While the investigation did not uncover physical evidence of these elusive entities, the null result serves as a critical milestone. By analyzing vast datasets collected between 2016 and 2018, the team successfully established new exclusion limits, ruling out specific theoretical parameters regarding quantum gravity and extra spatial dimensions up to approximately 12 Tera-electronvolts. Furthermore, the study marked the pioneering application of an innovative machine learning technique known as phase-space distance, setting a fresh benchmark for how physicists hunt for rare and exotic phenomena in complex collider data.
Searching for Clues to Quantum Gravity
The hypothetical black holes targeted by the UCSB researchers would be extraordinarily small, intensely energetic, and profoundly short-lived. Unlike the massive astrophysical black holes that populate the cosmos and consume entire stars, these quantum-scale anomalies would disintegrate the exact millisecond they formed. Yet, if they could be produced in a controlled laboratory setting like the LHC, their fleeting existence could provide researchers with an empirical bridge to address some of the deepest, most persistent mysteries surrounding spacetime and gravity.
"Had we found evidence, we could have begun to directly study quantum gravity," said Tamas Vami, a researcher in the 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."
Modern physics rests on two remarkably successful pillars that unfortunately speak entirely different languages. Quantum field theory and the Standard Model describe the behavior of the universe at its smallest scales with astonishing precision, while Albert Einstein’s general relativity governs the vast scales of gravity and massive cosmic structures. For decades, physicists have struggled to unify these frameworks. Because quantum mechanics applies to the extremely small and general relativity dominates the very massive, finding a single scenario where both forces matter simultaneously has proven exceptionally difficult. Microscopic black holes represent the ultimate meeting ground for these disciplines, possessing both quantum scale and enough mass-energy concentration for gravity to play a dominant role.
Although the search did not yield a detection, particle physicists emphasize that failing to find a signal is far from a dead-end. In experimental physics, ruling out where a phenomenon cannot exist is a vital part of narrowing down the architecture of 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."
The Theoretical Case for LHC Black Holes
The proposition that the LHC might be capable of generating tiny black holes first emerged roughly two decades ago. Theoretical physicists proposed that if enough energy were concentrated into an infinitesimally small volume, and if extra spatial dimensions predicted by string theory actually exist, quantum black holes could materialize during the trillions of high-energy proton-proton collisions orchestrated by the particle accelerator.
When these concepts were first introduced to the broader public, they sparked widespread misunderstandings and groundless safety concerns regarding the creation of stable, runaway black holes on Earth. Experts quickly clarified that the quantum black holes under scientific consideration would evaporate almost instantly via Hawking radiation.
"They wouldn’t stick around very long — if you made one, it would disintegrate immediately," explained UCSB physics theorist Steven Giddings, an expert in the paradoxical implications of combining quantum mechanics with gravity. Giddings was among the original scientists who proposed that such tiny voids in spacetime could theoretically exist under specific conditions. "People were more focused on the classical behavior of black holes," Giddings noted, contrasting the theoretical subatomic anomalies with the massive, stable singularities of astrophysics.
The creation of any black hole fundamentally requires compressing a significant amount of energy into an extremely localized region. According to Giddings and his colleagues, that requisite volume might extend through two or more hypothetical spatial dimensions that remain hidden from human perception within our standard three-dimensional space plus time reality.
These extra dimensions have long been proposed as a potential solution to the hierarchy problem—a major puzzle in fundamental physics that asks why gravity is dramatically weaker than electromagnetism and the strong and weak nuclear forces. One leading hypothesis suggests that gravity is not intrinsically weak at all. Instead, a portion of its fundamental strength could be leaking away into these imperceptible extra dimensions. If true, the true energy scale associated with gravity, known as the Planck scale, could be much closer to the energy levels that particle accelerators can experimentally reach.
Probing Distance Scales at Extreme Energies
To test these theories, the Large Hadron Collider accelerates beams of protons to near-light speeds before crashing them together with immense force. These high-energy impacts allow scientists to probe exceptionally 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."
At the LHC, researchers routinely probe distance scales as small as $10^-20$ meters—a scale so minuscule that the relationship between that distance and an atom is comparable to the relationship between an atom and a human being. If extra dimensions exist and gravity strengthens rapidly at those microscopic ranges, the extreme energy concentrations achieved in proton-proton collisions could theoretically cause spacetime to fold in on itself, producing a quantum black hole.
