The quest for the universe’s hidden secrets has always been a driving force in scientific exploration, pushing the boundaries of our understanding and leading us to ponder the very fabric of reality. For decades, physicists have been captivated by the enigma of dark matter, an invisible substance that constitutes a staggering 85% of the universe’s total mass, yet remains frustratingly elusive to direct detection. While the Standard Model of particle physics, our current reigning theory of fundamental particles and their interactions, has been remarkably successful in describing the known universe, it is incomplete. The existence of dark matter is one of the most compelling pieces of evidence suggesting that there are fundamental particles and forces at play that lie beyond our current theoretical grasp. This ongoing mystery has fueled a relentless pursuit of new physics, with numerous ambitious experiments and theoretical frameworks being developed and tested in the hope of finally unveiling the identity of this cosmic phantom. The implications of discovering dark matter are profound, potentially revolutionizing our understanding of cosmology, galaxy formation, and the fundamental laws governing the universe.
At the heart of this ongoing investigation lies the tantalizing possibility of axion-like particles (ALPs), a class of hypothetical elementary particles that have emerged as a leading candidate for dark matter. These ALPs, though similar in some respects to the theoretically proposed axion, possess a broader range of properties that make them particularly intriguing. The original axion was theorized to solve a problem in quantum chromodynamics (QCD), the theory describing the strong nuclear force, but ALPs are more general constructs that could arise from various theoretical extensions to the Standard Model. Their potential to be weakly interacting and to have survived from the early universe makes them prime candidates for forming the vast halos of dark matter that surround galaxies. The search for these elusive particles is not merely an academic exercise; it is a crucial step towards a more complete and accurate picture of the cosmos, and the recent advancements in experimental strategies are bringing us closer than ever to potentially detecting them.
The challenge in detecting ALPs lies not only in their inherent weakness of interaction but also in their potential to be “long-lived.” This means that instead of decaying almost instantaneously after their creation, ALPs might persist for a significant duration, traveling considerable distances before eventually transforming into more conventional particles, if they decay at all. This longevity is a key characteristic that experimental physicists are endeavoring to exploit. If ALPs are indeed the dark matter particles, their long-lived nature would allow them to travel from the extremely dense environments where they might have been produced in the early universe, or even within high-energy particle collisions, across the vast expanse of detectors. The signatures of such decay events, occurring away from the primary interaction point, are precisely what new research is focusing on.
This is where the groundbreaking work presented in the European Physical Journal C enters the picture, offering a novel and sophisticated approach to the hunt for ALPs. The researchers, led by CX. Yue and XY. Li and collaborators, propose a strategy that leverages the peculiar signature of “displaced vertices” at the High-Luminosity Large Hadron Collider (HL-LHC). A vertex, in particle physics, refers to the point in spacetime where particles are produced or interact. In typical high-energy collisions, these interactions occur at the very center of the detectors, producing particles that fly outward immediately. However, if ALPs are produced and then travel a measurable distance before decaying, their decay point, or secondary vertex, will be separated from the primary collision point. This displacement is the key.
The HL-LHC, an upgraded version of the already powerful Large Hadron Collider at CERN, is poised to deliver unprecedented levels of luminosity, meaning it will generate a vastly increased number of proton-proton collisions per second. This immense data-generating capability, coupled with the enhanced sensitivity of advanced detectors, creates an ideal environment for searching for rare and subtle signals, such as those produced by the decay of long-lived ALPs. The sheer volume of collisions means that even if ALP production is an infrequent event, the probability of observing several such events within the datasets collected by the HL-LHC becomes significantly higher. This increased collision rate is not just about seeing more; it’s about seeing more of the subtle, often hidden phenomena that whisper clues about the universe’s deepest mysteries.
The concept of displaced vertices is crucial to the proposed search strategy. Imagine a tiny explosion happening not right at the center of your explosion-detection apparatus, but a few millimeters or even centimeters away. That’s the essence of a displaced vertex. In the context of particle physics, if an ALP is produced in a high-energy collision and travels a short distance before decaying into detectable particles (like photons or electrons and positrons), the detector will register these decay products originating from a point away from the main collision point. This spatial separation acts as a powerful discriminator, helping to distinguish potential ALP decay signals from the overwhelming background of standard particle interactions that occur precisely at the interaction point.
The challenge with displaced vertices is that they are rare. Most particles produced in LHC collisions are short-lived, decaying very close to the interaction point. Identifying an event with a secondary vertex requires highly precise tracking capabilities within the detectors, along with sophisticated algorithms to reconstruct these tracks and pinpoint their origin. The existing LHC detectors, and even more so the upgraded ones planned for the HL-LHC, are designed with exactly this capability in mind. They are equipped with incredibly fine-grained silicon pixel detectors and sophisticated algorithms that can accurately measure the trajectories of charged particles, allowing for the reconstruction of vertices with very high precision, even if they are displaced.
