LH$\mu$C Concept: Energy-Frontier Antimuon-Proton Collisions at CERN

To understand the universe, scientists have long relied on smashing particles together at incredible speeds. When protons, the heavy building blocks of atoms, collide, they reveal the forces that hold matter together. But to truly see how these forces work, physicists often need to fire a lighter, more precise probe at the proton. For decades, they used electrons for this job. However, electrons are so light that they lose a tremendous amount of energy when they are forced to turn a corner in a circular machine, making it impossible to reach the highest energies needed to explore the deepest secrets of the atom. Muons are cousins of electrons, but they are about two hundred times heavier. Because of this extra weight, they do not lose energy as easily when turning, allowing them to be accelerated to much higher speeds in a ring. This paper explores a bold new idea: using a beam of antimuons, the mirror-image twins of muons, to crash into protons at the world's most powerful particle accelerator, the Large Hadron Collider at CERN. The goal is to create a machine that can peer into the structure of matter with an energy level far beyond what is currently possible, potentially solving mysteries about why the universe has mass and how the strong nuclear force behaves.

The researchers, Arda Goroglu and Saleh Sultansoy, present a detailed concept for a facility they call the LHµC. This machine would not be built from scratch but would be a clever addition to the existing Large Hadron Collider infrastructure. The core of the proposal involves generating a beam of antimuons and accelerating them to an energy of one trillion electron volts, or one tera-electron volt. These high-speed antimuons would then be steered into the path of the seven-tera-electron-volt protons already circulating in the main ring. When these two beams collide, they would create a center-of-mass energy of 5.3 tera-electron volts. This is a massive leap forward; it is nearly five times more energetic than the previous best electron-proton collider and significantly higher than any other proposed lepton-hadron machine for the coming decade. The team calculated that this setup could achieve a collision rate, or luminosity, of roughly 8.8 times 10 to the 33 collisions per square centimeter per second, a figure that would allow scientists to gather vast amounts of data in a single year.

The path to creating this beam relies on a specific technology involving "ultra-cold" antimuons. Unlike the difficult process of cooling standard muon beams, which requires complex and unproven methods, this approach uses a source that has already been developed for other experiments. The process begins by firing a powerful beam of protons at a target to create pions, which quickly decay into muons. These muons are then stopped in a special target where they are converted into a gas of ultra-cold antimuons. From there, a series of accelerators, including a booster ring and a main storage ring, would speed them up to their final velocity. The paper suggests that this booster could be installed in a tunnel that was originally designed for a different project, the LHeC, allowing for a staged construction where the facility could first be used for electron collisions and later upgraded for antimuon collisions. This reuse of existing space and the reliance on established ultra-cold muon technology make the project appear more feasible than other proposals that require waiting for entirely new cooling breakthroughs.

Once the machine is running, the physics potential is immense. The primary goal is to act as a powerful microscope for Quantum Chromodynamics, the theory that describes how quarks and gluons bind together to form protons and neutrons. Current experiments have only scratched the surface of how these particles behave at extremely high energies and very small distances. This new collider would allow scientists to probe regions of the proton that have never been seen before, potentially revealing how the strong force creates the mass of visible matter, which accounts for about 99 percent of the mass in the universe. Beyond standard physics, the machine offers a unique chance to search for new particles. The authors simulate how the collider would look for "color-octet muons," hypothetical heavy partners to the muon that would indicate the muon is not a fundamental particle but is made of smaller parts. Their calculations suggest that this machine could discover such particles at masses up to 4.4 tera-electron volts, a range that is significantly higher than what the current Large Hadron Collider can reach, and it could do so with far less data.

The paper also addresses the significant engineering hurdles that remain. The most critical challenge is the rapid acceleration of the antimuon beam. Because muons are unstable and decay into other particles in a fraction of a second, the entire acceleration process must happen incredibly fast, within about one millisecond, before the beam disappears. The team proposes using a special type of accelerator that does not require magnets to change their strength, keeping the beam intact as it speeds up. Another challenge is the radiation produced by the decaying muons, which creates a shower of neutrinos that could damage the surrounding equipment. The design includes a heavy tungsten nozzle to shield the detector and limit the acceptance of the beam, ensuring that only the most useful collisions are recorded. While the authors acknowledge these difficulties, they argue that the technology is advancing rapidly and that the scientific payoff justifies the effort.

In conclusion, this paper outlines a realistic and ambitious roadmap for the next generation of particle physics. By combining the high-energy proton beams of the Large Hadron Collider with a newly developed beam of ultra-cold antimuons, the LHµC concept promises to open a window into the fundamental structure of matter that has remained closed for decades. It is not a guaranteed success, as it depends on demonstrating the ability to produce enough antimuons and accelerating them fast enough, but the simulations show that if these technical steps are taken, the result would be a machine capable of exploring energies and phenomena that are currently out of reach. This facility would not only help explain why the universe has mass but also provide the precise data needed to interpret the results of future, even larger, particle colliders, securing its place as a vital tool for understanding the laws of nature.