Dark matter is one of the greatest mysteries in the universe. It neither emits nor reflects light, but its existence can be deduced from the gravitational effects it exerts on visible matter: it influences the rotation of galaxies, deflects light and conditions the large-scale structure of the cosmos. It is estimated to account for about 85% of the total.
Although it has been tried to detect for decades, its particles remain unidentified. Most theories propose that they are formed by WIMPs (Weakly Interacting Massive Particles), which barely interact with normal matter. This elusive nature has led to exploring different strategies for analysing them: producing them in colliders, capturing them in underground detectors or looking for indirect signals in space.
The search has not yielded conclusive results, so new ideas and more sensitive technologies are needed. Models that predict particles like the Higgsino, as explored in a study co-led by researcher José Zurita, of the Institute of Corpuscular Physics (IFIC), located in the scientific-academic area of the Science Park of the University of Valencia (PCUV), offer promising ways to detect those subtle signals that could reveal the nature of dark matter.
One of the great challenges in the search for dark matter is that, if it really consists of particles like 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 being overlooked by current experiments.
Some theoretical models suggest 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 could be related to each other by their own forces, different from those of the visible world.
A common feature in these scenarios is that dark particles would have very similar masses to each other, complicating their detection. When one of them, slightly heavier, disintegrates into another more stable, possible dark matter candidate, it releases only a minimal amount of energy, often in the form of a faint trace or a light particle like a pion.
"The pure Higgsino has traditionally been regarded as 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 trace that suddenly disappears when it disintegrates into a neutral particle, the neutralino, and a pion of low energy, which is normally discarded", IFIC researcher José Zurita
Such a signal is so weak that, in current colliders like the CERN Large Hadron Collider (LHC), it tends to be lost among the "noise" of other interactions, since pions are often common secondary products in collisions of heavier particles. In this context, however, they may be one of the few visible traces that reveal the existence of the dark sector. Therefore, detecting low-energy pions, although complex, can be key to discovering new particles and advancing the search for dark matter.
The new work co-led by IFIC proposes an innovative approach: take advantage of the cleaner and more controlled conditions of a possible muon collider, an infrastructure still in the design phase, but which is gaining more and more support and enthusiasm within the international community. This type of collider would make it possible to record more precisely those faint traces that are now invisible.
"The pure Higgsino has traditionally been regarded as 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 trace that suddenly disappears when it disintegrates into a neutral particle, the neutralino, and a low-energy pion, which is normally discarded", explains Zurita.
This search for the Higgsino is of great interest because it could confirm a theoretical scenario capable of accounting for dark matter. In particular, the neutralino, a very stable particle with a very weak interaction with ordinary matter, could be one of the ingredients, if not the only ingredient, of the dark matter that envelops the universe
Thus, researchers have shown that it is possible to address the existence of the Higgsino by analyzing the products of its possible disintegration. In particular, one of the particles involved in the process, the chargino, would disintegrate into a neutralino plus a low-energy pion. It is these low-energy pions that leave "soft tracks" or "faint traces" that could be detected in muon colliders, more sensitive to such low-energy particles.
This search for the Higgsino is of great interest because it could confirm a theoretical scenario capable of accounting for dark matter. In particular, the neutralino, a very stable particle with a very weak interaction with ordinary matter, could be one of the ingredients, if not the only ingredient, of the dark matter covering the universe.
Although the proposal depends on the future development of high-sensitivity detectors, it opens a realistic way to explore regions of the subatomic universe that have remained outside the experimental range. It also strengthens the scientific case for muon colliders as a key tool in particle physics of the future.
The work not only opens a realistic way to detect dark matter, but also reinforces the scientific case for building new particle colliders, such as the proposed muon collider, an infrastructure that is gaining momentum among the priorities of the international community.
In recent years, the possibility of building a muon collider has gone from being a theoretical idea to becoming one of the most studied options for the future of high-energy physics. Its main attraction lies in the fact that muons, being about 200 times heavier than electrons, can reach very high energies without emitting as much radiation as these, which would make it possible to build more compact accelerators than electron and positron colliders, and cleaner than proton colliders like the LHC. This more controlled environment would be ideal for looking for subtle signals, such as those produced by dark matter.
Agencies such as the U.S. Particle Physics Project Prioritization Panel (P5) have recently recognized the strategic potential of this technology. In addition, the creation of the Muon Collider Collaboration, an international network of research centres and accelerator experts, marks the start of an ambitious programme to assess its technical and scientific feasibility.
"The implementation of the collider would be key not only to the search for dark matter, but to other current mysteries such as the origin of neutrino mass, or the precise measurement of the properties of the Higgs boson, the asymmetry between matter and antimatter..." , José Zurita, IFIC researcher
In this context, work such as that led by the IFIC is crucial, since it not only proposes concrete methods to exploit the unique capabilities of these colliders, but also provides clear and measurable objectives to guide their development. Its implementation would be key "not only for the search of dark matter, but for other current mysteries such as the origin of neutrino mass, or the precise measurement of the properties of the Higgs boson, the asymmetry between matter and antimatter..." , explains Zurita, who therefore calls for the leap to this new generation of colliders.
The work has been carried out by an international team combining experience in particle theory and phenomenology: José Zurita (IFIC), Federico Meloni (DESY, Germany) and Rodolfo Capdevilla (Fermilab, United States). The study, published in the journal Physical Review Letters, proposes the innovative method detailed here to identify extremely weak signals in certain theoretical models of dark matter. The proposal is the result of a line of research that explores signals difficult to detect, but with great potential to reveal the nature of dark matter.
Source: IFIC