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 ambitious search also gave researchers a unique chance to test a novel analytical technique for finding rare and previously unknown particles in complex collider data.
"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."
While the comprehensive search did not uncover evidence of quantum black holes, particle physics often advances through such null results. Failing to detect something can still provide immensely important information by ruling out where it could theoretically exist within the universe.
"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."
Why Missing Black Holes Still Matter
One of the major puzzles in fundamental physics involves the enormous difference between the scale of the universe we experience in everyday life 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 massive difference. Crucially, some of those theoretical effects might appear at energy levels that the LHC can successfully reach.
Years of meticulous 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 contemporary researchers. But similar situations have happened repeatedly throughout the history of science. Periods in which existing theories struggled to explain observations have sometimes led to radically new frameworks, including Albert Einstein’s revolutionary theory of relativity.
For that reason, the researchers emphasize that null results remain an essential part of scientific progress. Each one reduces the number of viable possibilities and helps determine where future experiments should look next.
Vami’s and Zhang’s results are published in the journal Progress in High Energy Physics (PHEP).
Could the LHC Create Tiny Black Holes?
The possibility of producing microscopic black holes at the LHC emerged roughly two decades ago. Physicists proposed that if enough energy were concentrated into an extremely small region, and if extra spatial dimensions—which are already required in advanced string theory—actually exist, then quantum black holes might form during the trillions of high-energy proton-proton collisions created by the massive accelerator.
These objects would be nothing like the enormous astrophysical black holes found throughout the universe.
"They wouldn’t stick around very long — if you made one, it would disintegrate immediately," said UCSB physics theorist Steven Giddings, an expert in the paradoxical implications of combining quantum mechanics with gravity, and one of a few scientists at the time who proposed that under certain conditions these tiny voids in spacetime could exist.
When scientists first discussed the possibility years ago, the idea became widely misunderstood by the general public. Public concerns focused heavily on the possibility that the LHC might create stable, dangerous black holes, even though the quantum black holes being considered by physicists would disappear almost instantly upon creation.
"People were more focused on the classical behavior of black holes," said Giddings, referring to those massive voids in spacetime, areas of extreme gravity that can eat whole stars, grow, and merge.
The hypothetical black holes produced at the LHC would instead arise from proton-proton collisions combined with the effects of extra spatial dimensions that have never been directly observed.
Hidden Dimensions Could Make Gravity Stronger
Creating any black hole requires squeezing a large amount of energy into an extremely small region.
"So what do you need to make a black hole? Well, you have to compress some energy into a really small volume," Giddings explained.
That "really small volume" might extend through two or more hypothetical spatial dimensions that are far too small for humans to detect within our standard 3 plus 1 dimension reality.
Such extra dimensions have been proposed as one possible answer to the hierarchy problem, a longstanding question in physics that asks why gravity is dramatically weaker than all the other fundamental forces of nature.
One leading possibility is that gravity is not intrinsically as weak as it appears in our everyday lives. Instead, some of its true strength could be "leaking" into these hidden extra dimensions. If that hypothesis were true, the fundamental Planck scale could be much closer to the energy scales that physicists can experimentally reach.
"Basically, the gravitational force gets stronger, faster, as you go to shorter distances," Giddings said.
However, stronger gravity alone would not be enough to form a black hole. Scientists would also need to concentrate enormous energy into an exceptionally small volume.
That is where the Large Hadron Collider becomes vitally important to modern research.
Colliding Particles at Extreme Energies
The LHC accelerates protons to tremendous energies before smashing them together inside massive particle detectors. These high-energy collisions give physicists access to extremely small distance scales.
"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."
Researchers are probing scales as small as $10^-20$ meters at the LHC, a distance scale that is to an atom what a single atom is to an entire human being.
"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 tiny scales and enough energy were concentrated into a sufficiently small region, spacetime could theoretically fold in on itself and produce a quantum black hole.
Safety concerns surrounding this provocative idea were eventually addressed through detailed scientific reports and careful comparisons with ultra-high-energy cosmic rays. These naturally occurring particles have been striking Earth’s upper atmosphere and other astronomical objects at immense energies throughout cosmic history without producing any dangerous effects.
Those rigorous comparisons showed beyond doubt that high-energy particle collisions do not pose a black hole threat. Any quantum black holes produced under the proposed theoretical models would evaporate essentially immediately.
Even so, their extremely brief existence might leave detectable traces in the shower of particles produced as they decay.
Earlier searches by the ATLAS and CMS collaborations failed to find such evidence, but those previous studies had access to much smaller datasets.
With far more collision data now available from subsequent operational runs, researchers could search at higher energies and increase their chances of seeing an exceptionally rare quantum black hole event if such events actually occur in nature.
Where Quantum Physics Meets Gravity
The search is ultimately connected to one of the biggest and most enduring unresolved problems in modern theoretical physics.
"We have two big theories that describe nature," Tamas 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."
Physicists have spent decades trying to combine these two successful frameworks into a single, unified description of nature.
The fundamental difficulty is that quantum physics generally describes extremely small objects, while general relativity becomes most important for very massive, macroscopic ones.
