Probing transverse momentum dependent gluon distribution functions from hadronic quarkonium pair production

The inclusive hadronic production of eta(Q) (eta(c) eta(b)) pair is proposed to extract the transverse momentumdependent (TMD) gluon distribution functions. We use nonrelativistic QCD (NRQCD) for the production of eta(Q). Under the nonrelativistic limit, the TMD factorization for this process is assumed to make a lowest-order calculation. For unpolarized initial hadrons, unpolarized and linearly polarized gluon distributions can be extracted by studying different angular distributions.