A once-fantastical collider could answer physics’ biggest mysteries

In the realm of particle physics, Tova Holmes is a notable figure, having even designed a T-shirt inspired by her work. Her journey began in 2022 when she and her colleagues attended a particle physics meeting, advocating for the creation of a novel particle collider.

To promote their idea, they wore shirts featuring a circular accelerator design with the word “BUILD” to show their enthusiasm for a muon collider. Holmes, from the University of Tennessee, Knoxville, explains their intent was to visually express excitement for this project.

Proponents believe this new collider could rejuvenate particle physics, as the existing Large Hadron Collider (LHC) at CERN, Geneva, has not yielded groundbreaking discoveries in recent years. They argue for a shift from merely upgrading the LHC to creating a novel collider that would smash muons together.

Despite skepticism, primarily due to the fleeting nature of muons, technological advancements are making the idea more plausible, attracting interest from funding bodies. This raises the question of what it would take to construct this muon collider and what mysteries it might unravel.

The LHC confirmed the Higgs boson’s existence in 2012, a significant discovery that supported long-standing theories about the universe’s fundamental forces. The Higgs boson, arising from the Higgs field, imparts mass to certain particles, leaving others unaffected.

The discovery was a triumph yet unsettling, as the Higgs boson’s mass appeared oddly small, contradicting quantum field theory that predicted a much larger mass. This discrepancy leaves physicists like Patrick Meade from Stony Brook University pondering why this balance exists, suggesting it marks a new beginning for particle physics.

The next big discovery machine

However, experimental particle physics seems to have reached a standstill. Addressing questions raised by the Higgs boson requires a new machine capable of deeper exploration through advanced particle collisions.

One approach is to build a larger LHC version, like the proposed Future Circular Collider at CERN. This supercollider would be three to four times the LHC’s circumference and could smash protons with significantly more energy, enabling the discovery of new phenomena at higher energy levels.

Yet, protons, composed of quarks and gluons, create complex collision outcomes that require extensive analysis. Additionally, expanding the LHC would entail considerable costs.

Alternatively, there are electron-positron colliders like the Compact Linear Collider from CERN. These colliders produce cleaner collision results, but require a linear design to avoid energy loss from radiation, preventing particle reuse as in circular designs.

Another intriguing possibility is the muon collider. Muons, heavier than electrons but similarly charged, are not visible in normal matter but form briefly when cosmic rays hit Earth’s atmosphere. They lose less energy when directed around a collider ring, reaching higher energies without needing a large tunnel. Muons, as fundamental particles, produce cleaner collision data. A muon collider could surpass the current 13.6 TeV energy limit, according to design studies by the US Muon Collider Collaboration.

The concept of a muon collider dates back to the 1960s, yet its feasibility was questioned due to the difficulty in producing and stabilizing muons. Muons are generated by colliding protons with a target, creating pions that decay into muons, resulting in a chaotic spread of particles with different energies and trajectories. Converting this chaos into a focused beam is a significant challenge.

Muons are also unstable, decaying in 2.2 microseconds, making it difficult to accelerate them to high speeds before they decay, unlike protons which take about 20 minutes to reach full speed in the LHC.

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At some point, we need a new approach, and colliding muons may be that
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A muon collider is a race against time, requiring rapid capture and acceleration of particles before they decay. “You’re starting with a beam of muons that’s like the size of a beach ball, and you want to turn it into something the thickness of a human hair,” says Meade. This must be done swiftly to facilitate direct collision, generating high-energy Higgs bosons.

For years, this demanding combination of speed and precision sidelined the concept. However, the idea gained traction in the 2013 Snowmass process, although deemed infeasible at that time. Holmes was then pursuing her master’s degree, and over the next decade, technological advances have positioned the muon collider as a serious candidate for future discovery in her field.

Reviving the muon collider

Technological progress has significantly advanced the muon collider concept. Early designs envisioned modest collision energies, but recent plans aim for up to 30 TeV, far exceeding initial 1960s proposals. At these energies, muons travel near light speed, benefiting from Einstein’s theory of special relativity, appearing to live longer to an external observer.

