Scientists Hunt for Tiny Black Holes Hidden in LHC Collisions

Physicists found no tiny black holes at the LHC, but they just made the hiding place for new physics considerably smaller. Credit: SciTechDaily.comScientists have expanded the search for microscopic black holes at the Large Hadron Collider, looking for fleeting objects that could reveal hidden dimensions and clues to quantum gravity.Quantum black holes could give physicists a rare chance to study intense gravity on the scale of tiny particles. These hypothetical microscopic objects would bring together two aspects of nature that researchers can describe remarkably well separately, yet have struggled to combine in a single theory.At CERN’s Large Hadron Collider (LHC), researchers from UC Santa Barbara have been looking for the particles such black holes might leave behind. If one formed in a collision between protons, it would disappear almost immediately. Its brief existence could nevertheless provide clues about how gravity works in the quantum world.The latest search found no evidence of these objects, narrowing the possibilities for theories that predict them. It also demonstrated a new way to pick potential signals out of the enormous number of collisions recorded at the accelerator, a technique that could help physicists search for other unknown particles and processes.For Tamas Vami, a researcher on the Compact Muon Solenoid (CMS) experiment who is conducting his postdoctoral work under UCSB physics professor Joe Incandela, the motivation starts with a divide in our understanding of nature.The Compact Muon Solenoid particle detector at CERN. Credit: CERN“We have two big theories that describe nature,” Vami said. “If you want to describe things that are small, you go to quantum field theory. We have the Standard Model to describe all the particles, and it performs exceptionally well in practice. And when you go to the very, very big you have general relativity that would describe how big and massive objects behave.”Bringing those theories together requires understanding situations in which both matter, he said, “and that’s really hard to do because you don’t often have a situation which is really tiny but also extremely heavy.”Microscopic black holes could provide such a situation because quantum effects would become especially significant for them.“Had we found evidence, we could have begun to directly study quantum gravity,” Vami said. “It’s a step toward unifying all of the known fundamental forces, which has been a goal of physicists for more than a century.”Danyi Zhang, left, and Tamas Vami. Credit: Matt PerkoMachine learning sharpens black hole searchesThe new analysis, published in Progress in High Energy Physics, used data collected by the CMS detector between 2016 and 2018. Its results constrain quantum black hole production at energies up to about 12 tera-electron volts (TeV), under the assumptions of the theories tested.Earlier searches by CMS and ATLAS, another LHC experiment, had found no evidence of microscopic black holes in smaller datasets. With more collisions available to examine, the researchers could search higher energy regions and improve their chances of catching an extremely rare event, if these black holes can form.“They wouldn’t stick around very long — if you made one, it would disintegrate immediately,” said UCSB physics theorist Steven Giddings, who studies the problems that arise when quantum mechanics and gravity are combined.That distinction was often lost when the possibility of making black holes at the LHC first attracted public attention.“People were more focused on the classical behavior of black holes,” Giddings said.Astronomical black holes can swallow stars, grow, and merge. Safety reports and comparisons with naturally occurring collisions involving ultra-high-energy cosmic rays, which strike the upper atmosphere and other astronomical objects, established that the LHC’s high-energy collisions posed no threat.The muon system of the CMS experiment. Credit: CERNFor the researchers examining collision data, the practical question was how to recognize a black hole’s decay among events produced by already known physics.“We know that black holes are very high energy,” said Danyi Zhang, a graduate student researcher in Incandela’s laboratory. “So we basically just take the energy of these particles that are decay products of whatever was created in the collision and sum them together. And if the sum is large enough, we can say that this is the region where we are likely to find the signal.”The particles’ directions offer a second clue, described by a property called sphericity. Incandela explained, “You form a black hole, and it immediately disintegrates. But it has a very spherical decay signature, lots of things going in all directions.”A method developed by UCSB particle theorist Nathaniel Craig and collaborators allowed the team to compare collision patterns in another way. It uses phase space, a multidimensional mathematical map that represents a system of particles as a point, incorporating information about space, time, energy, and momentum. Measuring distances between events in that map helps researchers distinguish the structure of a potential signal from the background.“We developed the idea of the phase space between events, which can be combined with SVM to help the search,” Craig said.SVM stands for Support Vector Machine, a machine learning tool. The method converts comparisons between events into an SVM score, with higher scores indicating events more likely to resemble the signal researchers are seeking. This was the first application of phase-space distance to a particle physics data analysis.“We compared phase space distance with the sphericity variable and our conclusion is that phase space distance outperforms sphericity,” Zhang said.The approach uses supervised machine learning, and researchers can inspect the mathematics behind its results. That makes its workings accessible in a way that those of “black box” tools are not.Extra dimensions could strengthen gravityWhether the collider can produce microscopic black holes at all depends on how gravity behaves at distances far smaller than we can perceive. About two decades ago, theorists proposed that the LHC might reach the necessary conditions if space has additional dimensions. Giddings was among the scientists who explored how such black holes could form.