During the QCD parton evolution, partons are emitted giving rise to the Q2 dependence of the parton densities. Because of the multi-parton emissions, the partons receive a significant transverse momentum. In the traditional approach, those transverse momenta are neglected. However, when including parton showers into the calculation, as done for example in all Monte Carlo event generators, but also in the next-to-leading order (NLO) calculations matched with parton showers (i.e. POWHEG, MC@NLO, etc ), such transverse momenta appear and special gymnastics are needed to conserve energy-momentum.
The TMD approach
In the TMD (Transverse Momentum Dependent parton distributions), the transverse momenta generated during the QCD evolution are kept (and not neglected). With this one can define TMD parton densities and can perform calculations of cross sections which from the beginning conserve energy momentum, independent whether parton showers are explicitly simulated or not.
Our group is active in the development of the TMD parton evolution and the precision determination of the TMD parton densities from existing measurements.
We have developed a Library of transverse momentum dependent (TMD) parton densities: TMDlib.
The TMD parton densities (but also the usual collinear parton densities) can be visualized with TMDplotter