Inclusive b-jet production cross section measurement at LHCb

The LHCb detector features an outstanding capability to identify displaced secondary vertices with respect to the interaction point. The extraordinary performance of the whole LHCb tracking system makes possible accurate measurements for particles’ trajectories and momenta. Many lifetime and kinematic based properties of the decay of the long-lived B hadrons are at the basis of the identification of b-jets, thus the LHCb detector could prove to be suitable to perform QCD studies based on heavy-quark jets. The aim of the analysis is to calculate the inclusive bb quark pair production cross section inside the range ? (2.5, 4.0). For this purpose, a cone jet algorithm has been developed to reconstruct and identify jets from the B decay. It relies on how well the measured B hadron approximates the properties of the b quark in order to infer the cross section of b quarks production. Monte Carlo productions of fully simulated events in which a b quark pair has been produced have been used to study the correlation between the emerging quarks and the resulting B hadron pairs. Due to the large mass of the b quark, for which it turns to be less affected by non-perturbative QCD effects than lighter quarks, most of the properties of the b are retained by the corresponding B hadron. Thus may be safe to infere the b production cross section from inclusive B final states. The method used here is also expected to be less affected by non-perturbative effects than other measurements based on exclusive channel decays. The jet reconstruction tool developed to identify the jets originating from the b quark hadronisation is a cone-type seeded algorithm: the basic idea is to take an inclusive secondary vertex originating from the long-lived B hadron as seed for jet reconstruction. The position of the seed with respect to the primary verted establishes the direction of a cone of given aperture. The jet is then built collecting charged and neutral particles that are found to be inside the cone. The radius of the cone is expressed in terms of the dimensionless parameter Rin the (? , ? ) plane (azimuthal angle and pseudorapidity). Seed track candidates are properly preselected with requirements on transverse momentum, ? 2 of the resulting track fit and impact paramenter significance IPS (= IP/? IP) with respect to the PV. Goodness of vertex is ensured with cuts on the resulting vertex fit ? 2 and on the distance of closest approach between the two tracks (DOCA). The reconstructed seed are found to describe well the B hadron direction: the ? and ? resolution evaluated for signal seeds turns out to be (15.8 ± 0.2) mrad and (1.82 ± 0.18) mrad respectively. Jets are then built and their properties studied, such as their b-tagging efficiency and spatial overlapping. Monte Carlo simulations have also been used to study the LHCb trigger. A handful of trigger lines have been selected meeting the following requirements: good efficiency in selecting signal events, the inclusive feature of the analysis must be preserved, difficult evaluation of systematic effects has to be avoided. A jet energy correction has been studied and applied in order to account for energy loss due to undetected particles or wrong energy measurement and tracking. This allows to compare Monte Carlo and real data and define a fiducial cut on the transverse momentum of the jets. Events are finally selected requiring them: ? to have only one reconstructed primary vertex (to avoid ambiguous assignation of tracks to the proper primary vertex); ? to have at least one reconstructed secondary vertex; ? to be accepted by at least one line of each trigger stage. The measurement has been performed using 17.7 pb? 1 of data, about ½ of the total luminosity collected in 2010 at 7Tev. Background from c-jets and light quarks and gluons has been studied using LHCb Monte Carlo simulations. The bb? production cross has been found to be ? bb? = 80.6 ± 1.2 (stat) ± 11.4 (syst) ? b, compatible within 1 sigma with NLO predictions.