Shooting strings

Shooting strings.
Source: Meez Forums.

String theory, once hailed as theory of everything, now struggles to demonstrate its use for anything at all.

Most string theorists today, if not working for banks, study the gauge-gravity correspondence. This celebrated idea, arguably one of the most interesting findings in string theory, relates a strongly coupled field theory in flat space to weakly coupled gravity in a higher dimensional space. These higher-dimensional spaces do not resemble our universe, so the interesting applications of the gauge-gravity correspondence are analytical calculations in strongly coupled field theory. Notoriously difficult problems of the field theory can become manageable by reformulating them in the language of gravity.

The most widely promoted use of this gauge-gravity correspondence has been the quark gluon plasma, which is produced in highly energetic collisions of heavy ions, previously at RHIC and now at the LHC. There has been a lot of hype about the low viscosity that was analytically found using the gauge-gravity correspondence and that fit well with observations. But heavy ion physics isn’t just viscosity. There are many other observables that a good model must be able to explain.

One of these observables is the energy loss that elementary particles experience when they travel through the plasma. Just by chance it can happen that a particle pair is created but only one of the two particles travels through the plasma and loses energy. The primary particles are unstable and eventually decay to form stable hadrons. By measuring and summing up the momentum of the decay products one can infer the energy loss that happened in the plasma.

We previously saw that the gauge-gravity correspondence seems to work well for the RHIC data, but misses the mark when the more recent LHC is also taken into account. The prediction is far outside the error margin of the data, both in terms of magnitude and in terms of slope. The gauge-gravity correspondence predicts too much energy loss. I call that a bad fit to the data. String theorists call it “qualitatively correct” which seems to mean their prediction has an upward slope.

But heavy ion physics is a messy business where many different processes come together and that makes it difficult to draw unambiguous conclusions. Clearly that situation doesn’t look good. However, as I mentioned earlier, last year I heard a talk by Steven Gubser about an upcoming paper of his addressing the energy loss in the gauge-gravity correspondence. Ficnar, Gubser and Gyulassy now recently posted their new paper on the arxiv:

Shooting String Holography of Jet Quenching at RHIC and LHC
Andrej Ficnar, Steven S. Gubser, Miklos Gyulassy
arXiv:1311.6160 [hep-ph]

In this paper, the authors propose a new description on the gravity side for the particle which on the field theory side loses energy while traveling through the plasma. Previously, this particle was modeled by a string parallel to the boundary that fell towards the black hole horizon. Ficnar et al instead model the particle by a string that ‘shoots up’ away from the horizon. They calculate the energy loss of the endpoint and find that the energy loss is reduced relative to the previous scenario.

They do not motivate the gravitational description and I am left wondering if not there should be an unambiguous procedure to find the gravitational analogue. If one can just choose a different setup and get a different energy loss that does not exactly increase my faith in the predictive value of the model.

Be that as it may, with their new model a sufficient reduction of the energy loss can only be achieved by pushing the crucial parameter (λ, the ‘t Hooft coupling) into a limit where the approximation actually breaks down. This is no good because then the results cannot be trusted.

So then they add higher curvature terms on the gravity side. This introduces an additional parameter, and a suitable choice for this second parameter allows the coupling to remain just about in the okay range. One would expect these higher-order terms to be present, but in principle I’d think the coupling shouldn’t be an independent parameter. In any case, this still doesn’t fit both the RHIC and the LHC data.

Since the interpretation of the data depends on the reconstruction of the effective temperature at the collision, they then speculate that maybe the temperature values are off by 10% or so, in which case their calculation would fit the data just fine.

This model is clearly an improvement though I can’t say I am terribly convinced. What seems to become increasingly clear though is that any successful model for highly energetic heavy ion collisions must use a suitable combination of both weakly and strongly coupled physics. The gauge-gravity correspondence still has a good chance to prove its use for the strongly coupled physics, but that will necessitate getting into all the messy details.