Physicists at UC Santa Barbara have pushed the search for microscopic black holes at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) into new territory. These hypothetical black holes would be extraordinarily small and short-lived. If they could be produced at the LHC, their existence might help physicists address some of the deepest unanswered questions about spacetime and gravity.
The search also gave researchers a chance to test a new technique for finding rare and previously unknown particles. "Had we found evidence, we could have begun to directly study quantum gravity," said Tamas Vami, a researcher in the Compact Muon Solenoid (CMS) experiment who is conducting his postdoctoral work under the guidance of UCSB physics professor Joe Incandela. "It's a step toward unifying all of the known fundamental forces, which has been a goal of physicists for more than a century." The search did not uncover evidence of quantum black holes.
But in particle physics, failing to detect something can still provide important information by ruling out where it could exist. "It's not a dead-end," said Incandela Lab graduate student researcher Danyi Zhang. "The result is an exclusion limit, which is a real, publishable statement: 'If this thing existed with these properties, we'd have seen it.
We didn't, so we can rule it out here.' That's genuine knowledge about how the universe works." One of the major puzzles in fundamental physics involves the enormous difference between the scale of the universe we experience and the Planck scale, the fundamental energy scale associated with quantum gravity. Some physicists have proposed that new physics or an undiscovered symmetry could explain this difference. Crucially, some of those effects might appear at energy levels the LHC can reach.
Years of experiments have already eliminated many theoretical possibilities, and the continued absence of clear signs of new physics at the LHC has become a major challenge for researchers. But similar situations have happened before. Periods in which existing theories struggled to explain observations have sometimes led to radically new frameworks, including Einstein's theory of relativity.
For that reason, the researchers say null results are an important part of scientific progress. Each one reduces the number of viable possibilities and helps determine where future experiments should look. Vami's and Zhang's results are published in the journal Progress in High Energy Physics (PHEP).
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