Dark matter is one of the greatest mysteries of the universe. It neither emits nor reflects light, but its existence is inferred from the gravitational effects it exerts on visible matter: it influences the rotation of galaxies, bends light, and shapes the large-scale structure of the cosmos. It is estimated to account for about 85% of the total matter.

Although efforts to detect it have been underway for decades, its particles remain unidentified. Most theories propose that dark matter is composed of WIMPs (Weakly Interacting Massive Particles), which interact very weakly with normal matter. This elusive nature has led to exploring various strategies to study them: producing them in colliders, capturing them in underground detectors, or searching for indirect signals in space.

The search has yet to yield conclusive results, so new ideas and more sensitive technologies are needed. Models predicting particles like the Higgsino—such as the one explored in a study co-led by researcher José Zurita from the Instituto de Física Corpuscular (IFIC), a joint center of the CSIC and the Universitat de València—offer promising avenues to detect these subtle signals that could reveal the nature of dark matter.

Major challenges and solutions

One of the major challenges in searching for dark matter is that, if it is indeed made up of particles like the Higgsino, predicted by some theoretical extensions of the Standard Model of particle physics, its signals in existing detectors would be extremely faint, to the point of going unnoticed in current experiments.

Some theoretical models propose the existence of several particles within the so-called “dark sector,” a kind of parallel universe whose components interact very weakly with known matter. These particles might be related to each other by their own forces, distinct from those of the visible world.

A common feature in these scenarios is that dark particles would have very similar masses, which complicates their detection. When one of them, slightly heavier, decays into another more stable particle—possibly a dark matter candidate—it releases only a minimal amount of energy, often in the form of a faint track or a light particle such as a pion.

Such a signal is so weak that, in current colliders like the Large Hadron Collider (LHC) at CERN, it is usually lost among the “noise” of other interactions, since pions are common secondary products in collisions of heavier particles. However, in this context, they could be one of the few visible footprints revealing the existence of the dark sector. Therefore, detecting low-energy pions, although complex, could be key to discovering new particles and advancing the search for dark matter.

The new work, co-led by IFIC, proposes an innovative approach: taking advantage of the cleaner and more controlled conditions of a potential muon collider, an infrastructure still in the design phase but gaining increasing support and enthusiasm within the international community. This type of collider would allow more precise recording of those faint tracks that are currently invisible.

“The pure Higgsino has traditionally been considered a target for future accelerators, such as the Future Circular Collider (FCC-hh) or a muon collider. In both cases, this particle is expected to manifest as a charged track that suddenly disappears when it decays into a neutral particle, the neutralino, and a low-energy pion, which is normally discarded,” explains Zurita.

Thus, the researchers have demonstrated that it is possible to probe the existence of the Higgsino by analyzing the products of its possible decay. In particular, one of the particles involved in the process, the chargino, would decay into a neutralino plus a low-energy pion. These low-energy pions leave “soft tracks” that could be detected in muon colliders, which are more sensitive to particles of such low energy.

This search for the Higgsino is of great interest because, as mentioned, it could confirm a theoretical scenario capable of accounting for dark matter. In particular, the neutralino—a very stable particle with very weak interaction with ordinary matter—could be one of the components, if not the only one, of the dark matter that fills the universe.

Although the proposal depends on the future development of highly sensitive detectors, it opens a realistic pathway to explore regions of the subatomic universe that have remained out of experimental reach. Additionally, it strengthens the scientific case for muon colliders as a key tool in future particle physics.

The future of new colliders

The work not only opens a realistic avenue to detect dark matter but also reinforces the scientific rationale for building new particle colliders, such as the proposed muon collider, an infrastructure gaining momentum among the priorities of the international community.

In recent years, the possibility of building a muon collider has evolved from a theoretical idea to one of the most studied options for the future of high-energy physics. Its main appeal lies in the fact that muons, being about 200 times heavier than electrons, can reach very high energies without emitting as much radiation, allowing the construction of more compact accelerators than electron-positron colliders, and cleaner than proton colliders like the LHC. This more controlled environment would be ideal for searching for subtle signals, such as those produced by dark matter.

Organizations such as the U.S. Particle Physics Project Prioritization Panel (P5) have recently recognized the strategic potential of this technology. Moreover, the creation of the Muon Collider Collaboration, an international network of research centers and accelerator experts, marks the start of an ambitious program to evaluate its technical and scientific feasibility.

In this context, works like the one led by IFIC are fundamental, as they not only propose concrete methods to leverage the unique capabilities of these colliders but also provide clear and measurable objectives to guide their development. Its implementation would be key “not only for the search for dark matter but also for other current mysteries such as the origin of neutrino mass, or precise measurements of the Higgs boson properties, the matter-antimatter asymmetry…,” explains Zurita, who advocates for taking the leap into this new generation of colliders.

The work was carried out by an international team combining expertise in particle theory and phenomenology: José Zurita (IFIC, CSIC–Universitat de València), Federico Meloni (DESY, Germany), and Rodolfo Capdevilla (Fermilab, USA). The study, published in the journal Physical Review Letters, proposes the innovative method detailed here to identify extremely faint signals in certain theoretical models of dark matter. The proposal stems from a research line that explores difficult-to-detect signals but with great potential to reveal the nature of dark matter.

References:

R. Capdevilla, F. Meloni, and J. Zurita. Discovering Electroweak Interacting Dark Matter at Muon Colliders using Soft Tracks. Phys. Rev. Lett. DOI: 10.1103/PhysRevLett.134.181802

R. Capdevilla, F. Meloni, R. Simoniello, and J. Zurita. Hunting wino and Higgsino dark matter at the muon collider with disappearing tracks. JHEP 06. DOI: 10.1007/JHEP06(2021)133