Measurement of $\psi (2S)$ polarisation in $pp$ collisions at $\sqrt{s}=7$ TeV

The polarisation of prompt $\psi (2S)$ mesons is measured by performing an angular analysis of $\psi (2S)\to {\mu }^{+}{\mu }^{-}$ decays using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb${}^{-1}$, collected by the LHCb detector at a centre-of-mass energy of 7 TeV. The polarisation is measured in bins of transverse momentum $p_{\mathrm{T}}$ and rapidity $y$ in the kinematic region $3.5<p_{\mathrm{T}}<15$ GeV$\mathrm{/}c$ and $2.0<y<4.5$, and is compared to theoretical models. No significant polarisation is observed.