Receiving a Signal from the Heart of Darkness
If we take a fleeting glance around us, everything we see consists of matter that makes up the world. But is our universe, like our immediate surroundings, filled with visible matter and is it taken for granted? Let us step away from this visible matter for a moment and venture into a more enigmatic realm. Imagine leaving Earth behind and traveling so far that galaxies appear merely as tiny dots among countless others. From that vantage point, we can observe the entire observable universe within a single frame. But is everything before us truly composed of visible matter?
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In this frame, by observing the motion of galaxies, we uncover the existence of a substance known as dark matter—a form of matter that neither emits light nor is easily detectable, yet plays a crucial role in the structure of the cosmos. It is an invisible part of space that possesses gravity, and scientists observe its gravitational effects on visible celestial bodies. Dark matter acts as the invisible glue that holds solar systems, galaxies, and galaxy clusters together. It binds stars, dust, and gas within a galaxy, making up the majority of its mass. In essence, it forms the foundation of the cosmic structure that defines our universe.
Accompanying this matter is an even more dominant component—dark energy—whose presence surpasses that of dark matter. Together, these two entities prevent ordinary matter from constituting more than 5% of the entire universe. In reality, about 95% of what exists remains unseen, lying beyond our senses. For this reason, we refer to it as "dark". This invisible and enigmatic realm serves as a veil, behind which hidden secrets await discovery. For years, scientists have meticulously strived to illuminate this cosmic mystery, seeking to unveil the unknown forces shaping our universe.
Exploring this darkness is a pursuit that has captivated many scientists, including Yahya Tayalati, whose passion for research has driven significant advancements in the field. Tayalati is one of the pioneers in theoretical physics, contributing to the unraveling of dark matter's mysteries through a unique approach—studying light-by-light scattering. His work has played a crucial role in shedding light on the hidden aspects of the universe. Before delving into the details of light-by-light scattering observations, it is essential to understand a fundamental concept: Objects become visible to us because they scatter light. The rough and uneven surfaces of objects scatter incident light in all directions, allowing the human eye to perceive them.
Which Particle Constitutes Dark Matter?
At the beginning of his career, Tayalati worked on the ATLAS experiment at the Large Hadron Collider (LHC) at CERN. His objective in the ATLAS project was to explore light-by-light scattering and search for axion-like particles (ALPs)—hypothetical fundamental particles that are strong candidates for dark matter. Today, scientists believe that axions or axion-like particles, which are lightweight and electrically neutral, could be the very components that make up dark matter due to their unique properties.
To better understand light-by-light scattering and axion-like particles (ALPs), it is useful to first review the Standard Model (SM) of particle physics. To account for all known matter in the universe, scientists predicted that all that there is in the world consists of several fundamental building blocks known as elementary particles, governed by four fundamental forces. Over time, extensive experiments confirmed the existence of these particles, and the Standard Model became an established theoretical framework describing them. In this model, if a particle is a constituent of matter, it is classified as a fundamental fermion—such as electrons, quarks, and neutrinos. If a particle serves as a mediator of interactions or forces between matter particles, it is categorized as a fundamental boson—such as photons, gluons, and others. Despite its success in explaining a vast range of experimental data, the Standard Model has a critical limitation: It does not account for dark matter. Furthermore, it fails to explain the formation of galaxies and the dark energy responsible for the accelerated expansion of the universe. This fundamental gap in the model has driven scientists to search for an additional, undiscovered particle—one that could finally explain the enigmatic nature of dark matter.
Before now, scientists have posited various hypotheses regarding the nature of dark matter; such as Weakly Interacting Massive Particles (WIMPs), Neutralinos, Sterile Neutrinos, and Neutrinos, among others. However, axions and axion-like particles (ALPs) emerged as the most promising candidates for this mysterious form of matter. Axions are theorized to have extremely low mass and no electric charge. Therefore, they cannot be readily observed through electromagnetic radiation, making them suitable candidates for dark matter particles. In other words, they do not interact with light, which makes them invisible despite their existence in the world and galaxies.
Searching for Axion-Like Particles (ALPs) at the LHC
Tayalati took his first steps at CERN using the Liquid Argon Calorimeter Prototype, an advanced detector designed to identify photons and electrons. One of his major achievements, along with the ATLAS collaboration, was the first-ever observation of light-by-light scattering at the LHC in 2019. Light-by-light scattering is an extremely rare phenomenon, making its measurement highly challenging and nearly impossible. For decades, the direct observation of high energy light-by-light scattering remained unsuccessful. However, at the Large Hadron Collider (LHC)—the world's most powerful particle collider —this elusive process was finally detected using lead ion collisions.
Heavy-ion collisions provide a uniquely clean environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as ‘ultra-peripheral collisions’. In this special type of interaction, the distance between the lead ions must be at least twice the radius of a single lead ion. During these photon-photon interactions, various new particles with different energy levels can be produced. Scientists hypothesized that under certain conditions, axion-like particles (ALPs) could emerge from these collisions. Experimental data provided strong statistical evidence supporting the existence of such particles, marking a significant step in the search for dark matter.
What Signals the Production of Axion-Like Particles?
In the ATLAS experiment, strong magnetic fields exist within the LHC. Axion-like particles (ALPs), produced in photon-photon interactions, can themselves decay into photons through interaction with these strong magnetic fields. This photon emission serves as the key signal indicating the presence of axion-like particles.
Through his work, Tayalati was able to extract a distinct and identifiable signal from the vast amount of data collected at the LHC—a signal that could potentially serve as evidence for dark matter. His research, along with the efforts of other scientists in the field, is not only advancing the search for dark matter but also addressing fundamental questions about the universe. The discovery of light-by-light scattering marked a major breakthrough, paving the way for new physics beyond the Standard Model. However, the journey is far from over—it remains long and filled with challenges. Are axion-like particles the long-sought missing pieces that constitute dark matter? Answering this question, along with many others, requires further investigations and experiments. The quest for understanding the dark universe continues.
"This material was originally published in the second issue of the International Observatory Journal."
