The question of the mass gap


Some years ago I proposed a set of solutions to the classical Yang-Mills equations displaying a massive behavior. For a massless theory this is somewhat unexpected. After a criticism by Terry Tao I had to admit that, for a generic gauge, such solutions are just asymptotic ones assuming the coupling runs to infinity (see here and here). Although my arguments on Yang-Mills theory were not changed by this, I have found such a conclusion somewhat unsatisfactory. The reason is that if you have classical solutions to Yang-Mills equations that display a mass gap, their quantization cannot change such a conclusion. Rather, one should eventually expect a superimposed quantum spectrum. But working with asymptotic classical solutions can make things somewhat involved. This forced me to choose the gauge to be always Lorenz because in such a case the solutions were exact. Besides, it is a great success for a physicist to find exact solutions to fundamental equations of physics as these yield an immediate idea of what is going on in a theory. Even in such case we would get a conclusive representation of the way the mass gap can form.

Finally, after some years of struggle, I was able to get such a set of exact solutions to the classical Yang-Mills theory displaying a mass gap (see here). Such solutions confirm both the Tao’s argument that an all equal component solution for Yang-Mills equations cannot hold in any gauge and also my original argument that an all equal component solution holds, in a general case, only asymptotically with the coupling running to infinity. But classically, there exist solutions displaying a mass gap that arises from the nonlinearity of the equations of motion. The mass gap goes to zero as the coupling does. Translating this in the quantum realm is straightforward as I showed for the Lorenz (Landau) gauge. I hope all this will help to better elucidate all the physics around strong interactions. My efforts since 2005 went in that direction and are still going on.

Marco Frasca (2009). Mapping a Massless Scalar Field Theory on a Yang-Mills Theory: Classical Case Mod. Phys. Lett. A 24, 2425-2432 (2009) arXiv: 0903.2357v4

Marco Frasca (2014). Exact solutions for classical Yang-Mills fields arXiv arXiv: 1409.2351v1

Dispersive Wiki


Since I was seventeen my great passion has been the solution of partial differential equations. I used an old book written by Italian mathematicians to face for the first time the technique of variable separation applied to the free Schrödinger equation. The article was written by Paolo Straneo, professor at University of Genova in the first part of the last century and Einstein’s friend, and from it I was exposed to quantum theories in a not too simpler way. At eighteen, some friends of mine, during my vacation in Camdridge, gave to me my first book of mathematics on PDEs: François Treves, Basic Linear Partial Differential Equations. You can find this book at low cost from Dover (see here).

Since then I have never given up with my passion with this fundamental part of mathematics and today I am a professional in this area of research.  As a professional in this area, important references come from the work of Terry Tao (see also his blog), the Fields medalist. Terry, together with Jim Colliander at University of Toronto, manage a Wiki, Dispersive Wiki, with the aim to collect all the knowledge about differential equations that are at the foundation of dispersive effects. Most of you have been exposed at their infancy with the wave equation. Well, this represents a very good starting point. On the other side, it would be helpful to add some contributions for Einstein or Yang-Mills equations. Indeed, Dispersive Wiki is open to all people that, like me, is addicted to PDEs and all matter around them.

I have had the chance to write some contributions to Dispersive Wiki. Currently, I am putting down some lines on Yang-Mills equations (I did it before but this was recognized as self-promotion… just look at the discussion there), Dirac-Klein-Gordon equations and other articles. I think it would be important to help Jim and Terry in their endeavor as PDEs are the bread and butter of our profession and to have on-line such a bookkeeping of results would be extremely  useful. Just take your time to give a look.

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