Safety apprehensions surrounding these collision experiments were thoroughly addressed years ago through rigorous safety reports. Scientists pointed to natural phenomena such as ultra-high-energy cosmic rays, which have continuously bombarded Earth’s upper atmosphere and other astronomical bodies throughout cosmic history at energies far exceeding human technological capabilities, without triggering catastrophic events. These comparisons confirmed that high-energy particle collisions pose no threat, and any theoretical quantum black holes produced would evaporate harmlessly and almost instantaneously.
While earlier searches conducted by the ATLAS and CMS collaborations found no trace of these events, those projects relied on significantly smaller datasets. With modern upgrades providing vastly larger volumes of collision data, researchers are now equipped to search at higher energy thresholds, drastically improving their sensitivity to rare phenomena.
Machine Learning and the Search for Signatures
To analyze the immense volume of data gathered by the CMS detector between 2016 and 2018, the UCSB researchers deployed two distinct analytical strategies. The first relied on a classic property known as sphericity. When a quantum black hole disintegrates immediately upon formation, its decay products are expected to spray outward in all directions with a highly spherical energy distribution.
The second strategy focused on total energy aggregation. Because black holes are inherently high-energy objects, researchers summed the energy levels of the particles emerging from individual collisions. If the aggregate energy surpassed a certain threshold, the event was flagged as a potential signal candidate.
Crucially, this study integrated a cutting-edge analytical framework known as "phase-space distance," developed by UCSB particle theorist Nathaniel Craig and his collaborators. This method partners with a machine learning architecture called a Support Vector Machine (SVM) to help researchers differentiate genuine signal events from the overwhelming background noise of conventional particle collisions. In physics, phase space is a multidimensional mathematical representation that maps out properties such as position, momentum, energy, and time for a system of particles.
"We developed the idea of the phase space between events, which can be combined with SVM to help the search," Craig said.
This research marks the first time the phase-space distance approach has been successfully implemented in a published particle physics data analysis. When the team evaluated its performance against traditional metrics, the results were clear.
"We compared phase space distance with the sphericity variable and our conclusion is that phase space distance outperforms sphericity," Zhang noted. Furthermore, unlike opaque "black box" machine learning algorithms that provide conclusions without explanation, this supervised approach allows scientists to inspect the underlying mathematics, ensuring complete transparency in data processing.
Refining the Map of Fundamental Physics
Ultimately, the comprehensive analysis revealed no evidence of quantum black hole production. The resulting exclusion limits indicate that quantum black holes are unlikely to exist below energy thresholds of approximately 12 Tera-electronvolts within the parameters of the models tested. These restrictions also place tight constraints on theories involving extra spatial dimensions.
String theory, for instance, typically requires a framework of ten total dimensions. However, these latest empirical measurements suggest that, under the evaluated model parameters, no more than two extra dimensions can be accommodated in this manner.
"Theories don’t predict one exact answer," Zhang explained. "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 long driven progress in high-energy physics. The landmark discovery of the Higgs boson at CERN in 2012 was achieved only after decades of preceding experiments progressively ruled out one energy range after another. By systematically closing off theoretical loopholes, physicists refine their models and design more sophisticated detectors for future exploration.
Expanding the Hunt to Other Exotic Phenomena
Taking advantage of their robust analytical framework, the UCSB researchers also utilized the same dataset to hunt for another theoretical construct known as sphalerons.
Sphalerons are not particles; rather, they represent unstable configurations of particle fields that, much like quantum black holes, would be expected to generate relatively spherical energy signatures upon transition. Probing for sphalerons is of paramount interest because they could potentially help solve the matter-antimatter asymmetry problem—one of the most glaring cosmological puzzles of our time. According to standard cosmological models, the Big Bang should have produced equal quantities of matter and antimatter, which ought to have completely annihilated one another, leaving behind a universe devoid of stable matter.
As with the black hole search, the analysis found no direct evidence of sphaleron processes, allowing the team to establish stringent new upper limits on how frequently such anomalous transitions could occur in nature.
With the LHC currently shut down for a comprehensive phase of hardware upgrades designed to transform it into the High Luminosity Large Hadron Collider (HL-LHC), researchers are already looking ahead to the next generation of data collection. The upcoming high-luminosity era will supply scientists with dramatically larger datasets, offering unprecedented opportunities to probe the foundational components of matter and potentially unmask the hidden mechanisms that shaped the early universe.
"We will be putting constraints on what theories can be true," Zhang said, looking forward to the resumption of experimental operations.
