While the investigation did not uncover evidence of these elusive, subatomic phenomena, the researchers successfully established rigorous new exclusion limits that narrow the scope of where undiscovered physics might hide. Published in the journal Progress in High Energy Physics (PHEP), the study marks an important step forward in humanity’s ongoing quest to understand the fundamental laws governing spacetime, gravity, and the building blocks of the universe.
The hypothetical black holes targeted by the research team would be extraordinarily small and short-lived, fundamentally distinct from the massive astrophysical black holes scattered throughout the cosmos. If such microscopic entities could ever be produced within the high-energy environment of the LHC, their fleeting existence might provide researchers with the long-sought-after keys to addressing some of the deepest, most perplexing questions about gravity and quantum mechanics.
"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."
Although the search ultimately yielded a null result, members of the research team emphasized that failing to detect a predicted phenomenon remains a vital component of the scientific method. In the specialized realm of particle physics, ruling out specific scenarios provides concrete, publishable knowledge about the fundamental nature of reality.
"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 most persistent puzzles in fundamental physics involves the enormous discrepancy between the macro-scale universe we experience daily and the Planck scale, which is the fundamental energy threshold associated with quantum gravity. For decades, theorists have proposed that new physics or previously undiscovered symmetries could bridge this gap. Crucially, some of those theoretical effects might manifest at energy levels that modern particle accelerators, such as the LHC, are finally capable of reaching.
Years of meticulous experimentation have already eliminated numerous theoretical possibilities. The continued absence of clear, unambiguous signs of new physics at the LHC has undeniably presented a major challenge for modern researchers. However, similar stagnation periods have occurred historically before leading to paradigm-shifting frameworks, including Albert Einstein’s theory of general relativity.
For that reason, the researchers maintain that null results are an indispensable part of scientific progress. Each exclusion systematically reduces the number of viable possibilities, helping the scientific community determine where future experiments should focus their resources.
Could the LHC Create Tiny Black Holes?
The provocative possibility of producing microscopic black holes at the LHC first emerged roughly two decades ago. Physicists hypothesized that if a sufficient amount of energy could be concentrated into an infinitesimal region, and if extra spatial dimensions—which are already required by string theory—actually exist, then quantum black holes might briefly form during the trillions of high-energy proton-proton collisions generated inside the circular accelerator.
These objects would bear no resemblance to the massive gravitational titans found in outer space. "They wouldn’t stick around very long — if you made one, it would disintegrate immediately," said UCSB physics theorist Steven Giddings, a recognized expert in the paradoxical implications of combining quantum mechanics with gravity. Giddings was among the small group of scientists who initially proposed that such tiny voids in spacetime could theoretically exist under specific conditions.
When the concept was first widely discussed among scientists two decades ago, it occasionally sparked misunderstandings among the general public. Public concerns at the time mistakenly focused on the remote possibility that the LHC might generate stable, runaway black holes. In reality, the quantum black holes considered by researchers would dissolve almost instantly upon formation.
"People were more focused on the classical behavior of black holes," said Giddings, contrasting the hypothetical collider-produced phenomena with massive astrophysical singularities capable of consuming whole stars. Instead, the microscopic phenomena theorized for the LHC would arise purely from high-energy proton-proton collisions interacting via hidden, extra spatial dimensions.
Hidden Dimensions Could Make Gravity Stronger
Creating any black hole inherently requires compressing a tremendous amount of energy into an exceptionally tight volume. "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 intensely compressed volume might extend through two or more hypothetical spatial dimensions that remain invisible within humanity’s standard three-plus-one dimensional reality. Such extra dimensions have long been proposed as a potential solution to the hierarchy problem, a longstanding mystery in physics that questions why gravity is dramatically weaker than electromagnetism and the strong and weak nuclear forces.
One leading hypothesis suggests that gravity is not intrinsically as weak as it appears in our everyday lives. Instead, a significant portion of its fundamental strength could be "leaking" into these hidden extra dimensions. If that scenario is correct, the true Planck scale could reside much closer to the energy scales that modern physicists can access experimentally. "Basically, the gravitational force gets stronger, faster, as you go to shorter distances," Giddings noted.
However, stronger gravity alone is not enough; scientists must also concentrate immense energy into an exceptionally minute volume. This requirement is precisely where the Large Hadron Collider plays an indispensable role.
Colliding Particles at Extreme Energies
The LHC accelerates beams of protons to near-light-speed velocities before smashing them together at tremendous energies. These violent collisions grant researchers access to extraordinarily 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."
Today, researchers are probing distance scales as small as $10^-20$ meters at the LHC—a scale where the ratio of that distance to an atom is roughly equivalent to the ratio of an atom to a 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 truly becomes sufficiently robust at those microscopic scales and enough energy is concentrated into a tiny region, spacetime could theoretically fold in on itself, producing a fleeting quantum black hole. Early safety concerns regarding this scenario were thoroughly addressed and dismissed through rigorous scientific reports comparing collider operations to ultra-high-energy cosmic rays. These natural particles constantly strike Earth’s upper atmosphere at immense energies without causing adverse effects, proving that high-energy collisions pose no environmental threat. Any quantum black holes produced under these theoretical models would evaporate almost instantaneously.
Even so, their extraordinarily brief existence should leave distinct, detectable signatures within the particle debris produced as they decay. While earlier searches conducted by the ATLAS and CMS collaborations failed to uncover such evidence, those studies relied on much smaller datasets. With vastly more collision data accumulated over recent years, researchers were finally equipped to search at higher energies, significantly improving their odds of spotting an exceptionally rare quantum black hole event.
