[sem theorie] Heavy flavour and direct photon probes of the gluon Sivers function

by Abhiram Kaushik (IISc Bangalore)

Wednesday 3 Apr 2019, 14:00 → 15:00 Europe/Paris

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Traditionally the production of open and closed heavy flavour quarks, as well as direct photons, has been used to constrain the gluon density inside the proton. The gluon Sivers function (GSF) is a transverse-momentum dependent parton distribution function that describes the azimuthal distribution (in momentum space) of unpolarised partons inside a transversely polarised proton. Since the existence of a non-zero Sivers function requires 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), which are azimuthal asymmetries in collisions involving transversely polarised protons. In my talk, I will discuss five different processes which can potentially shed light on the GSF. These are: the production of closed-charm (J/\psi) in polarised ep and pp collisions, the production of open-charm (D-mesons) in ep and pp collisions, and finally the production of direct photons in pp collisions. Each of these processes comes with 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.