Measurement of the $J/\psi$ production cross-section at $\sqrt{s}$ = 7 TeV in LHCb

The production of J/$\psi$ mesons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV is studied with the LHCb detector. The differential production cross-section is measured as a function of the J/$\psi$ transverse momentum $p_{T}$ in the range $p_{T} \in$ [0;10] GeV/$c$ and rapidity in the range $y \in$ [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/$\psi$ mesons are reconstructed in the decay mode J/$\psi \to \mu^+\mu^-$ and J/$\psi$ from b-hadron decays are separated from prompt J/$\psi$ using the J/$\psi$ pseudo-propertime. From a fit to the $\mu^+\mu^-$ invariant mass distribution and to the pseudo-propertime distribution, we measure: \begin{align} &\sigma\left({\rm inclusive}\, J/\psi, p_{\rm 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_{\rm 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 extrapolating to the LHCb acceptance and to the full angular acceptance with PYTHIA 6.4, we measure: \begin{align} &\sigma(pp\to H_bX, \,2<\eta(H_b)<6) = 84.5 \pm 6.3 \pm 15.6 \, {\mu} {\rm b} \\ &\sigma(pp \to b\overline{b}X) = 319 \pm 24 \pm 59 \, {\mu}{\rm b}, \end{align} where the first result gives the average cross-section to produce ${b}$-flavoured or $\overline{b}$-flavoured hadrons, $H_b$, in the pseudo-rapidity region 2 < $\eta$ < 6 and the second result gives the total $b\overline{b}$ production cross section at $\sqrt{s}$ = 7 TeV. The first error is statistical and the second one systematic. No additional uncertainty is assigned to the extrapolation method.