Asymmetries in heavy quark pair and dijet production at an EIC

Daniël Boer,Cristian Pisano,Jian Zhou,Piet J Mulders

doi:10.1007/jhep08(2016)001

Abstract

Asymmetries in heavy quark pair and dijet production in electron-proton collisions allow studies of gluon TMDs in close analogy to studies of quark TMDs in semi-inclusive DIS. Here we present expressions for azimuthal asymmetries for both unpolarized and transversely polarized proton cases and consider the maximal asymmetries allowed. The latter are found to be rather sizeable, except in certain kinematic limits which are pointed out. In addition, we consider the small-x limit and expectations from a McLerran-Venugopalan model for unpolarized and linearly polarized gluons and from a perturbative, large transverse momentum calculation for the T-odd gluon TMDs. Comparison to related observables at RHIC and LHC is expected to provide valuable information about the process dependence of the gluon TMDs. In particular this will offer the possibility of a sign change test of the gluon Sivers TMD and two other T-odd gluon TMDs. This provides additional motivation for studies of azimuthal asymmetries in heavy quark pair and dijet production at a future Electron-Ion Collider.

Highlights

In principle allows to cross check the results obtained at LHC and EIC for unpolarized protons and perhaps for transversely polarized protons too in case a polarized fixed target experiment, called AFTER@LHC [6, 7], will be performed in the future at LHC
This implies that the transverse spin asymmetries in heavy quark pair and dijet production in electron-proton collisions will become suppressed in the small-x limit
We have studied the azimuthal asymmetries in heavy quark pair and dijet production in DIS process, which provide direct access to the WW-type gluon Transverse momentum dependent distribution functions (TMDs)

Summary

Gluon TMD definitions and small-x expressions

For an unpolarized (U ) and transversely polarized (T ) proton, the following parametrizations for the correlator in terms of gluon. Which, despite its name, is not related to the well-known transversity distribution hq for quarks that has no gluonic analogue Both hq and hg denote helicity flip distributions, the quark distribution is chiral-odd, T-even, and survives after transverse momentum integration, while the gluon distribution is chiral-even, T-odd, and vanishes upon integration over transverse momentum. Both distributions require another helicity flip somewhere else in the process, which will require consideration of different types of processes. Venugopalan (MV) model [17], the WW type linearly polarized gluon distribution is given by [18, 19]

Λ2QCD and

Heavy quark pair production

Sign change test of the T-odd gluon TMDs

Dijet production

Conclusions