Physicists at UC Santa Barbara have looked again for quantum black holes in collisions recorded by the Compact Muon Solenoid detector at the Large Hadron Collider (LHC), the proton collider at CERN. A quantum black hole, in this setting, would be a tiny, short-lived collapse of spacetime made when enough energy is packed into a very small volume. It would evaporate at once and leave a burst of particles rather than grow like an astronomical black hole. The analysis, published in Progress in High Energy Physics, uses data from 2016 to 2018. No signal was found.
In some theoretical models, gravity might leak into extra spatial dimensions, directions too small to notice in ordinary life, and allow microscopic black holes at the LHC.
A new way to sort rare events
Two handles were used to hunt the signal. One is sphericity, a measure of whether decay products spray evenly in all directions, as expected if a black hole explodes. The other is a large sum of the energies of those products. The physicists also applied a phase-space distance method with a support vector machine, a learning tool that draws a boundary between classes of examples. Distances among events are turned into one score that is higher when an event looks more like the sought signal than like ordinary background. The paper says this is the first use of that method on particle-physics data, that it beat sphericity in their comparison, and that the math can be inspected rather than left opaque.
The search excludes quantum black holes up to about 12 TeV in the models considered and, under those same assumptions, leaves little room for more than two extra dimensions. It also limits sphalerons, unstable field configurations with spherical energy patterns that some ideas invoke to help explain why the universe contains more matter than antimatter. The authors treat the empty result as a map of where new physics is not, and as a general tool for later hunts, including with the High-Luminosity LHC.