Novel collider signatures in the type-I 2HDM+$a$ model

The universe we can see—stars, planets, and the people on Earth—makes up only a tiny fraction of everything that exists. The rest is dark matter, an invisible substance that does not emit light but reveals itself through its gravity, holding galaxies together. For decades, physicists have searched for this hidden material, suspecting it might interact with the visible world through a "portal" of new particles. One leading idea involves a family of particles called Higgs bosons. While we have found one Higgs boson, theories suggest there could be a whole family of them, some of which might act as bridges between our world and the dark sector. If these extra particles exist, they could be produced in the high-energy collisions of the Large Hadron Collider, the massive particle accelerator at CERN, and then decay into invisible dark matter particles that fly away undetected.

A team of researchers has now taken a fresh look at how these extra Higgs particles might behave, challenging a long-held assumption about their masses. For years, experiments at the collider have searched for these particles under the belief that they would all be heavy and nearly identical in weight, a scenario that limits the types of signals scientists can look for. The researchers behind this new study propose a different possibility: that these extra particles could be much lighter, perhaps even lighter than the Higgs boson we already know, and that they could have very different masses from one another. By exploring this lighter, more varied landscape, they have uncovered a suite of new ways these particles could reveal themselves, offering a roadmap for future searches that could finally catch a glimpse of the dark universe.

The study focuses on a specific theoretical framework known as the type-I two-Higgs-doublet model with an additional pseudoscalar. In simpler terms, this is a model that adds new particles to the Standard Model of physics, which describes how matter and forces work. The researchers began by examining the rules that govern these particles, specifically looking at how they interact with ordinary matter like quarks and electrons. They found that if the model is of a certain type, called "type-I," the strict limits that usually force these new particles to be very heavy no longer apply. This opens the door to a much wider range of possibilities, where the new particles could be light enough to be produced in large numbers at the collider, yet remain hidden because they decay into dark matter.

To understand what this means for the search for dark matter, the team mapped out how these light particles would decay. They discovered that when the masses of the new particles are not all the same, the ways they break apart change dramatically. Instead of just disappearing into invisible dark matter, they could decay into other new particles that then produce visible signals, such as jets of bottom quarks, pairs of Z bosons, or even pairs of Higgs bosons, all accompanied by a missing energy signature. These missing energy signatures are crucial because they indicate that invisible particles have been created and carried away energy, a hallmark of dark matter production. The researchers identified several distinct patterns, or "signatures," that would appear in the detector data if these light, varied particles exist.

The team then simulated what these signals would look like using the data from the collider's previous runs. They tested four specific scenarios, or "benchmarks," to see if existing searches could find them. They found that current searches for a single Higgs boson accompanied by missing energy are sensitive to some of these new scenarios, particularly when the new particles have specific mass combinations. However, they also discovered that many other scenarios remain completely unexplored. For instance, signals involving pairs of bottom quarks and a Z boson, or pairs of bottom quarks with missing energy, have not been thoroughly searched for in the context of these light, non-identical particles. The researchers developed new analysis strategies to target these specific signals, showing that with the data already collected, the collider could rule out or discover these particles in regions of mass that were previously thought to be safe.

One of the most significant findings is that the assumption that these new particles must be heavy and identical is not a requirement of the theory, but rather a choice made for convenience in previous studies. By relaxing this assumption, the researchers showed that the extra Higgs bosons could be as light as 50 GeV, well below the mass of the known 125 GeV Higgs boson. This lightness allows for new decay paths that were previously forbidden, such as one heavy particle decaying into a lighter one and a dark matter carrier. These new paths create complex but distinct patterns in the data, such as a Z boson appearing alongside missing energy, or a pair of Higgs bosons appearing with missing energy. The study suggests that these patterns are not just theoretical curiosities but are within the reach of current and upcoming experiments.

The researchers also looked at the constraints imposed by other areas of physics, such as the precision measurements of how particles interact with the weak force. They found that while these measurements do prefer particles to have similar masses, they do not strictly forbid the mass differences they are proposing. The allowed differences are large enough to create the new decay patterns they are interested in. Furthermore, they checked the stability of the vacuum, ensuring that their proposed particle masses do not lead to a universe that would collapse. They confirmed that their scenarios are theoretically sound and consistent with all known laws of physics.

In their simulations, the team calculated how likely these signals are to be seen given the amount of data the collider has collected so far. They found that for some of the lightest scenarios, the current data is already powerful enough to exclude them, meaning that if these particles exist, they must be heavier or have different properties than those specific scenarios. For other scenarios, the data is not yet sufficient, but the researchers showed that the upcoming runs of the collider, which will collect significantly more data, will be able to probe these regions. They also noted that some signals, like those involving four leptons and missing energy, are harder to see but could become visible with the full dataset expected from the next phase of the collider.

The study concludes by emphasizing the importance of looking beyond the standard assumptions. By considering a wider variety of masses and decay patterns, the search for dark matter becomes much more comprehensive. The researchers recommend that experimental teams at the collider update their search strategies to include these new signatures. They suggest that looking for specific combinations of particles, such as bottom quarks and Z bosons, or pairs of Higgs bosons, could unlock the discovery of dark matter in ways that were previously overlooked. This work does not prove that these light particles exist, but it provides a clear and detailed guide on where to look and what to look for, turning a blind spot in the search into a promising new avenue for discovery.

Ultimately, this paper represents a shift in perspective. It moves away from the idea that new physics must be heavy and simple, suggesting instead that it could be light and complex. By mapping out the consequences of this complexity, the researchers have provided a new set of tools for experimentalists. Their work highlights that the dark sector might be hiding in plain sight, not in the heavy, rare events that have been the focus of the search for so long, but in the lighter, more frequent interactions that have been waiting to be recognized. As the collider continues to operate, these new strategies will be essential in the ongoing quest to understand the invisible half of our universe.