${\eta }_c$ production at LHC and indications on the understanding of $J\mathrm{/}\psi$ production

We present a complete evaluation for the prompt ${\eta }_c$ production at the LHC at next-to-leading order in ${\alpha }_s$ in nonrelativistic QCD. By assuming heavy quark spin symmetry, the recently observed ${\eta }_c$ production data by LHCb results in a very strong constraint on the upper bound of the color-octet long distance matrix element $1S0$ of $J\mathrm{/}\psi$. We find this upper bound is consistent with our previous study of the $J\mathrm{/}\psi$ yield and polarization and can give good descriptions for the measurements, but inconsistent with some other theoretical estimates. This may provide important information for understanding the nonrelativistic QCD factorization formulism.