In the realm of particle physics, Tova Holmes has been a driving force, even designing the very attire that signifies her dedication. Her journey began in 2022, collaborating with peers to advocate for a novel particle accelerator. Their message was clear, conveyed through T-shirts featuring a circular accelerator motif and the emphatic word: BUILD.
“We sought a way for individuals to visibly express their enthusiasm for a muon collider,” Holmes, based at the University of Tennessee, Knoxville, explained. This proposed collider, advocates suggest, could revitalize particle physics, a field yearning for fresh discoveries. Unlike the renowned Large Hadron Collider (LHC) at CERN, which, despite its capabilities, has not yielded groundbreaking findings in recent years, the new approach focuses on a different particle: the muon.
Physicists are eager to discover new particles, a pursuit hampered by current collider limitations. The proposed muon collider offers a new avenue to explore a hidden facet of reality and potentially locate these elusive entities.
While this concept long seemed ambitious, primarily due to the muon’s fleeting existence, recent technological advancements are making it increasingly viable. Funding organizations are also expressing serious interest, prompting a closer examination of the challenges and potential revelations associated with building this advanced muon machine.
The Higgs Boson and the Dawn of a New Era
The year 2012 marked a significant milestone when the LHC confirmed the existence of the Higgs boson. This particle, theorized nearly fifty years prior, was proposed to explain the initial separation of fundamental forces in the early universe. The Higgs boson arises from an excitation in the Higgs field, a mechanism responsible for imparting mass to certain particles, including the W and Z bosons that mediate the weak force, while leaving others, like the photon, massless.
This discovery served as a powerful validation of physicists’ theoretical models. However, it also presented a puzzle: the Higgs boson’s mass appeared unusually small. Quantum field theory predicted a far greater mass, yet it settled at a precise, seemingly stable level. This delicate balance raised questions about its configuration. Patrick Meade at Stony Brook University noted that the Higgs discovery, often seen as a culmination, was instead a perplexing beginning.
The Search for the Next Great Accelerator
If the Higgs discovery signaled a new chapter, the progress in experimental particle physics appears to have slowed. Addressing the profound questions stemming from the Higgs boson necessitates a new generation of instruments capable of deeper exploration through more powerful particle collisions.
One direct approach involves constructing an enhanced version of the LHC. This is the principle behind the Future Circular Collider project at CERN, which envisions a proton supercollider with a ring significantly larger than the LHC’s. Such a machine could achieve higher collision energies, enabling the discovery of particles or phenomena that manifest only at elevated energy levels and revealing more fundamental structures of matter.
However, protons are not elementary particles; they are composite. When protons collide, their constituent quarks and gluons interact, generating complex sprays of secondary particles that demand extensive analysis. Furthermore, scaling up the LHC’s design would incur substantial financial costs.
Exploring Alternatives: Electron-Positron and Muon Colliders
An alternative approach involves electron-positron colliders, such as the proposed Compact Linear Collider from CERN. Electrons and positrons, being fundamental point-like particles with opposite charges, produce cleaner, more interpretable collision data. A challenge arises from the energy loss due to radiation when these particles are accelerated in a circular path at high energies. Linear colliders address this by accelerating particles along a straight line, but they lack the particle recycling capability of circular designs.
Emerging as a compelling candidate is the muon collider. Muons are analogous to electrons but are approximately 200 times more massive, retaining their negative charge. While not constituents of ordinary matter, they are transiently produced when high-energy cosmic rays interact with atmospheric molecules.
Their greater mass results in less energy loss through radiation when bent in a collider ring, permitting higher energies within a more compact tunnel. As fundamental particles, their collisions, like those of electrons, offer cleaner outcomes. Design studies from the US Muon Collider Collaboration suggest that a muon collider could surpass the current energy frontier of 13.6 teraelectronvolts (TeV) by a factor of four, within a ring comparable in size to the LHC.
The ‘Impossible’ Particle and Cosmic Enigmas
Neutrinos, consistently challenging to explain, may now offer clues to the universe’s most perplexing mysteries, particularly after the detection of one possessing unexpected energy.
Overcoming the Muon’s Challenges
The concept of a muon collider is not new, with initial proposals dating back to the 1960s. The primary hurdle was the production of muons. Unlike protons or electrons, muons cannot be readily extracted from atoms; they are generated by smashing protons into targets, creating secondary particles that then decay into muons. This process yields a dispersed spray of particles with varying energies and trajectories, making the creation of a focused beam a significant technical challenge.
An additional complication is the muon’s instability. At rest, muons decay within 2.2 microseconds. This is an extraordinarily short period compared to the approximately 20 minutes required to accelerate protons to full speed in the LHC’s main ring.
Consequently, a muon collider operates as a race against time. Physicists must capture a chaotic collection of newly created particles, compress them, and accelerate them before they decay. As Patrick Meade describes it, the process transforms a diffuse beam the size of a beach ball into one as thin as a human hair, executed with extreme speed and precision. The challenge intensifies when steering these compressed beams for a direct high-energy collision.
For decades, this demand for speed and precision relegated muon colliders to the periphery. They were considered unfeasible even during the 2013 Snowmass process, a decadal review of future priorities in US particle physics.
Holmes, then early in her master’s studies, witnessed this assessment. However, over the subsequent decade, technological breakthroughs began to elevate the muon collider’s status as a potential cornerstone of future discovery.
Reviving the Muon Collider Concept
Technological advancements have played a pivotal role. Earlier designs for muon colliders proposed energies considerably lower than current projections. Recent plans target the 30 TeV range, a hundredfold increase from the 1960s proposals. At these energies, muons travel near light speed, where Einstein’s theory of special relativity extends their apparent lifespan. The faster they move, the longer they seem to exist from an external perspective.
