Every minute, countless muons rain down from space, passing through everything—including us—before penetrating deep below the Earth’s surface. The discovery of these particles, created when cosmic rays hit our planet’s atmosphere, was one of the biggest surprises in the history of particle physics.
In the mid-1930s, two sets of physicists independently found evidence of muons in tracks left behind by cosmic rays. At the time, only a handful of particles—such as electrons, protons and photons—were known. The muon, a heavier and shorter-lived cousin of the electron, was an unexpected new member of the subatomic family. Upon first hearing evidence that muons might exist, the physicist and Nobel laureate Isidor Isaac Rabi famously quipped, “Who ordered that?”
“This was an era where physicists thought they had the parts needed to explain what the world was made of,” says David Kaiser, a physicist and historian of science at the Massachusetts Institute of Technology. “It took a long time for experimentalists and theorists to agree that there really was a need for a new particle.”
The discovery of the muon led to the realization that the universe likely contained more unknown particles, many which physicists discovered in the decades that followed. With these, scientists eventually formulated the Standard Model, the framework modern particle physicists use to describe the building blocks of the universe. This model has stood the test of time, providing the best mathematical explanation to date of how the universe works at the subatomic level.
“Muons are one of the particles that proves that research should not be hyperspecialized. There are many opportunities that have been open by muons—both in looking for new physics, but also in applications."
But it has some gaps. It fails to explain certain phenomena, such as the antimatter-matter asymmetry in our universe and the presence of dark matter. Over the years, physicists have carefully studied muons’ properties, hunting for signs of physics beyond the Standard Model.
Muons have proven beneficial in other domains, such as volcanology and material science. And many scientists also believe that muons may even play an important new role in the future of particle physics.
From ancient artefacts to volcanoes
Particle physics involves building gigantic experiments. In the process, scientists often come up with clever ideas for new instruments, some of which end up being useful for other applications. One such example is in medical imaging: Hospitals around the world house particle accelerators for both diagnosis and treatment.
Many researchers working in this area have been involved in R&D for particle detectors, says Andrea Giammanco, a particle physicist at the Université Catholique de Louvain. When Giammanco began exploring how to apply imaging with muons outside the realm of basic particle physics research, he says, “the many potential applications I had never thought about blew my mind.”
The ability of muons to penetrate deeply into materials makes them useful for reconstructing the insides of large structures. In 2023, scientists announced the discovery of a hidden corridor within the 4,500-year-old Great Pyramid of Giza. They had probed the structure using measurements of muons in the cosmic ray particles that passed through it.
Known as muography, this type of muon imaging has been used to examine the interior of other structures, such as an ancient Chinese wall and nuclear reactors. The MUon RAdiography of VESuvius (MURAVES) project aims to use muography to visualize the inner workings of Mount Vesuvius, the active volcano in Italy that famously destroyed and buried the city of Pompeii.
In recent years, Giammanco and others have also explored the possibility of using muon imaging to study ancient artefacts or to scan cargo for illegal goods. These applications use a method known as muon scattering tomography, which takes advantage of the fact that muons are sensitive to electromagnetism: Because they carry a charge, when they encounter the nucleus of an atom, they are kicked away by its electric field. By analyzing the overall distribution of deflections from many muon interactions, scientists can infer what materials an item contains.
“Muons are one of the particles that proves that research should not be hyperspecialized,” says Cristina Carloganu, a senior researcher at the National Institute of Nuclear and Particle Physics in France. “There are many opportunities that have been open by muons—both in looking for new physics, but also in applications.”
The search for new physics
Many physicists also see muons as essential to the future of particle accelerators.
Particle accelerators provide invaluable insights into the inner workings of the universe: By examining the aftermath of particle collisions, scientists have been able to uncover new particles and gain valuable insights into their properties.
But existing accelerators have limitations that make it difficult—and perhaps impossible—to address some of particle physicists’ most pressing questions. Accelerators that collide electrons and positrons, both elementary particles that cannot be broken down into smaller components, can directly convert all the energy from a collision into a new set of particles with little background noise. However, due to their small masses, these particles are extremely difficult to accelerate to very high energies.
Physicists have overcome this obstacle by building machines that smash beams of more massive hadrons or charged atoms instead. But due to their composite nature, only some of their constituents collide, making only a fraction of their energy available to produce new particles. The datasets they generate are also messier, and new physics may get lost in the noise.
Muons offer the best of both worlds: They are both fundamental particles and much more massive than electrons—meaning a muon collider could produce clean collisions at very high energies from which physicists may be able pinpoint previously unseen particles. “The nature of muons makes them perfect candidates for creating particle collisions that go beyond what’s achievable at the LHC,” says Sergo Jindariani, a senior scientist at the US Department of Energy’s Fermi National Accelerator Laboratory. “A muon collider really seems like the right tool for understanding the questions that are facing particle physics now.”
Using such an instrument, scientists hope to tackle some of the biggest mysteries in particles physics: Why does the Higgs field, which gives particles their mass, persist in a vacuum? Do certain dark matter candidates, such as weakly interacting massive particles, exist? What did the Standard Model look like during the early days of our universe?
Physicists around the world are drawing up designs for a future muon collider. But they have their work cut out for them: Unlike electrons or protons, which may live forever, muons decay after a mere 2.2 microseconds. This means that scientists need to produce, accelerate and collide muons in an extremely short period of time.
A key challenge is to develop and demonstrate the technology to “cool” muon beams. When bunches of particles are circulated through an accelerator, physicists must ensure that these bunches are very small in order to maximize the number of interesting collisions. Reducing the space occupied by particle bunches is referred to as “cooling”. Scientists have theorized several possible methods to achieve this goal, says Nathaniel Craig, a particle theorist at the University of California, Santa Barbara. “Now, it’s a practical challenge of, can we do that fast enough?”
“To me, what’s really important about the muon is that it’s really the emblem of this idea that we go to higher energies not necessarily because we’re guaranteed to find something, but because nature is filled with all sorts of things that we didn’t have any reason to expect,” Craig says.
“The muon represents the idea that discovery, more often than not, proceeds through exploration rather than following some absolutely clear theoretical line to some destination.”
