One of the puzzzles that interests me about high energy perturbative processes is the role of Regge physics. Regge particles are somewhat out of fashion, being a major topic of the 1960s – long ago. However, from that time and from continuing work by Lipatov Fadin and their school, it is clear that Regge physics is real and that it occurs in QCD. These Regge studies exists as distinct subfield of QCD.
This is a problem for SCET. So far, Regge physics has not been found in SCET. This is a clear conflict, since SCET is supposed to describe the high energy limit of QCD and Regge physics should be part of this. This is of personal interest since one of my old papers using Regge physics on final state interactions is in apparent conflict with classic perturbative work by BBNS on factorization. In fact there is not a direct conflict, because Regge physics can convert perturbative logs into powers s^alpha. (To explain this fully needs a longer conversation.) But the conclusions that the community draws draw from the two works are different and conflicting.
Daniel Wyler and I have explored a part of this problem – the kinematics. This was done in the spring of 2008 but we finally got a written version out in our new paper. This leads to an plausible understanding of why Regge physics has been missed in SCET. There needs to an infinite number of scales involved, as well as one step that involves a Glauber or Coulombic gluon.
This summary of the kinematics is only part of the answer. Daniel and I would like to also understand how to match the Regge manifestation onto perturbative QCD. We have not yet done this, and this is a key task that needs to be done in SCET. I would argue that SCET is incomplete, and many of its predictions are uncertain, until the power corrections due to Regge physics are incorporated into the theory.