Where Quantum Physics Meets Gravity
This specialized search directly addresses one of the most profound unresolved problems in modern theoretical physics. "We have two big theories that describe nature," Tamas Vami observed. "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 merge these two monumental frameworks into a single, cohesive description of nature. The central difficulty lies in the fact that quantum mechanics typically governs extremely small objects, whereas general relativity dominates massive celestial bodies.
The ultimate goal is to forge a unified theory, 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 theoretically provide that exact intersection, remaining small enough for quantum effects to govern them while simultaneously concentrating enough mass and energy for gravity to play a decisive role.
Searching for the Signature of a Black Hole
To hunt for these events, the UCSB researchers analyzed CMS detector data collected between 2016 and 2018, employing two distinct strategies to identify potential quantum black hole signatures. One approach relied on a geometric property known as sphericity.
"You form a black hole, and it immediately disintegrates. But it has a very spherical decay signature, lots of things going in all directions," Vami explained. Another key indicator would be an unusually massive concentration of energy among the particles emerging from a collision.
"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."
These distinctive event patterns also presented an ideal opportunity to test a novel analytical framework called "phase-space distance," which was recently developed by UCSB particle theorist Nathaniel Craig and his collaborators. This method integrates with a machine learning architecture known as a Support Vector Machine (SVM) to help researchers reliably distinguish hypothetical signal events from the massive background noise of conventional high-energy particle collisions. In physics, phase space serves as a multidimensional mathematical representation incorporating properties such as space, time, energy, and momentum to comprehensively describe 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 explained. The innovative technique converts calculated distances between events into a single quantitative metric called an SVM score. Events yielding higher scores exhibit characteristics that more closely resemble the sought-after signal.
This study marked the first time the phase space distance method has ever been deployed in an active particle physics data analysis. "We compared phase space distance with the sphericity variable and our conclusion is that phase space distance outperforms sphericity," Zhang stated. Furthermore, this approach avoids the opacity of traditional "black box" machine learning models because it remains fully supervised, allowing researchers to examine the precise mathematics driving the results rather than simply trusting an unexplained output.
New Limits on Quantum Black Holes
Ultimately, the comprehensive data analysis uncovered no definitive evidence of quantum black hole production. Based on the specific theoretical models examined, the findings indicate that quantum black holes are highly unlikely to exist at energy thresholds up to approximately 12 Tera-electronvolts (TeV), while also placing strict constraints on theories involving extra spatial dimensions.
These empirical restrictions hold immense scientific value because they systematically eliminate portions of the parameter space where theoretical models could otherwise operate. String theory, for instance, conventionally requires a total of 10 dimensions to remain mathematically consistent.
"लेकिन these measurements say that, assuming the parameters of the theories we considered, you cannot have more than two," Vami noted regarding the extra spatial dimensions accessible to these specific effects. "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 methodical process of elimination has historically guided particle physics toward major breakthroughs. The famous discovery of the Higgs boson in 2012, for example, was achieved only after decades of preceding experiments progressively excluded one energy region after another. By systematically removing unviable possibilities, physicists can refine existing theories, construct more advanced models, and design superior detection hardware.
The Mystery of Weak Gravity Remains
For the time being, the overarching hierarchy problem remains persistently unresolved. Without the existence of extra spatial dimensions to siphon off gravitational force, theorists estimate that particle collisions would need to achieve roughly a million billion times the energy currently attainable at the LHC to successfully produce even the tiniest black holes, which would possess 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 acknowledged. "The best guide is experimental data, and that’s what we’d really like to have," he added, emphasizing the need to tackle quantum gravity, which he regards as "the most profound problem in theoretical physics."
By probing particle collisions at some of the highest energy levels currently achievable on Earth, Vami and Zhang have successfully pushed the Standard Model to its absolute operational limits. Their results deliver crucial new benchmarks for future searches targeting quantum black holes and potential solutions to the hierarchy problem. Equally important, the research demonstrates that the phase space distance machine learning method can be effectively deployed across broader particle physics investigations to hunt for unfamiliar particles, anomalous interactions, and other rare phenomena.
Searching for Another Exotic Phenomenon
Taking advantage of their robust analytical framework, the researchers also utilized the study to search for another theoretical phenomenon known as sphalerons. Unlike elementary particles, sphalerons represent unstable configurations of particle fields that, much like quantum black holes, would be expected to generate relatively spherical energy signatures upon decaying.
The investigation of sphalerons could potentially help resolve another massive cosmological mystery: why the visible universe is composed overwhelmingly of matter rather than antimatter. According to current cosmological models, the Big Bang should have produced equal quantities of matter and antimatter, which ought to have completely annihilated one another and left behind a sterile sea of pure energy instead of a universe filled with stars, planets, and galaxies. This glaring mismatch is known as the matter-antimatter asymmetry problem.
The UCSB research team found no empirical evidence of sphaleron processes in the CMS data either, an absence that allowed them to place strict quantitative limits on how frequently such anomalous particle interactions could possibly occur.
A More Powerful LHC Is Coming
Future experimental runs promise to push these exploratory boundaries significantly further. "We will be putting constraints on what theories can be true," Zhang said, expressing optimism regarding upcoming data collection efforts at the LHC, which is currently undergoing a scheduled, long-term shutdown to install an ambitious suite of technological upgrades.
The upcoming High Luminosity Large Hadron Collider (HL-LHC) will equip scientists with vastly larger datasets and significantly higher collision rates, vastly expanding their capacity to spot extremely rare events. These future experiments will ultimately allow researchers to study the fundamental components of matter in unprecedented detail, potentially uncovering elusive processes that reveal how the early universe evolved from a primordial soup into the complex cosmos observed today.