Even a 10-TeV muon collider could extend muon lifetimes to a tenth of a second, 45,000 times longer than normal, providing crucial time for beam control.

Researchers have learned to exploit this extended time. In 2020, the Muon Ionization Cooling Experiment, led by Kenneth Long at Imperial College London, demonstrated “ionisation cooling,” where muons pass through materials like liquid hydrogen, reducing momentum in all directions before acceleration, transforming a diffuse spray into a compact, high-speed bunch.

Jesse Thaler of MIT, initially skeptical, now finds the muon collider concept increasingly feasible with detailed studies. Over time, researchers have also gained practical experience handling muons. The Muon g-2 experiment at Fermilab, starting in 2017, precisely measured muon behavior, providing insights into controlling muons on a large scale.

By 2022, at the subsequent Snowmass meeting, Holmes and her colleagues, wearing their T-shirts, saw the muon collider as a leading future project. Europe’s CERN-backed International Muon Collider Collaboration (IMCC) has initiated parallel studies, with US physicists favoring Fermilab for a potential site, while European counterparts explore CERN as a host.

“The muon collider is quite an old concept,” says Steinar Stapnes at the University of Oslo, part of the IMCC. “Now, everybody thinks it is very interesting — scientifically and technically.”

It’s an open field, with each collider proposal needing technical studies and pilot demonstrations before governments decide on funding. Meanwhile, advocates will vie for their machine to shape the next era of particle physics.

“A machine like this would be around the middle of the century,” says Holmes. “That’s if we get given a whole lot of funding.”

Sergo Jindariani, leading the US Muon Collider Collaboration’s feasibility studies, believes a new approach is needed: “We’ve been doing things the same way for many decades. At some point, we need a new approach, and colliding muons may be that.”

Window into the Higgs

If constructed, the muon collider aims to delve deeper into the Higgs boson than any previous machine. Despite its discovery over a decade ago, the Higgs remains an enigma. “In the standard model, there are over a dozen particles, but none of them has properties like the Higgs. It’s very unique,” says Jindariani.

Physicists believe the Higgs field influenced the early universe, activating during a transition that separated the unified electroweak force into today’s electromagnetic and weak forces. The nature of this transition might explain why matter exists while antimatter doesn’t.

Today, the Higgs field might not be entirely stable. Some theories suggest our universe is precariously balanced, with the Higgs field not at its lowest energy. A quantum fluctuation could potentially trigger a shift to a deeper energy state, causing instant changes across the universe.

“All fundamental particles that have mass would get heavier, and presumably completely reorder our elements and cause total chaos,” says Holmes.

“Essentially, it’s like somebody turning the lights on or off in the universe. If they’re off, none of us exists. If they’re on, we can live,” says Meade.

The suspicion of an issue persists among physicists. Quantum theory suggests interactions with heavy particles should increase the Higgs boson’s mass. However, it remains at a manageable 125 gigaelectronvolts, requiring precise adjustments to align with theoretical predictions.

Physicists have long proposed solutions to this discrepancy. One idea is multiple Higgs bosons, where each particle in the standard model, including the Higgs, has a heavier counterpart, offsetting expected mass increases. Another theory is the Higgs as a composite particle, composed of bound smaller elements akin to protons.

These options could leave detectable signs that a muon collider might uncover by examining the Higgs’s interactions with other particles and itself at high energies, according to Holmes. This capability gives the muon collider an edge over Higgs factories—electron-positron colliders designed to produce numerous Higgs bosons but at lower energies than a muon collider could achieve.

Before building a full-scale muon collider, researchers need to validate its key technologies. The next step involves a demonstrator facility to test muon beam preparation and control. The IMCC is planning such a machine at CERN, while the US Muon Collider Collaboration, in collaboration with the IMCC, is considering a similar project at Fermilab. Detailed technical designs are expected by around 2030, with a demonstrator potentially operational in the early 2030s if governments approve and fund it, providing the necessary proof of concept for a full collider.

Holmes and her team are committed to the long-term vision of the muon collider as a groundbreaking project. She is encouraged by the growing support from the physics community, as evidenced by the increasing appearances of muon collider T-shirts in various departments.

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