At the end of 2023, the US Particle Physics Projects Prioritization Panel (P5) published its recommendations to the Department of Energy. Among them was the support for the study of a novel high-energy frontier machine, the muon collider.

The panel recommended that the United States join the International Muon Collider Collaboration (IMCC) to perform common R&D and also prepares a potential bid to host the facility.

The IMCC is an international collaboration hosted at CERN that is developing the muon collider concept, which provides a unique, sustainable road to the high-energy frontier in particle physics. It has been created in 2021 as a result of the last strategy process for particle physics in Europe, where the concept has been recognised as important.

In the past, high-energy colliders have typically either collided electrons with positrons or protons with protons or anti-protons, also the combination of protons and electrons existed.

Electrons and positrons are point-like particles and make their full energy available for the production of new particles in the detector. In contrast, protons – as for example used in the LHC – are composite particles and only a fraction of their energy is available for the production of new particles. Therefore electron-positron colliders require less energy for the same discovery potential and can also provide more precise measurements. However, the energy of conventional lepton colliders is limited. If they are build as a circle, the electrons radiate x-rays when forced around the arcs, this effect increases strongly with energy. If they are built straight as in a linear collider, the beams can only collide once.

These limitations are solved by a collider that uses muons, which can be considered as heavy electrons, to produce the physics events in the detector. Muons have the same advantages as electrons and positrons in the collisions and therefore require much less energy than a proton collider.

In addition, they are about 200 times heavier than electrons, which implies that the radiation in an arc is reduced by more than a factor of one billion. This allows to accelerate and collide muons multiple times in rings. Unfortunately, muons have one drawback; they are instable particles and at rest decay within microseconds. A muon collider has therefore to rapidly produce and accelerate the muons, which increases their lifetime. An additional challenge arises from the production of muons by crushing protons into a target. The produced muons need to be captured and cooled to form a beam before they can be accelerated.

IMCC is addressing these challenges of the muon collider concept through an R&D programme that has been developed by the international community, including scientists from all regions of the world.

This R&D effort that is required to make the muon collider a reality is important. Several of the technologies are synergetic with those required for other particle physics projects, for example the detector development. Others are not generally used in particle physics facilities but are important on a much larger scale. A prominent example are solenoids, a special form of magnet, based on novel high-temperature superconductors. Their further development is critical for the muon production and cooling. The same technology is also important for fusion reactors on the one hand and the power generators of offshore windmills on the other hand.

Thankfully, CERN and many national laboratories and universities already support the muon collider study and also the European Union granted additional funds. The total budget remains small at this moment compared to that for the preparation of other future facilites. Nevertheless these funds combined with the enthusiasm and strong dedication of senior and even more importantly young scientists, engineers and technicians has enabled remarkable progress.

First informal discussions between representatives of the Department of Energy, the US ministry that is also in charge of high energy physics, and the muon collider collaboration took place and the collaboration will continue to work with the United States to develop an integrated programme.

This participation of the US and an anticipated ramp-up of European and Asian contributions will accelerate the programme. It should enable the collaboration to develop a staged approach to the collider to be implemented in any of the regions. The first stage is expected to be able to start providing physics by 2050.

By calling it "our muon shot", the US strategy compared the muon collider effort to the first mission to the moon. And indeed this is a novel concept and hard challenges have to be addressed and inovations be made. But facing these hardships will allow the young generation to develop and build their collider to continue the journey to understand the fundamentals of our universe.