The ultimate goal is to somehow merge the two theories into a single, unified framework, Vami noted, "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 exactly that elusive combination. They would be small enough for quantum effects to become critically important while also concentrating enough mass and energy for gravity to matter profoundly.
Searching for the Signature of a Black Hole
The researchers analyzed CMS detector data collected between 2016 and 2018 during the LHC’s second major operational run, using two distinct analytical approaches to look for evidence of quantum black holes.
One method involved a property called sphericity, which measures how evenly energy is distributed across the collision debris.
"You form a black hole, and it immediately disintegrates. But it has a very spherical decay signature, lots of things going in all directions," researchers explained.
Another crucial clue would be an unusually large amount of total energy contained within the particles produced by a single collision event.
"We know that black holes are very high energy," Danyi Zhang said. "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."
Those unusual event patterns also provided a timely opportunity to test a novel analytical method known as "phase-space distance," which was developed by UCSB particle theorist Nathaniel Craig and his collaborators.
The method works in tandem with an advanced machine learning system called a Support Vector Machine (SVM), which helps researchers distinguish possible signal events from the enormous background noise of conventional high-energy particle collisions.
In particle physics, "phase space" is a multidimensional mathematical representation incorporating properties such as space, time, energy, and momentum into a comprehensive description of a particle system.
Machine Learning Joins the Search
"We developed the idea of the phase space between events, which can be combined with SVM to help the search," Craig said.
The method successfully converts the complex distances between collision events into a single measurement called an SVM score. Events with larger scores are more likely to resemble the specific signal scientists are searching for.
This recent study marked the very first time that the phase space distance method had ever been applied in a major 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 innovative approach also differs significantly from some opaque "black box" machine learning systems because it remains supervised. Researchers can carefully examine the underlying mathematics that produced the result rather than simply accepting an unexplained output from an algorithm.
New Limits on Quantum Black Holes
The thorough search ultimately found no evidence for quantum black hole production in the analyzed data.
Based on the theoretical models examined by the team, the result means quantum black holes are unlikely to exist up to energy thresholds of about 12 Tera-electron volts (TeV). It also strictly constrains certain theoretical models involving extra spatial dimensions.
Those valuable restrictions are scientifically meaningful because they systematically eliminate parts of the range in which these theoretical models could otherwise operate.
String theory, for instance, assumes a total of 10 dimensions to remain mathematically consistent.
"But these measurements say that, assuming the parameters of the theories we considered, you cannot have more than two," Vami said.
"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."
That rigorous process of elimination has repeatedly played a foundational role in particle physics history. The iconic Higgs boson was discovered in 2012 only after decades of painstaking experiments gradually excluded one energy region after another.
By continually eliminating possibilities, physicists can improve their overarching theories, develop fresh models, and design better experiments and detectors for future campaigns.
The Mystery of Weak Gravity Remains
For now, the overarching hierarchy problem remains entirely unresolved.
Without extra dimensions to alter the math, Giddings estimates that particle collisions would need to reach roughly a million billion times the energy currently achieved at the LHC to produce even the absolute smallest black holes, which would possess microscopic masses measured in micrograms.
"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 added, emphasizing the need to study quantum gravity, which he describes as "the most profound problem in theoretical physics."
By testing particle collisions at some of the highest energies currently available to humanity, Vami and Zhang have successfully pushed the Standard Model toward its absolute limits.
Their results provide vital new guidance for future searches for quantum black holes as a possible solution to the hierarchy problem. Just as importantly, the work demonstrates that the phase space distance machine learning method could be utilized more broadly to search for unfamiliar particles, unusual interactions, and other exceedingly rare phenomena in future datasets.
Searching for Another Exotic Phenomenon
The researchers also utilized the same comprehensive study to hunt for another theoretical phenomenon known as sphalerons.
Sphalerons are not actual particles. Instead, they represent theoretical unstable configurations of particle fields that, much like quantum black holes, would be expected to produce relatively spherical energy patterns upon transitioning.
They could potentially help resolve another major cosmological mystery: why the visible universe contains matter at all.
According to our current understanding of physics, the Big Bang should have produced matter and antimatter in exactly equal amounts. Those two opposing forms of matter should then have completely annihilated one another, leaving behind only ambient energy rather than the matter-filled universe we observe around us today.
This profound mismatch is known to researchers as the matter-antimatter asymmetry problem.
The UCSB researchers found no evidence of sphaleron processes in the data either. That absence allowed them to place strict new limits on how many particle interactions could potentially involve sphaleron transitions in the cosmos.
A More Powerful LHC Is Coming
Future experimental campaigns could push the boundaries of these searches considerably further in the coming years.
"We will be putting constraints on what theories can be true," said Zhang, who is eagerly looking forward to obtaining fresh data at the LHC, which is currently shut down for the installation of an ambitious, comprehensive set of hardware upgrades.
The upcoming High Luminosity Large Hadron Collider (HL-LHC) will provide scientists with vastly larger datasets and significantly more opportunities to detect extremely rare physical events.
Those future experiments will allow researchers "to study fundamental components of matter in more detail," including elusive processes that may ultimately reveal how the early universe evolved from its earliest moments.