Microscopic black holes would be extraordinarily small and short-lived. If the LHC could produce them, their existence might help scientists address some of the deepest unanswered questions about spacetime, gravity and the relationship between quantum mechanics and general relativity.

“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 absence of a detection is still scientifically meaningful. In particle physics, a null result can rule out specific possibilities and narrow the range of theories that remain viable.

“It’s not a dead-end,” said Danyi Zhang, a graduate student researcher in the Incandela Lab. “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 the Search for Microscopic Black Holes Matters

One of the major puzzles in fundamental physics is the enormous difference between the scale of everyday phenomena 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. Importantly, certain effects predicted by these ideas might appear at energy levels that the LHC can reach.

Years of experiments have already eliminated many theoretical possibilities. The continued absence of clear signs of new physics at the LHC has created a major challenge for researchers, but null results have played an important role in science 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 that negative results are an essential 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).

Could the LHC Create Tiny Black Holes?

The possibility of producing black holes at the LHC emerged roughly two decades ago. Physicists proposed that quantum black holes might form if enough energy were concentrated into an extremely small region and if extra spatial dimensions—already required in string theory—exist.

The LHC produces trillions of proton-proton collisions, creating conditions in which such rare processes could theoretically occur. However, these hypothetical objects would be completely different from 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. He is an expert on the implications of combining quantum mechanics with gravity and was one of the scientists who proposed that tiny black holes might exist under certain conditions.

When the idea was first discussed, it was widely misunderstood. Public concerns focused on the possibility that the LHC could create stable black holes, even though the quantum black holes considered by physicists would disappear almost instantly.

“People were more focused on the classical behavior of black holes,” Giddings said, referring to the massive objects formed by extreme gravity that can consume stars, grow and merge.

By contrast, hypothetical black holes produced at the LHC would arise from proton-proton collisions and the possible effects of extra spatial dimensions that have never been observed.

Could Hidden Dimensions Make Gravity Stronger?

Creating a black hole requires compressing 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 volume might extend through two or more hypothetical spatial dimensions that are too small for humans to detect within our familiar 3 + 1-dimensional reality.

Extra dimensions have been proposed as one possible solution to the hierarchy problem, a longstanding question about why gravity appears dramatically weaker than the other fundamental forces.

One possibility is that gravity is not intrinsically weak. Instead, some of its strength could be “leaking” into extra dimensions. If so, the Planck scale could be much closer to the energy levels that particle accelerators can reach.

“Basically, the gravitational force gets stronger, faster, as you go to shorter distances,” Giddings said.

Stronger gravity alone would not be enough, however. Scientists would also need to concentrate enormous energy into an exceptionally small volume. That is where the LHC becomes important.

How the Large Hadron Collider Probes Extreme Physics

The LHC accelerates protons to tremendous energies before smashing them together. These collisions allow physicists to investigate 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 distances as small as 10^-20 meters at the LHC—a scale comparable to the relationship between an atom and 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 scales and enough energy were concentrated into a tiny region, spacetime could theoretically fold in on itself and produce a quantum black hole.

Safety concerns about this possibility were addressed through detailed reports and comparisons with ultra-high-energy cosmic rays. These naturally occurring particles have struck Earth’s upper atmosphere and other astronomical objects at immense energies without producing dangerous effects.

Those comparisons showed that high-energy particle collisions do not pose a black-hole threat. Any quantum black holes produced under the proposed models would evaporate essentially immediately.

Even such a brief existence could leave detectable traces in the particles created as the black hole decayed.

Earlier searches by the ATLAS and CMS experiments found no evidence of quantum black holes, but those analyses used much smaller datasets. With substantially more collision data available, researchers could search at higher energies and improve their chances of detecting an exceptionally rare event.

Quantum Black Holes and the Search for Quantum Gravity

The search is connected to one of the biggest unresolved problems in modern physics: how to combine quantum mechanics with general relativity.

“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.”

Physicists have spent decades trying to combine these frameworks into a single description of nature. The challenge is that quantum physics generally describes extremely small objects, while general relativity becomes most important for very massive ones.

“That’s really hard to do because you don’t often have a situation which is really tiny but also extremely heavy,” Vami said.

