Colour sextet baryons in composite Higgs models

The universe as we understand it is built on a foundation of particles and forces, a framework known as the Standard Model. For decades, this model has successfully predicted how matter behaves, yet it leaves a profound mystery unsolved: why does the Higgs boson, the particle responsible for giving other particles their mass, have the specific weight it does? Theoretical physicists suspect the answer lies in a deeper layer of reality, where the Higgs is not a fundamental building block but a composite object, much like a proton is made of smaller quarks. In these "composite Higgs" theories, new, heavy particles exist at energy scales far beyond what we have currently seen. Among these hidden inhabitants are exotic forms of matter that carry a unique type of charge called "color," which binds them together in ways that differ from the familiar particles we know.

A recent study by Manuel Kunkel at the University of Würzburg explores a specific and surprising prediction within this family of theories: the existence of "sextet" baryons. In the standard world, particles that feel the strong nuclear force usually come in groups of three, known as triplets. However, these new models suggest that under certain conditions, nature could also allow for groups of six. These sextet baryons would be heavy, unstable particles that decay rapidly into other known particles. The research focuses on how these exotic six-particle groups would behave if they were created in the high-energy collisions of the Large Hadron Collider, the massive machine buried beneath the border of France and Switzerland. By mapping out their likely decay paths and comparing them against data already collected, the study determines how heavy these particles could be before they would have already been spotted by current detectors.

The theoretical landscape Kunkel investigates suggests that these sextet baryons are not solitary wanderers but part of a larger, interconnected family of particles. They are formed from even more fundamental constituents called hyperquarks, which are bound together by a new, powerful force. When these sextets are created, they do not simply vanish; they break apart in specific, predictable ways. The most likely scenario involves the sextet decaying into a heavy particle and a new type of scalar particle, which then immediately splits into top quarks. The top quark is the heaviest known fundamental particle, and because the sextet is so massive, its decay would produce a shower of these heavy tops. In some cases, a single collision creating a pair of sextets could result in a final state containing as many as six top quarks, a signature that is incredibly rare and difficult to produce by any other means.

However, the story changes if the mass of the sextet is arranged differently relative to other particles in the theory. In an alternative scenario, the sextet might decay into bottom quarks and a stable, invisible particle that carries no electric charge and does not interact with light. This invisible particle would escape the detector unseen, leaving behind a signature of missing energy alongside the visible bottom quarks. The researchers carefully analyzed both of these possibilities, calculating how often these events would occur and what they would look like to the sensors of the Large Hadron Collider. They used computer simulations to model the collisions, generating millions of potential events to see which ones would survive the strict filters used by experimentalists to find new physics.

To test these ideas, the team took the simulated events and ran them through the same analysis tools used by the ATLAS and CMS experiments, the two giant detectors at the collider. They looked for the specific patterns of top quarks or bottom quarks that their models predicted. By comparing their simulated signals against the actual data collected from over 139 inverse femtobarns of collisions, they were able to set strict limits on the existence of these particles. The results were clear: if these sextet baryons exist, they must be heavier than 2.64 tera-electronvolts. This is a mass roughly 2,600 times that of a proton, pushing the boundaries of what the current machine can probe. The study also projected what would happen if the collider were to run at even higher intensities in the future, suggesting that with more data, the search could extend to masses approaching 3 tera-electronvolts.

The findings serve as a crucial guide for the ongoing search for new physics. By defining exactly what the detectors should be looking for and establishing a lower bound on the mass of these exotic particles, the work helps experimentalists refine their search strategies. It confirms that while these sextet baryons have not yet been found, they are not ruled out entirely; they simply hide at energies higher than previously thought. The research also highlights the importance of looking for unusual combinations of particles, such as events with multiple top quarks or significant amounts of missing energy, which might be the only way to reveal the presence of these heavy, composite states. As the Large Hadron Collider continues to operate and collect data, the window for discovering these exotic six-particle groups remains open, waiting for the next collision to reveal a glimpse of this deeper, composite reality.