The proposed research focuses on specific decay channels for ALPs. While ALPs can decay into various particles, researchers often prioritize channels that are easier to detect and reconstruct. For instance, the decay of an ALP into two photons (a diphoton resonance) or into an electron-positron pair (a dilepton resonance) are prime targets. These decay products are relatively clean signals that can be meticulously analyzed by the detector systems. The precise measurement of their energy, momentum, and arrival direction allows physicists to reconstruct the properties of the parent particle, including its mass and decay length.
The specific theoretical framework underpinning this search involves considering ALPs with masses that fall within a particular range and decay lengths that are also observable within the HL-LHC detectors. If an ALP is too light, it might travel too far, potentially escaping the detector before decaying. Conversely, if it’s too heavy or decays too quickly, its decay vertex might be too close to the primary interaction point to be clearly distinguished. The researchers explore a parameter space where ALPs would produce a detectable number of displaced vertices within the expected performance of the HL-LHC. This involves intricate theoretical calculations and simulations to predict the expected signals.
The power of the HL-LHC in this context cannot be overstated. The sheer increase in the number of collisions from the nominal LHC to the HL-LHC is staggering, often quoted as being up to ten times greater. This means that the integrated luminosity, a measure of the total number of collisions delivered and recorded by the experiments, will be significantly higher. This higher integrated luminosity translates directly into an increased sensitivity for discovering rare processes. For a signal that is intrinsically rare, like the production and decay of ALPs leading to displaced vertices, a factor of ten increase in luminosity can dramatically extend the accessible parameter space for these particles, potentially allowing us to probe masses and coupling strengths that were previously out of reach.
Beyond the luminosity, upgrades to the detectors themselves are critical. New technologies in tracking detectors, such as advanced silicon pixel sensors with higher granularity and radiation hardness, will be crucial for accurately reconstructing the trajectories of particles originating from displaced vertices. Furthermore, enhancements in trigger systems, which are responsible for selecting potentially interesting events in real-time from the immense deluge of data, will be vital for not missing these rare signals. The ability to precisely identify and isolate events with displaced vertices amidst a sea of billions of proton-proton interactions is a technological tour de force.
The significance of finding ALPs goes far beyond solving the dark matter puzzle. If ALPs are discovered, it would represent a profound breakthrough in our understanding of fundamental physics, potentially opening up new avenues of theoretical research and leading to a paradigm shift in how we view the universe. It could indicate the existence of new fundamental symmetries or dimensions, or provide evidence for theories that attempt to unify gravity with other fundamental forces. The discovery would mark a monumental stride towards a “Theory of Everything,” a unified description of all fundamental forces and particles in the universe.
The research highlights the synergistic relationship between theoretical predictions and experimental capabilities. Theoretical models predict the existence of ALPs and their potential properties, guiding experimentalists in designing searches. In turn, experimental results, whether they lead to a discovery or set stringent limits, provide crucial feedback to theorists, refining their models and pointing towards new directions for investigation. This iterative process is the engine of progress in particle physics, constantly pushing the boundaries of our knowledge and refining our understanding of the fundamental constituents of the cosmos.
The proposed search strategy at the HL-LHC for long-lived ALPs via displaced vertices represents a sophisticated and forward-thinking approach to one of the most pressing mysteries in modern physics. By combining the unprecedented data rates of the HL-LHC with the advanced capabilities of next-generation detectors and cutting-edge analysis techniques, physicists are well-positioned to potentially uncover evidence for these elusive particles. The implications of such a discovery would be far-reaching, not only solving the enigma of dark matter but also potentially reshaping our fundamental understanding of the universe and the laws that govern it, marking a new era in particle physics.
The work emphasizes the intricate interplay between theory and experiment, where theoretical predictions for ALPs serve as a roadmap for experimentalists. The specific mass ranges and decay properties of ALPs considered in this study are informed by various theoretical models beyond the Standard Model, such as those arising from string theory or supersymmetry. By targeting ALPs that would decay within the fiducial volume of the HL-LHC detectors, the research maximizes the chances of detection and provides a concrete, actionable strategy for the experimental collaborations. This precise targeting is crucial for efficiently utilizing the collider’s resources and maximizing the scientific output of future data.
The very act of conducting such a search at the HL-LHC speaks to the ingenuity and perseverance of the scientific community. The technical challenges in reconstructing displaced vertices are immense, requiring extremely precise alignment of detector components, sophisticated calibration procedures, and advanced machine learning algorithms to sift through the data. The success of this proposed search will hinge on the meticulous execution of these technical aspects, pushing the limits of detector technology and data analysis techniques to their absolute extreme. It is a testament to human curiosity and our relentless drive to unravel the universe’s deepest secrets.
Subject of Research: Searching for long-lived axion-like particles (ALPs) as a dark matter candidate.
Article Title: Searching for long-lived axion-like particles via displaced vertices at the HL-LHC.
Article References: Yue, CX., Li, XY., Yang, S. et al. Searching for long-lived axion-like particles via displaced vertices at the HL-LHC. Eur. Phys. J. C 85, 1442 (2025).
Image Credits: AI Generated
DOI: https://doi.org/10.1140/epjc/s10052-025-15190-x
Keywords: Axion-like particles, dark matter, displaced vertices, HL-LHC, particle physics, beyond the Standard Model
