Posts Tagged ‘qcd’

Thursday, November 3rd, 2011

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, $N$, the AdS description requires both $N$ and the coupling times the number of colors, $\alpha N$, to be large, and it is the latter condition that fails in the real world.

• Systematics of the charged-hadron $P_T$ spectrum and the nuclear suppression factor in heavy-ion collisions from $\sqrt{s}=200$ GeV to $\sqrt{s} =2.76$ 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

Monday, March 6th, 2006

the rho tragectory from Karch et. al.

A new paper by Karch, Katz, Son, and Stephanov attempts to modify the 5D AdS background that is used to model QCD in order to obtain the behaviour of a string state with large spin or excitation number.  They find that this can occur in a non-trivial dilaton background.

Chiral Phase Transition

Monday, December 12th, 2005

QCD phase boundary from Braun and Gies

A new paper by Jens Braun and Holger Gies gives a nice picture of the chiral phase transition that links the finite temperature phase transition with the chiral phase transition that appears as the number of quark flavors is varied at zero temperature.  The idea is that in the chiral symmetric phase there is an infrared fixed point for the gauge coupling.  As the temperature or number of flavors in lowered, the fixed point coupling moves to stronger coupling.  Eventually the fixed point coupling is large enough that four quark operators become relevant (i.e have scaling dimensions less than four) and produce chiral symmetry breaking and destroy the infrared fixed point.  It would be nice if lattice gauge theorists who do Monte Carlo simulations would try to verify this picture.