Jet Flavour Tagging and Top Production at LHCb

Top quark production in the forward region in proton-proton collisions is observed for the first time. The $W+b$ final state with $W\rightarrow \mu\nu$ is reconstructed using muons with a transverse momentum, $p_{\rm T}$, larger than 25 GeV in the pseudorapidity range $2.0 < \eta < 4.5$. The $b$ jets are required to have $50 < p_{\rm T} < 100$ GeV and $2.2 < \eta < 4.2$, while the transverse component of the sum of the muon and $b{-}$jet momenta must satisfy $p_{\rm T} > 20$ GeV. The algorithm was developed to select $b{-}$ and $c{-}$quark jets. The efficiency for identification a $b(c)$ jet is about 65% (25%) with a probability for misidentifying a light-parton jet of 0.3% for jets with $p_{\rm T} > 20$ GeV. The results are based on data corresponding to integrated luminosity of 1.0 and 2.0 fb$^{-1}$ collected at the center-of-mass energies of 7 and 8 TeV by LHCb. The inclusive top quark production cross-sections in the fiducial region are \begin{align*} \sigma\text{(top)[7 TeV]} \;=\; 239 \pm 53 \;\text{(stat)} \pm 33 \;\text{(syst)} \pm 24 \;\text{(theory) fb},\\ \sigma\text{(top)[8 TeV]} \;=\; 289 \pm 43 \;\text{(stat)} \pm 40 \;\text{(syst)} \pm 29 \;\text{(theory) fb}. \end{align*} These results, along with the observed differential yields and charge asymmetries, are in agreement with next-to-leading order Standard Model predictions.