Today in arxiv appeared a work by Christian Fischer, Axel Maas and Jan Pawlowski (see here). This work is relevant because, for the first time, three authors that defended functional method so strongly now acknowledge the very existence of another solution in the infrared for the Dyson-Schwinger equations for Yang-Mills theory. Indeed they properly cite the work of Boucaud et al. (see recent preprint) and Aguilar, Binosi and Papavassiliou (see here) that found such solution with Dyson-Schwinger equations. I would like to say that these authors find a scenario that agreed with mine and lattice computations (see here and my contribution here). This approach has the fault that does not give a closed form to the gluon propagator permitting computations or, at least, to prove the existence of a mass gap. This means that the utility of such understanding, while very important, has some limitations.
Fischer, Maas and Pawlowski paper contains an important new result that none considered before. They prove that the solution that agreed with the scenario seen on the lattice violates BRST invariance. The reason why this is a striking result is that this is what one expects if the particles in the theory acquire a mass. They correctly observe that the gluon acquires a kind of screening mass and cannot be considered a true massive particle. This is truly beautiful because we know that the true carriers of the strong force in the infrared are bounded states of gluons that should be better named glueballs. They also show, but this was an expected result, that the gluon propagator in the infrared has the same behavior independently on the number of colors.
I would like to conclude with praise to this beatiful work that I hope it will appear soon in the published literature. The change of point of view of these researchers has been of great moment.