“So what do you need to make a black hole? Well, you have to compress some energy into a really small volume,” Giddings explained.The difficulty is getting gravity to become strong enough within that volume. In some theories, two or more extra spatial dimensions could change its behavior over very short distances. These dimensions would be too small for us to register in our familiar world of three spatial dimensions and one time dimension.Extra dimensions, which also appear in string theory, have been proposed as an explanation for the hierarchy problem. One aspect of that problem is why gravity is so much weaker than the other fundamental forces. It belongs to a broader puzzle about why the masses and energies we observe are so far below the Planck scale, a fundamental energy scale associated with gravity and quantum physics.An undiscovered symmetry or new physics might explain this mismatch. Some proposed explanations could produce effects within the LHC’s reach.In the extra-dimension scenario, gravity might appear weak because it is “leaking” into dimensions we cannot see. Its underlying strength could be much greater, bringing the relevant Planck scale closer to accessible energies.“Basically, the gravitational force gets stronger, faster, as you go to shorter distances,” Giddings said.The LHC allows physicists to investigate those short distances by concentrating energy in collisions between protons.“At the LHC, we’re colliding particles at extremely high energy, which corresponds to tiny distance scales,” Incandela said. “As with microscopy, higher energies mean smaller wavelengths, allowing one to probe smaller distances.”The accelerator probes distances as small as 10⁻²⁰ meters. Roughly speaking, that distance is to an atom what an atom is to a human.“The extra dimensions wouldn’t have to be that small,” Incandela continued, “meaning that the LHC proton-proton collisions could be affected by them.”If gravity became sufficiently strong at those distances, the collision’s concentrated energy could theoretically make spacetime fold in on itself, creating a quantum black hole.Black hole theories face tighter limitsThe results also constrain extra dimensions within the particular models and parameters tested. Discussing those dimensions, Vami noted that string theory assumes 10 dimensions in total, “but these measurements say that, assuming the parameters of the theories we considered, you cannot have more than two.”“It’s not a dead-end,” Zhang said. “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.”The LHC’s continued lack of evidence for new physics has left researchers with a difficult question about where their existing descriptions need to change. Earlier searches have already excluded a large range of theoretical possibilities. The researchers point to past impasses that preceded major shifts in understanding, including Einstein’s relativity.“Theories don’t predict one exact answer,” Zhang added. “They predict a whole range of places a particle could be hiding. Each search clears out part of that range and says ‘not here,’ and over time the map of where new physics could still be, shrinks.”She pointed to the Higgs boson, discovered in 2012 after decades of searches that progressively excluded possible energy regions. The same process helps shape new theoretical models and decisions about future experiments and detectors.Without extra dimensions, producing a microscopic black hole would remain far beyond the LHC’s capabilities. Giddings estimates that creating the smallest black holes, with masses on the order of a microgram, would require collisions with a million billion times the energy currently available at the accelerator.“Theorists will continue to generate ideas and maybe we will do better in figuring things out without experimental data, but it will be difficult,” Giddings said. “The best guide is experimental data, and that’s what we’d really like to have,” he said, to investigate quantum gravity, which he calls “the most profound problem in theoretical physics.”Searching for clues to matter’s survivalThe collision patterns examined in the study could also expose processes connected to a different mystery, the existence of matter in the universe. In the same paper, the team searched for sphalerons, hypothetical unstable configurations of particle fields. These are not particles, but processes involving them could produce roughly spherical energy signatures resembling those sought in the black hole search.Sphalerons could help explain the imbalance between matter and antimatter.

The Big Bang is thought to have created equal amounts of both, which should have annihilated one another and left only energy. The researchers found no evidence of sphaleron processes, setting limits on the fraction of interactions that could produce sphaleron transitions.Those searches demonstrate how the phase-space distance method could be applied beyond microscopic black holes, including to other exotic particles, uncommon interactions, and unusual physical conditions.With the LHC shut down for an extensive set of upgrades, Zhang is looking ahead to the data the future High Luminosity Large Hadron Collider (HL-LHC) will provide.“We will be putting constraints on what theories can be true,” she said.The upgraded accelerator will allow researchers “to study fundamental components of matter in more detail,” including processes that may reveal how the early universe evolved.Reference: “Search for Black Holes and Sphalerons Using Novel Machine Learning Techniques at CMS” by Tamas Almos Vami and Danyi Zhang, 5 May 2026, Progress in High Energy Physics -PHEP. DOI: 10.31526/PHEP.2026.21Never miss a breakthrough: Join the SciTechDaily newsletter.Follow us on Google and Google News.