Looking for microscopic black holes

Gravity is by far the weakest of the fundamental forces. In elementary-particle interactions, it’s thought to become comparable to the other forces only at collision energies of 10¹⁶ TeV—the so-called Planck energy. The perplexing disparity between the Planck energy and the TeV energy scale of electroweak unification is called the hierarchy problem. Several appealing attempts to address that problem posit extra spatial dimensions that make gravity only seem to be intrinsically weak (see Physics Today, February 2002, page 35 ); gravity’s true energy scale would be in the TeV regime. Such theories predict that the threshold for producing microscopic black holes is somewhere in the TeV collision-energy range of CERN’s Large Hadron Collider (LHC, shown in the photo) rather than at the inaccessible Planck energy. Now the collaborations that run the LHC’s CMS and ATLAS detectors have both reported the results of their searches for evidence of the production and prompt decay of perhaps a few hundred microscopic black holes in some 10¹⁵ proton–proton collisions at 8 TeV. Having looked at final states into which black holes with masses just above threshold are expected to decay, neither team finds evidence of black hole production. For a variety of models and final states, the CMS analysis constrained the black hole threshold mass to be above 6.2 TeV. And for one important class of final states not previously examined, the ATLAS analysis sets a lower mass limit of 5.3 TeV. With the LHC scheduled to run next year with a collision energy of 13 TeV, the search will resume. (S. Chatrchyan et al., CMS collaboration, J. High Energy Phys. 178 (7), 1, 2013 doi:10.1007/JHEP07(2013)178 ; G. Aad et al., ATLAS collaboration, Phys. Rev. Lett. 112, 091804, 2014 doi:10.1103/PhysRevLett.112.091804 .)