Speaker
Prof.
Yuri Kovchegov
Description
We set up a formalism for calculating transverse-momentum-dependent parton distribution functions (TMDs) using the tools of saturation physics: quasi-classical approximation and quantum evolution. In the quasi-classical approximation we show how to calculate the quark TMDs of a nucleus at large-x: the unpolarized quark distribution, the Sivers function, and the quark Boer-Mulders distribution. We observe that spin-orbit coupling leads to mixing between different TMDs of the nucleus and of the nucleons. We then consider the evolution of TMDs: at large-x, in the double-logarithmic approximation, we obtain the Sudakov form factor. At small-x the evolution of unpolarized-target quark TMDs is governed by BK/JIMWLK evolution, while the small-x evolution of polarized-target quark TMDs appears to be different, involving the QCD Reggeon mixing with non-BFKL gluon ladders. The small-x evolution of the quark helicity TMD appears to give a steep rise of the TMD with decreasing x thus suggesting that there is a lot of spin at small-x. This conclusion may lead to the eventual resolution of the spin puzzle.
Author
Prof.
Yuri Kovchegov
Co-author
Dr
Matthew Sievert
(Brookhaven National Laboratory)
