugate mode as well. 0370-2693/  2010 CERN. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2010.10.010