Constructing $SU(N)$ strongly coupled dark sectors by extending the Standard Model: mixed representations, flavor structure, global symmetries and various portals

The universe is filled with things we can see and touch, from the stars above to the atoms that make up our own bodies. Yet, astronomers and physicists know that the visible world is only a small fraction of reality. The rest is hidden in a vast, invisible realm known as the dark sector. This shadowy domain is thought to contain dark matter, the mysterious substance that holds galaxies together, but its nature remains one of the greatest puzzles in modern science. While we know dark matter exists because of its gravitational pull, we have no idea what particles it is made of or how those particles interact with one another. Some scientists suspect that dark matter might not be a single, lonely particle, but rather a complex community of particles that interact through their own forces, much like the protons and neutrons in our own atoms are held together by the strong nuclear force. This idea of a "dark QCD" sector suggests a hidden world with its own rules, its own chemistry, and perhaps even its own version of the periodic table.

In a recent study, a researcher at the Institute for Basic Science in Korea has proposed a bold new way to build this hidden world. Instead of inventing a completely separate set of rules for the dark sector, the study suggests that the dark world might be a direct extension of the Standard Model, the well-tested theory that describes all known particles and forces. The author constructs a theoretical framework where the dark sector emerges naturally from the same mathematical structures that govern the visible universe. By extending the Standard Model gauge group to an enlarged SU(N + 5) × SU(N + 1) × U(1), the study shows how a new, strongly interacting dark sector can arise, complete with its own versions of quarks, its own generations of particles, and its own unique conservation laws. This approach does not just add a few random particles to the mix; it weaves the dark sector into the very fabric of the Standard Model, suggesting that the dark and visible worlds are deeply connected siblings rather than strangers.

The core of this construction involves extending the mathematical groups that describe how particles interact. In the visible world, particles are organized into families and carry specific charges that dictate how they respond to forces. The researcher takes the existing families of particles and adds new dimensions to their mathematical description. This expansion automatically generates new particles that are invisible to our current detectors because they do not carry the electric or color charges we are familiar with. These new particles, which the study calls dark quarks, come in three distinct representations: fundamental, antisymmetric, and symmetric, mirroring the variety found in the visible world. They appear in three generations, just like the electrons and quarks that make up ordinary matter, and they possess a rich internal structure that includes both familiar and exotic forms. This setup ensures that the dark sector is not a chaotic collection of random particles but a structured, organized system with its own flavor and symmetry.

One of the most striking features of this model is how it handles the stability of dark matter. In our world, protons are stable because of a rule called baryon number conservation, which prevents them from decaying into lighter particles. The study identifies a similar rule for the dark sector, a "dark baryon number" that keeps the lightest dark particles from disappearing. However, the model goes further by introducing a second, more exotic rule. The researcher identifies an exotic dark baryon number, denoted as eD, which arises from a conserved (B − L) − eD symmetry. This new symmetry allows for the existence of "chimera" particles, which are made of a mix of different types of dark quarks. These exotic particles are stable not because of the standard dark baryon number, but because of this new, hidden symmetry. This discovery suggests that the dark sector could contain a diverse population of stable particles, some of which might be the dark matter we are searching for, while others could play a role in the early history of the universe.

The study also maps out how this hidden world might talk to our visible world. Even though the dark sector is invisible, it must have some way of interacting with ordinary matter, or else we would never be able to detect it. The researcher identifies several "portals," or bridges, that connect the two realms. These bridges are formed by heavy particles that act as messengers, carrying forces between the dark quarks and the particles we know, such as electrons and quarks. The study categorizes these portals into specific types, including the quark portal, lepton portal, Higgs and Z portal, heavy Z′ portal, kinetic mixing portal, and dipole portal. Each portal offers a different way for the dark sector to reveal itself, whether through the decay of the Higgs boson, the scattering of dark matter off atomic nuclei, or the production of new particles in high-energy collisions at facilities like the Large Hadron Collider.

What makes this approach particularly powerful is that it does not treat these connections as arbitrary additions. Instead, the strength and structure of these portals are dictated by the same mathematical rules that determine the masses of the particles we already know. Because the dark sector is built as an extension of the Standard Model, the researcher can predict how strongly the dark particles should interact with visible matter. For instance, the model suggests that the interaction between dark quarks and ordinary quarks might be strongest for heavy particles like the top quark, while interactions with lighter particles might be much weaker. This provides a clear guide for experimentalists, telling them exactly where to look for signs of the dark sector. It also implies that the dark sector is not just a theoretical curiosity but a testable hypothesis with specific, measurable predictions.

The study further explores the consequences of these interactions for the early universe. If the dark sector was created in the same way as ordinary matter, it might have inherited a similar imbalance between matter and antimatter. The model suggests that the decay of heavy right-handed neutrinos, particles introduced to explain why neutrinos have mass, could have generated an asymmetry in the dark sector just as it did in the visible one. This process, known as leptogenesis, could have produced a surplus of dark matter particles over anti-dark matter particles, explaining why the universe is filled with dark matter today. The existence of the exotic dark baryon number plays a crucial role here, ensuring that this asymmetry is preserved and that the dark matter we see today is a relic of these ancient processes.

While the model is built on a foundation of mathematical consistency, the researcher acknowledges that the details of the dark sector's low-energy behavior depend on how the theory breaks down at different energy scales. The study considers various scenarios for how the heavy particles might acquire their masses and how the different types of dark quarks might behave as the universe cools. In some scenarios, the dark sector might be dominated by light particles that form bound states similar to the pions in our own world. In others, the heavy mediators might be so massive that they are difficult to detect directly. The paper systematically analyzes these possibilities, showing how different assumptions about the mass of the mediators would change the experimental signatures. This flexibility allows the model to be tested against a wide range of current and future experiments, from direct detection experiments that look for dark matter hitting atoms to collider experiments that try to create dark particles from scratch.

The implications of this work extend beyond just finding dark matter. By showing that a complex, strongly coupled dark sector can arise naturally from an extension of the Standard Model, the study provides a new perspective on the structure of the universe. It suggests that the dark sector is not a random collection of particles but a highly organized system with its own rich history and dynamics. The identification of the exotic dark baryon number and the various portals connecting the two worlds offers a roadmap for future research. It gives experimentalists a clear set of targets to aim for and theorists a framework to build upon. The study does not claim to have solved the mystery of dark matter, but it offers a compelling and concrete path forward, turning a vague idea into a detailed, testable theory.

Ultimately, this research represents a significant step in our understanding of the hidden universe. It moves the conversation from "what if" to "what if this specific structure is true," providing a concrete framework that can be scrutinized and tested. By grounding the dark sector in the same principles that govern the visible world, the study bridges the gap between the known and the unknown. It invites us to imagine a universe where the dark sector is not just a shadowy backdrop but a vibrant, complex world with its own laws and history, waiting to be discovered through the careful application of the tools of particle physics. The work stands as a testament to the power of theoretical physics to illuminate the darkest corners of our reality, offering hope that the secrets of the dark sector are within our reach.