Quarkonium is the bound state of a heavy flavor (charm/bottom) quark-antiquark pair. Due to their characteristic dilepton decays, quarkonia are relatively easy to detect and are proposed as a probe to the transverse momentum dependent (TMD) observables, which encode information about the three-dimensional distribution of partons in the nucleon. However, there are many theoretical challenges and open questions that prevent us from using quarkonia as a reliable probe to TMD physics. Particularly, quarkonium production is usually studied within the factorization framework of non-relativistic QCD (NRQCD), this factorization is reliable only when the quarkonium transverse momentum is comparable to or larger than the heavy quark mass. In the small transverse momentum limit, soft gluon emissions spoil the perturbative expansion and the NRQCD factorization breaks down. In this proposal, building upon the latest developments on NRQCD and soft collinear effective theory (SCET), we suggest a new effective field theory (EFT) that will allow for both the restoration of factorization and soft gluon resummation in TMD observables involving quarkonium production. The new factorization involves the standard TMD parton distribution functions (TMDPDFs) and a new quarkonium TMD shape function. With this approach one can measure and/or constrain the gluon TMDs with processes that are relatively easy to access experimentally. Although in this proposal we are primarily interested in processes sensitive to the gluon TMDPDFs, the EFT can be extended in processes involving quarkonium production/decay with measurements sensitive to other TMDs. Yiannis Makris has extensive experience in applications of EFTs such as NRQCD and SCET and theoretical and phenomenological studies of quarkonium production at hadron colliders. In addition, INFN hosts some of the world-leading researchers on the field of TMD physics and thus is ideally placed for the successful completion of this proposal.

Yiannis Makris – INFN PAVIA