Terning's View of Physics

As described previously here, there are good theoretical reasons to think that the so called AdS/QCD correspondence should provide a poor description of the collisions of strongly interacting particles like the proton, or their internal quarks and gluons.  The idea for the correspondence was inspired by string theory where is can be shown that special (strongly interacting, supersymmetric, scale invariant)  theories with gluons can be simply described by calculations on a curved 5D space called anti-de Sitter space, and abbreviated as AdS.  While the theory of quantum chromodynamics (QCD) does contain gluons, it is not supersymmetric, not scale invariant, and, it turns out, not strongly interacting enough for the correspondence to work. The problem can be seen fairly easily in collisions.  In QCD collisions of quarks and gluons tend to produce narrow sprays of particles, known as jets, that look something like this:

Jets of particles. The length of the line shows the energy of the particle.

While in AdS theories the produced particle spread out uniformily in all directions like this:

spherical spray of particles

Some theorists have shrugged their shoulders about this problem and tried to apply the AdS/QCD correspondence to heavy ion collision data pointing out that some particular measurements happened to agree with the AdS/QCD prediction.

The BackReaction blog points out that the latest data from the LHC again points to the inadequacies of the AdS/QCD correspondence.

The ratio of the probability of finding a jet in lead-lead collisions to the same probability in proton-proton collisions as a function of the momentum of the jet away from the beam line (aka transverse momemtum P_T). Image from Thorsten Renk, Slide 17 of this presentention

The data most closely follow a model of ordinary QCD jet  production, labelled YaJEM for Yet another Jet Energy-loss Model, rather than the AdS calculation.  For the experts: while the jetty description of QCD continues to work at large number of colors, , the AdS description requires both and the coupling times the number of colors, , to be large, and it is the latter condition that fails in the real world.

Further reading:

• Systematics of the charged-hadron spectrum and the nuclear suppression factor in heavy-ion collisions from GeV to TeV
   arxiv.org/abs/1103.5308v2
• Pathlength dependence of energy loss within in-medium showers
    arxiv.org/abs/1010.4116
• The AdS/QCD Correspondence: Still Undelivered
   arxiv.org/abs/0811.3001