A new search at the Large Hadron Collider (LHC) has not found any evidence of microscopic quantum black holes, which are hypothetical objects that could arise under certain theories involving extra dimensions and quantum gravity. Researchers from the University of California, Santa Barbara (UCSB) analyzed data from the CMS experiment between 2016 and 2018 using two different methods. One method focused on a property called sphericity, which relates to the shape of energy patterns produced in collisions. The other looked for unusually high energy levels in the particles generated by these collisions. The study also introduced a new analytical technique known as "phase-space distance," which combines machine learning with a system called a Support Vector Machine (SVM) to help distinguish potential signals from the vast background of regular particle interactions. This method had never been used in particle physics data analysis before. The search found no evidence of quantum black holes, which means that, according to the models examined, such black holes are unlikely to form at energy levels up to about 12 TeV (Tera-electron volts). This result also limits some theories involving extra spatial dimensions. String theory, for example, posits the existence of 10 dimensions, but the study suggests that, based on the models tested, only two spatial dimensions are likely to exist. "Theories don't predict one exact answer," said Danyi Zhang, a graduate student researcher at UCSB. "They predict a range of possibilities, and each search helps narrow down where new physics might be hiding." In addition to searching for quantum black holes, the researchers also looked for sphalerons—unstable configurations of particle fields that could potentially explain the universe's matter-antimatter asymmetry. According to current understanding, the Big Bang should have created equal amounts of matter and antimatter, which would have annihilated each other, leaving only energy. Instead, the universe is filled with matter, a mystery that sphalerons might help explain. However, the study found no evidence of sphaleron processes, which placed limits on how often these interactions could occur. The idea of creating black holes at the LHC emerged about 20 years ago, when physicists proposed that if enough energy were concentrated into an extremely small region and if extra spatial dimensions existed, quantum black holes might form during the high-energy proton collisions created by the accelerator. These hypothetical black holes would be very different from the large astrophysical black holes found in space. "They wouldn't last very long—if you made one, it would disintegrate immediately," said Steven Giddings, a UCSB physics theorist. Safety concerns were eventually addressed by comparing LHC collisions with natural high-energy cosmic rays, which have been striking Earth without causing harm. Any quantum black holes created would evaporate almost instantly, leaving behind detectable traces in the particles they produce. The search for quantum black holes is part of a broader effort to unify two major theories in physics: quantum field theory, which describes the behavior of small particles, and general relativity, which describes the behavior of large, massive objects. The challenge is that quantum physics applies to the very small, while general relativity applies to the very large. Microscopic black holes could bridge this gap by being small enough for quantum effects to matter and massive enough for gravity to play a role. Researchers hope that future experiments, such as the upgraded High Luminosity LHC, will provide more data to explore these questions. For now, the search continues, pushing the boundaries of what is known about the fundamental nature of the universe.