Heavy flavour and direct photon probes of the gluon Sivers function

Abstract

Traditionally the production of heavy quarkonia, heavy-flavour mesons as well as of direct photons, has been used to constrain the gluon density inside the proton. In this thesis, I have assessed various heavy flavour and direct photon probes of the poorly constrained gluon Sivers function (GSF) in a transversely polarised proton. The GSF is a transverse-momentum dependent parton distribution function that describes the momentum distribution of unpolarised partons (quarks and gluons) inside a transversely polarised proton. Since the existence of a non-zero Sivers function requires non-zero parton orbital angular momentum, the measurement of a non-zero GSF would provide confirmation of non-zero orbital angular momentum inside the proton, about which much debate exists at present. The Sivers function can be accessed through transverse single-spin asymmetries (SSAs). In this thesis, I have studied five different processes which can probed the gluon Sivers function. The first two processes involve production of closed-charm, in particular J/ψ in ep and pp collisions respectively. The second two processes involve production of open-charm (D0 mesons), in ep and pp collisions, and the last process involves the production of prompt-photons in pp collisions. Each of these processes has its own advantages and disadvantages in terms of providing access to the GSF. These involve production rates, theoretical uncertainties regarding the mechanism behind SSA formation, potential non-universality of TMDs etc. I will show how these processes can discriminate between existing fits and models of the GSF using data from current and upcoming experiments.