Measurement of $\Upsilon$ production in $pp$ collisions at $\sqrt{s}=2.76$ TeV

The production of $\Upsilon(1S)$, $\Upsilon(2S)$ and $\Upsilon(3S)$ mesons decaying into the dimuon final state is studied with the LHCb detector using a data sample corresponding to an integrated luminosity of 3.3 $pb^{-1}$ collected in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=2.76$ TeV. The differential production cross-sections times dimuon branching fractions are measured as functions of the $\Upsilon$ transverse momentum and rapidity, over the ranges $p_{\rm T}<15$ GeV/$c$ and $2.0<y<4.5$. The total cross-sections in this kinematic region, assuming unpolarised production, are measured to be \begin{eqnarray*} \sigma \left( pp \rightarrow \Upsilon(1S) \mathrm{X} \right) \times B\left( \Upsilon(1S) \rightarrow \mu^{+}\mu^{-} \right) & = & 1.111 \pm 0.043 \pm 0.044 nb, \\ \sigma \left( pp \rightarrow \Upsilon(2S) \mathrm{X} \right) \times B\left(\Upsilon(2S) \rightarrow \mu^{+}\mu^{-} \right) & = & 0.264 \pm 0.023 \pm 0.011 nb, \\ \sigma \left( pp \rightarrow \Upsilon(3S) \mathrm{X} \right) \times B\left(\Upsilon(3S) \rightarrow \mu^{+}\mu^{-} \right) & = & 0.159 \pm 0.020 \pm 0.007 nb, \end{eqnarray*} where the first uncertainty is statistical and the second systematic.