The detector system of the SHiP/NA67 experiment at CERN

In the vast landscape of modern physics, scientists have long known that the standard model of particle physics, while incredibly successful, is incomplete. It cannot explain why the universe is made of matter rather than antimatter, nor does it account for the invisible substance known as dark matter. For decades, the primary strategy to find new physics has been to build machines that smash particles together at higher and higher energies, hoping to create heavy, unknown particles. However, there is another possibility that has remained elusive: particles that are light but interact so weakly with ordinary matter that they slip right through our detectors. These are known as feebly interacting particles. Because they are so difficult to catch, finding them requires a different approach. Instead of looking for rare, high-energy collisions, scientists must generate an enormous number of particles and wait for the rare ones to decay into something visible. This shift from searching for the heavy to hunting the faint is the core of the intensity frontier, a new direction in physics that relies on volume and precision rather than brute force.

At the CERN research center in Switzerland, a team of physicists is preparing to lead this charge with an experiment called SHiP, also known as NA67. The goal is to build a specialized facility capable of producing a massive stream of protons and using them to create a hidden sector of particles that might explain the universe's deepest mysteries. The team has recently detailed the design of the detector system that will make this search possible. This system is not a single machine but a complex assembly of different instruments, each engineered to solve a specific problem: how to create a clean environment where a faint signal can be seen against a backdrop of overwhelming noise. The experiment is scheduled to begin its first physics runs in 2032, and the design presented in the paper represents the culmination of years of planning to ensure that when the beam turns on, the detectors are ready to capture the first signs of new physics.

The experiment begins with a beam of protons traveling at high speed, which is fired into a target made of pure tungsten. This target is designed to be as efficient as possible, creating a shower of new particles when the protons hit it. To handle the intense heat generated by this process, the target is cooled by high-pressure helium gas circulating through slots in the metal blocks. Immediately behind the target sits a powerful magnetic shield, a chain of magnets that acts like a giant filter. Its job is to sweep away the vast majority of muons, which are heavy cousins of the electron, that are produced in the collision. Without this shield, these muons would flood the rest of the experiment, creating a background so loud that any new signal would be drowned out. The shield reduces the number of muons by a factor of a million, leaving a much quieter path for the particles the scientists are actually interested in.

Once the muons are filtered out, the remaining particles enter a long, empty tunnel filled with helium gas. This is the decay volume, a fifty-meter-long space where the feebly interacting particles, if they exist, are expected to travel and eventually decay into ordinary matter that can be detected. Because the particles interact so weakly, they can travel this entire distance without hitting anything. The helium inside is kept in a thin polymer balloon to minimize any chance of the particles bumping into gas atoms before they decay. Surrounding this entire tunnel is a sophisticated wall of sensors known as the background tagger. This wall is filled with a special liquid that glows when a charged particle passes through it. Its purpose is to catch any stray particles that might sneak in from the sides or interact with the walls of the tunnel itself. If a signal appears in the center of the tunnel at the exact same time as a particle hits the wall, the system knows it is just background noise and ignores it. This system is so precise that it can distinguish between a real event and a fake one with a timing accuracy of one billionth of a second.

At the end of the tunnel, the experiment looks for the debris of any decays that occurred inside. This is where the hidden sector spectrometer comes in. It is a large tracking system made of thousands of thin, straw-like tubes filled with gas. As charged particles fly through these straws, they leave a trail of electrical signals that allow scientists to reconstruct the path of the particle with extreme precision. A large magnet bends these paths, which helps scientists determine the momentum and identity of the particles. Following the tracker is a timing detector made of scintillating bars, which acts as a stopwatch to ensure that the particles being observed actually came from the decay volume and not from some other source. Finally, the particles enter a calorimeter, a device that measures their energy and identifies what kind of particle they are. This final section is crucial for distinguishing between different types of decays, such as those that produce light particles versus those that produce heavier ones.

The entire system is designed to operate without a traditional trigger, meaning it records data continuously rather than waiting for a specific signal to start recording. This is necessary because the experiment runs for long periods, and the events of interest are so rare that a standard trigger might miss them. Instead, the system timestamps every single piece of data, allowing scientists to reconstruct events later by looking at the time windows where interesting patterns appear. The design also includes a separate detector inside the muon shield to study neutrinos, a type of particle that is notoriously difficult to detect. This part of the experiment aims to observe thousands of neutrino interactions, providing a wealth of data on how these particles behave.

The paper outlines that the design for the target and the muon shield is essentially complete, with prototypes having been tested to ensure they can withstand the harsh conditions of the beam. The background taggers and the spectrometer are currently in the prototyping phase, with tests underway to confirm their performance. The team is confident that by the time the facility is commissioned in 2033, the detectors will be ready to search for these elusive particles with world-leading sensitivity. If the feebly interacting particles exist within the range of masses and couplings the experiment is designed to probe, the SHiP detector system should be able to find them. If they do not, the experiment will set the most stringent limits yet on their existence, effectively ruling out a large portion of the theoretical possibilities for new physics. The work represents a careful balance of engineering and physics, creating a machine that is quiet enough to hear the faintest whisper of the universe's hidden secrets.