MoEDAL-MAPP: the first dedicated search experiment at the LHC

The universe is built on a set of rules that scientists have spent a century deciphering, a framework known as the Standard Model. This model successfully explains how the smallest known particles interact, but it leaves some profound questions unanswered. For instance, it does not explain why electric charge always comes in specific, discrete amounts, nor does it account for the invisible substance known as dark matter that holds galaxies together. To find answers, physicists look for cracks in the current theory, searching for particles that should exist if nature is more complex than we think. Among the most intriguing possibilities are magnetic monopoles, particles that carry a single magnetic pole, much like how an electron carries a single electric charge. While every magnet we have ever seen has a north and a south pole, a monopole would be a particle with only one. Finding one would rewrite the laws of physics and explain why electric charge is quantized. Alongside these, scientists are also hunting for other strange, heavy particles that interact very weakly with normal matter, potentially revealing hidden sectors of the universe.

At the Large Hadron Collider in Switzerland, where protons smash together at nearly the speed of light, a specialized team is on the hunt for these elusive objects. This effort is led by an experiment called MoEDAL, which stands for Monopole and Exotics Detector at the LHC. Unlike the massive, complex detectors that record billions of electronic signals every second, MoEDAL is a unique, largely passive device. It does not rely on computers to trigger a recording or electronic sensors to capture a flash of light. Instead, it uses solid materials that act like a silent witness to the passage of rare, highly energetic particles. When a heavy, slow-moving particle passes through these materials, it leaves a permanent physical mark, a microscopic trail of damage that can be examined later under a microscope. The experiment has recently expanded its reach with a new component called MAPP, designed to catch particles that interact so weakly they might slip right past other detectors. A recent collection of research papers brings together the latest theoretical ideas and experimental results from this team, offering a comprehensive look at how they are searching for the unknown.

The core of this work involves understanding what these strange particles might be and how to find them. Theoretical physicists have spent years calculating whether magnetic monopoles could be stable enough to survive the journey from the collision point to the detector. If a monopole were to decay instantly, it would vanish before it could be seen, leaving only its decay products behind. However, if it is stable or long-lived, it would travel through the detector, leaving a distinct signature. Recent studies have examined the internal forces within these theoretical particles to see if they would hold together or fall apart. One analysis looked at the mechanical stresses inside a specific type of monopole model, finding that while it might have some internal rotational tendencies, it does not necessarily collapse. Another study used mathematical techniques to test if small disturbances would cause the particle to break apart, finding that the model appears stable against such perturbations. These findings do not prove that monopoles exist, but they confirm that the theoretical models describing them are robust enough to be taken seriously in experimental searches.

On the experimental side, the team is refining the tools used to catch these particles. The primary method involves sheets of a special plastic that record the path of a charged particle as a tiny, etched pit. However, the environment inside the collider is filled with radiation that can fog these sheets, making it hard to see the faint tracks left by the rare particles they seek. To solve this, researchers are testing new ways to read the data. One approach involves coating the plastic sheets with a thin layer of liquid and using advanced camera systems to scan for the pits, which helps clear away the visual noise caused by radiation. Another team is using thermal cameras to scan the sheets, looking for heat signatures that reveal defects in the material. These technological upgrades are crucial because they allow the experiment to distinguish between the background noise of the collider and the clear, sharp signal of a new particle. The researchers are also mapping the radiation environment in detail, using a network of silicon detectors to measure the flow of neutrons and other particles, ensuring they know exactly what kind of background they are working against.

The search has already begun to yield results, even in the absence of a discovery. By analyzing the data from collisions, the team has been able to set strict limits on the existence of magnetic monopoles and other heavy, charged particles. They have determined that if these particles exist, they must be heavier than a certain threshold, effectively ruling out the possibility of finding lighter versions in the current energy range. For example, they have excluded the existence of monopoles with certain magnetic charges up to a mass of several trillion electron volts. This is a significant achievement because it narrows down the search space, telling theorists where to look next. The experiment is also uniquely positioned to find particles that move slowly, a trait that other large detectors often miss because they are tuned to spot fast-moving debris. This ability to catch slow, heavy particles makes MoEDAL a vital complement to the other major detectors at the collider.

Looking beyond the immediate search for monopoles, the experiment is expanding its scope to find "millicharged" particles. These are hypothetical particles that carry a tiny fraction of the electric charge of an electron, making them incredibly difficult to detect. They are predicted by theories that suggest the existence of a "dark sector," a hidden world of particles that interact with our visible universe only through gravity or very weak forces. The new MAPP extension is designed specifically to catch these faint signals. By placing a detector in a shielded tunnel away from the main collision point, the team hopes to spot these particles as they drift away from the collision. Theoretical work suggests that if these particles exist, they could explain anomalies seen in other astronomical observations, such as the way hydrogen gas absorbs light in the early universe. The team has calculated that with the upcoming upgrades to the collider, their detector could reach masses up to 200 GeV, opening a new window into this hidden realm.

The research also explores other exotic possibilities, such as particles that carry an electric dipole moment but no net charge. These neutral particles could still interact with matter and leave a trace, though their signal would be much weaker than that of a charged particle. Calculations show that a heavy neutrino with such a property would lose energy as it passes through the detector, creating a faint ionization signal that the MAPP detector could potentially identify. Similarly, the team is investigating the possibility of finding heavy right-handed neutrinos, which are predicted by theories that try to explain why neutrinos have mass. By simulating how these particles would behave in the detector, the researchers have shown that their setup could be sensitive to very small mixing strengths between known and unknown particles.

Ultimately, this body of work represents a concerted effort to push the boundaries of what we know about the fundamental building blocks of nature. The MoEDAL-MAPP experiment stands as a unique sentinel at the edge of discovery, using simple, robust methods to hunt for the most complex and elusive particles. While no new particle has been confirmed yet, the experiment has successfully mapped the terrain, ruling out vast regions of possibility and refining the tools needed for the next generation of searches. The combination of theoretical insight and experimental innovation ensures that the search continues, driven by the conviction that the universe holds secrets that are just waiting to be revealed. As the collider reaches higher energies, the potential for discovery grows, and this dedicated team remains ready to catch the first glimpse of physics beyond our current understanding.