Heavy quarkonium physics with early ATLAS data

Results are reported on an analysis on simulated data samples for production of heavy quarkonium states $J/\psi \to \mu \mu$ and $\Upsilon \to \mu \mu$, corresponding to an integrated luminosity of $10$~pb$^{-1}$. It is shown that the $p_T$ dependence of the cross-section for both $J/\psi$ and $\Upsilon$ should be measured reasonably well in a wide range of transverse momenta, $p_T\simeq 10 -50$ GeV. The precision of $J/\psi$ polarisation measurement is expected to reach $0.02-0.06$, while the projected error on $\Upsilon$ polarisation is around 0.2. Observation of radiative decays of $\chi_c$ states, and the feasibility of observing $\chi_b \to J/ \psi, J/ \psi$ decays are also discussed.