Nobel Intent: LHC spots no black holes, eliminates a version of string theory

LHC spots no black holes, eliminates some versions of string theory

By John Timmer | Last updated a day ago

The results continue to pour out of the LHC's first production run. This week, the folks behind the CMS detector have announced the submission of a paper to Physics Letters that describes a test of some forms of string theory. If this form of the theory were right, the LHC should have been able to produce small black holes that would instantly decay (and not, as some had feared, devour the Earth). But a look at the data obtained by CMS shows that a signature of the black holes' decay is notably absent.

String theory is an attempt to deal with the fact that the two major theories in physics, quantum mechanics and relativity, are fundamentally incompatible. It manages to merge the two by positing a set of extra dimensions beyond the usual four. We don't see these because they're tightly wrapped within a tiny radius that is inapproachable at normal energies.

In one form of string theory—the paper calls it the ADD model because Arkani-Hamed, Dimopoulos, and Dvali proposed it—this unification has consequences for gravity. Normally, gravity is very weak relative to the other forces, such that it could only become unified with the rest of them at energies many orders of magnitude higher than the LHC could reach. But, in the ADD model, gravity only looks weak because portions of it are caught up in the remaining dimensions. This drops the energies down to something right in the heart of the LHC's capabilities.

If everything went as the model proposes, particles that collided at energies above this cutoff could close to within a distance that's smaller than the space occupied by the additional dimensions. Once that happens, they'd feel the full force of gravity, and immediately merge to form a tiny black hole. So tiny, in fact, that it would instantly decay via Hawking radiation. This decay would be visible as jets of particles; physicists I've seen asked about these have more or less said "you couldn't miss it."

What you could do, however, is mistake something else for a black hole. Interactions governed by quantum chromodynamics will also produce jets at a certain frequency, so the black hole events would have to stand out above this background. So that's what the new analysis looks for. The authors model what the jets from both string and quantum theories should look like in order to allow them to pull out and save jet events. (This actually involved the same modeling software used by the people who evaluated the TSA's scanners.)

They then use an area of the LHC's energy spectrum that's too low to produce black holes to figure out the level of background jet production via quantum chromodynamics. Next, they extend the analysis into the energy range where black holes should appear, and see whether any signal stands out above this background. It doesn't. "We can exclude the production of black holes with minimum mass of 3.5-4.5eV for values of the multidimensional Planck scale up to 3.5TeV at 95 percent CL," the authors conclude.

The results are also useful for studies beyond string theory. Mini black holes aren't the only hypothetical items predicted to decay into jets, so the lack of a signal that's much above background puts some very severe constraints on the physics there, too.

One other nice thing here is that the energies involved are completely out of reach of the Tevatron. So, even if the older collider beats the new machine to the punch on the Higgs, we're clearly getting some useful physics out of the LHC.

Contrary to some reports, this result doesn't mean the death of string theory, only the particular flavor that predicted black holes at these energies. Eliminating some models is a critical process of narrowing down what's possible, but most theoretical constructs have a range of possible models, and string theory is no different. In fact, it's entirely possible that the ADD model was generated simply because physicists were looking for something they could possibly test in the LHC.

via arstechnica.com