This relativistic effect is substantial. In a 10-TeV muon collider, muons could persist for up to a tenth of a second, approximately 45,000 times their natural lifespan. This temporal extension, paradoxically gained by increasing speed, provides crucial additional microseconds for beam manipulation.
Researchers have capitalized on this borrowed time. In 2020, the Muon Ionization Cooling Experiment, led by Kenneth Long at Imperial College London, demonstrated “ionization cooling.” This technique involved passing muons through materials like liquid hydrogen or lithium hydride, which reduced their momentum in all directions. Subsequently, electric fields accelerated them forward, compressing the diffuse spray into a focused, high-velocity bunch.
Jesse Thaler from the Massachusetts Institute of Technology, who was initially skeptical, remarked, “It sounds completely crazy because the back of the envelope just tells you that it’s not possible. But actually, going beyond the back of the envelope, with more scientific study, it starts to look more and more plausible.”
Furthermore, physicists have accumulated practical experience in handling muons. Since 2017, Fermilab’s Muon g-2 experiment has measured subtle deviations in muon behavior within a magnetic field, values predicted with high precision by theorists. While early results hinted at a divergence from the Standard Model, suggesting new physics, more refined calculations subsequently aligned the observations. Nevertheless, the experiment provided invaluable expertise in producing, storing, and controlling muons on a large scale.
By 2022, the muon collider had emerged as a leading contender for the field’s next major research instrument. In Europe, CERN’s International Muon Collider Collaboration (IMCC) initiated parallel studies. In the US, many physicists favor building a future muon collider at Fermilab, while their European counterparts are exploring CERN as a potential host site.
Steinar Stapnes of the University of Oslo, an IMCC member, noted, “The muon collider is quite an old concept. Now, everybody thinks it is very interesting — scientifically and technically.”
The current landscape is open to interpretation. Each collider proposal must undergo technical evaluations and pilot demonstrations before governments commit billions in funding. In parallel, proponents will vigorously advocate for their respective machines to define the future of particle physics.
“A machine like this would be around the middle of the century,” Holmes stated, contingent on securing substantial funding. Sergo Jindariani, head of the US Muon Collider Collaboration, leading early feasibility studies, commented, “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.”
A Window into the Higgs Boson
Should a muon collider be constructed, its primary objective would be to investigate the Higgs boson with unprecedented detail. Despite its discovery over a decade ago, the Higgs boson remains enigmatic. Jindariani explained, “In the standard model, there are over a dozen particles, but none of them has properties like the Higgs. It’s very unique.”
Physicists hypothesize that the Higgs field played a crucial role in shaping the early universe. As the cosmos cooled after the Big Bang, the field transitioned, separating the unified electroweak force into the distinct electromagnetic and weak forces observed today. The intensity of this transition could shed light on a profound cosmic mystery: the vanishing of antimatter while matter persisted.
Even currently, the Higgs field may not be entirely stable. Some theoretical calculations suggest that our universe might exist in a precarious state, with the Higgs field not at its lowest energy level. A quantum fluctuation could potentially trigger a transition to a deeper energy state, a phenomenon known as vacuum decay. Such an event would instantaneously alter all aspects of our universe.
“All fundamental particles that have mass would get heavier, and presumably completely reorder our elements and cause total chaos,” Holmes warned. Meade echoed this sentiment, likening the scenario to a universal switch, where the absence of the field means non-existence.
Physicists already suspect anomalies. Quantum theory predicts that interactions with massive particles should significantly increase the Higgs boson’s mass. Instead, it measures a relatively modest 125 gigaelectronvolts. Achieving this consistency requires an extraordinary level of fine-tuning.
Various theoretical frameworks have been proposed to resolve this discrepancy. One posits the existence of multiple Higgs bosons, suggesting that if every known particle, including the Higgs itself, possesses a heavier counterpart, it could counterbalance the effects that theoretically inflate the Higgs mass. Another theory proposes that the Higgs is not fundamental but composite, formed from smaller constituents analogous to how protons are composed of quarks.
The Universe’s Precarious Existence
The possibility of a cataclysmic quantum fluctuation arising at any moment threatens existence. The fact that humanity persists suggests the presence of underlying cosmic realities.
Each of these theoretical possibilities would leave detectable experimental signatures. A muon collider could identify these by measuring the Higgs boson’s interactions with other particles and itself at high energies, according to Holmes. This capability offers an advantage over dedicated “Higgs factories,” often electron-positron colliders designed to produce large quantities of Higgs bosons, but at lower energies than a muon collider could achieve.
The Path Forward: Demonstrators and Future Prospects
Before a full-scale muon collider can be realized, researchers must demonstrate the practical efficacy of its core technologies. The immediate next step involves a demonstrator facility to verify the ability to prepare and control muon beams sufficiently for collision. The IMCC is developing plans for such a facility at CERN, while the US Muon Collider Collaboration, in conjunction with the IMCC, is exploring a similar demonstrator at Fermilab. The objective is to finalize detailed technical designs by approximately 2030. If authorized and funded, a demonstrator could commence operation in the early 2030s, providing the essential proof of principle for a full-scale collider.
Scientists like Holmes are committed to this long-term endeavor, believing the muon collider will ultimately be selected as the world’s next major scientific undertaking. The growing prevalence of muon collider T-shirts, as Holmes observes, indicates an increasing consensus among physicists: “I’m delighted to see how often I show up at another department and see them already there.”