Microscopic black holes could provide precisely that combination. They would be small enough for quantum effects to matter while concentrating enough mass and energy for gravity to become important.

How Researchers Searched for a Black Hole Signature

The researchers analyzed CMS detector data collected between 2016 and 2018. They used two approaches to search for the distinctive signatures that quantum black holes might produce when they decayed.

One approach examined a property called sphericity. A rapidly decaying black hole could produce particles moving in many directions, creating an unusually spherical event pattern.

“You form a black hole, and it immediately disintegrates. But it has a very spherical decay signature, lots of things going in all directions,” Zhang said.

Another possible clue would be an unusually large amount of energy in the particles produced by the collision.

“We know that black holes are very high energy,” 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.”

These unusual event patterns also enabled the researchers to test a new analytical method called “phase-space distance,” developed by UCSB particle theorist Nathaniel Craig and collaborators.

The method works with a machine-learning system called a Support Vector Machine (SVM). It helps researchers distinguish possible signal events from the enormous background of conventional high-energy particle collisions.

In particle physics, phase space is a multidimensional mathematical representation that incorporates properties such as space, time, energy and momentum into a description of a particle system.

Machine Learning Improves the Search for Rare Particles

“We developed the idea of the phase space between events, which can be combined with SVM to help the search,” Craig said.

The method converts the distances between events into a single measurement called an SVM score. Events with larger scores are more likely to resemble the signal being investigated.

This study marked the first use of the phase-space distance method in 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 also differs from some “black box” machine-learning systems because it is supervised. Researchers can examine the mathematics behind the result instead of simply accepting an unexplained output.

New Limits on Quantum Black Holes and Extra Dimensions

The search found no evidence of quantum black hole production.

Based on the theoretical models examined, the results indicate that quantum black holes are unlikely up to about 12 TeV, or tera-electron volts. The analysis also constrains certain theories involving extra spatial dimensions.

These limits are scientifically valuable because they eliminate portions of the range in which the theoretical models could operate.

String theory, for example, assumes a total of 10 dimensions.

“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.”

This process of elimination has repeatedly shaped particle physics. The Higgs boson was discovered in 2012 only after decades of experiments gradually excluded one energy region after another.

By eliminating possibilities, physicists can refine theories, develop new models and design more effective experiments and detectors.

The Mystery of Weak Gravity Remains Unsolved

The hierarchy problem remains unresolved. Without extra dimensions, 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 smallest black holes, which would have 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, referring to the opportunity to study quantum gravity, which he calls “the most profound problem in theoretical physics.”

By testing collisions at some of the highest energies currently available, Vami and Zhang have pushed the Standard Model toward its limits.

Their results provide new guidance for future searches for quantum black holes as a possible solution to the hierarchy problem. The study also demonstrates that phase-space distance could be used more broadly to search for unfamiliar particles, unusual interactions and other rare phenomena.

The Same Method Searches for Sphalerons

The researchers used the same study to search for sphalerons.

Sphalerons are not particles. They are theoretical, unstable configurations of particle fields that, like quantum black holes, could produce relatively spherical energy patterns.

Sphalerons might help explain another major mystery: why the universe contains matter.

According to current understanding, the Big Bang should have produced matter and antimatter in equal amounts. The two forms of matter should then have annihilated one another, leaving energy rather than the matter-filled universe observed today.

This mismatch is known as the matter-antimatter asymmetry problem.

The researchers found no evidence of sphaleron processes either. That absence allowed them to place limits on how many particle interactions could potentially involve sphaleron transitions.

How the High-Luminosity LHC Could Extend the Search

Future experiments could push the search considerably further.

“We will be putting constraints on what theories can be true,” Zhang said. He is looking forward to new data from the LHC, which is currently shut down for the installation of an ambitious set of upgrades.

The future High-Luminosity Large Hadron Collider (HL-LHC) will provide scientists with much larger datasets and more opportunities to detect extremely rare events.

Those experiments will allow researchers “to study fundamental components of matter in more detail,” including processes that may reveal how the early universe evolved.

For now, the LHC has not revealed microscopic black holes, hidden dimensions or sphalerons. But by narrowing the possibilities and introducing a new way to identify unusual collision events, the research offers a clearer map for the next search for physics beyond the Standard Model.