Prompt J$\psi$ and b $\rightarrow$ J/$\psi$X production in pp collisions at $\sqrt{s}$ = 7 TeV

The production of $J/\psi$ mesons in proton-proton collisions at $\sqrt{s}$=7 TeV is studied with the LHCb detector using the decay mode $J/\psi\to\mu^+\mu^-$. The differential production cross-section $d\sigma/d p_T$ is measured with the $J/\psi$ transverse momentum in the range $p_T\in$[2.5;4]. The analysis is based on a sample of 14.2 nb$^{-1}$ collected between April and June 2010 at the Large Hadron Collider. The $J/\psi$ from b-hadron decays are separated from prompt $J/\psi$ using the $J/\psi$ pseudo-propertime. From a lift to the invariant mass and to the pseudo-propertime distributions, we measure: $\begin{align} &\sigma\left({\rm inclusive}\, J/\psi, p_T <10\,{\rm GeV}/c,\,2.5<y<4\right) \, = \, 7.65\pm 0.19\pm 1.10^{+0.87}_{-1.27}\,{\mu}{\rm b} \\ &\sigma\left(J/\psi {\rm from}\,\, b,\, p_T <10\,{\rm GeV}/c,\,2.5<y<4\right) \, = \, 0.81 \pm 0.06\pm 0.13\,{\mu}{\rm b} \end{align}$ where the first error is statistical and the second one systematic. For the inclusive cross-section, the third error indicates the acceptance uncertainty due to the unknown $J/\psi$ polarization. By using PYTHIA 6.4 the above result is extrapolated to the average cross-section to produce b or $\rm{\overline{b}}$-flavoured hadrons, $H_b$, in the pseudorapidity region covered by LHCb 2 < $\eta$ < 6 and to the full angular acceptance: $\begin{align} &\sigma\left(pp\to H_bX, \,2<\eta(H_b)<6\right)\, = \,84.5\pm 6.3 \pm 15.6 \mu b \\ &\sigma\ left (pp \to b\overline{b} X)\right)\, = \,319 \pm 24\pm 59\mu b \end{align}$ where the first error is statistical and the second one systematic and no additional uncertainty is assigned to the extrapolation method.