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Illustration of a Higgs Factory "Linear Collider" "Circular Collider" "Muon Collider" "Gamma Collider"
Illustration by Sandbox Studio, Chicago

A bouquet of options: Higgs factory ideas bloom

Now that a Higgs-like boson has been discovered at the Large Hadron Collider, proposals to build colliders that churn out the new particle are gathering momentum.

If you hurl two oranges together at close to the speed of light, there’s going to be a lot of pulp. But, somewhere in the gooey mess will be the rare splinters left over from two seeds colliding.

The Large Hadron Collider at CERN works in a similar way. Protons, each made of quarks and gluons, collide and produce other particles. Roughly once every 5 billion proton collisions, everything aligns and a Higgs-like boson pops out.

It’s a messy and inexact process. But that messiness gives scientists a way to observe a broad range of physics. Because all that pulp and the occasional seed collide in a different way almost every time, a huge array of particles can appear.

“It’s a broadband machine,” says Lyn Evans, the former director of the LHC. “Wherever the Higgs is, it will pop out, out of the noise.”

Now that a boson with Higgs-like qualities has been found, physicists are calling for something more precise: a Higgs factory that would collide elementary particles to produce Higgs bosons in droves without all the distracting pulp. By colliding particles that don’t break down into composite parts as they produce Higgs-like particles, a Higgs factory could allow a more precise view of the new boson.

The concept of a Higgs factory originated far before LHC experiments first spied the Higgs-like particle. As early as the mid-1990s, scientists were floating ideas for small, relatively simple machines to aid dedicated study of the hypothesized boson. In the world of high-energy physics, where colliders can cost billions of dollars and require vast international collaborations, it’s the norm to begin planning decades in advance. The research and development simply takes that long.

Now that the Higgs-like particle is known to have a mass of about 125 billion electronvolts, scientists know that it is within reach of a variety of proposed colliders, both small and large. As a result, proposals for Higgs factories have emerged for colliders that smash electrons with positrons, muons with muons, or photons with photons.

When opposites collide

Electron–positron colliders make use of the electron—a negatively charged particle roughly 2000 times less massive than a proton—and their antiparticle, the positron, which carries a positive charge. When electrons and positrons collide at high energies, they are capable of producing W and Z bosons, which in turn can emit a Higgs.

Linear electron–positron colliders are among the largest and most expensive Higgs factories because they are designed to be versatile. Two proposed machines, known as the International Linear Collider and the Compact Linear Collider, would be 3.4 miles and 1.35 miles long respectively. It would cost at least $5 billion to build the ILC or CLIC, though some estimates predict much higher than that. One or the other would only operate as a Higgs factory temporarily; after researchers have a good handle on the Higgs, the machines could be extended to be 18.5 and 31 miles long respectively and upgraded to higher energies for other studies. 

“One thing that is good about a linear collider is it can evolve. You can extend it. You can incrementally increase the energy,” Evans says. 

ILC and CLIC were recently combined under one organization, which Evans leads. The group will continue to develop both designs for now.

The ILC could reach energies up to 1 trillion electronvolts, while CLIC could go as high as 3 trillion electronvolts. CLIC would be more powerful, but the ILC is further along in development. 

“ILC is rather mature technology,” Evans says. “If we were given approval to build an ILC using superconducting technology, we could build it today. CLIC is quite novel technology that is very CERN-specific. We need to do a substantial amount of research and development to prove it.”

There is some momentum to build the ILC in Japan with support from international partners; scientists from the high-energy physics community in Japan announced at a conference in Krakow, Poland, last September that they would like to build the ILC. But first they have to secure government approval and funding. If all goes according to plan, an ILC located in Japan could produce a Higgs as early as 2025.

“Within one year from now, a much stronger recommendation... will appear from the bigger body which combines key persons of almost all Japanese social communities,” says KEK director Atsuto Suzuki.

New life for the LHC tunnel

Electron–positron colliders can also be circular. The LHC tunnel was originally built for the Large Electron–Positron collider, which produced the first precise measurements of the W and Z bosons in the 1980s.

One proposal, called LEP3, would build a Higgs factory in the LHC tunnel, most likely after the LHC shuts down. It would cut costs by using existing infrastructure, such as some of the particle detectors and the cryogenics system. 

CERN theoretical physicist John Ellis, one of the authors of the LEP3 proposal, says it is one of the most developed options for a Higgs factory, as it is based on familiar technology.

“No one has ever built a high-energy linear electron–positron collider,” Ellis says. “I think the LEP3 technology is more sure and could give you more collisions than any other collider.” 

Light particles such as electrons lose a lot of energy as they make turns around a ring, so a LEP3 built in the LHC tunnel could not study particles much heavier than the Higgs. The current plan calls for a machine designed to collide particles at up to 240 billion electronvolts—the sweet spot to produce a Higgs.

Part of LEP3’s fate depends on what discoveries the LHC makes. If it continues to find particles, the LHC’s lifetime could be extended into the 2030s. LEP3 and the LHC could not operate at the same time due to interference, so researchers would need to alternate running the two machines or wait until the LHC is shut down. Alternatively, LEP3 could be built at a different location. 

Another electron–positron collider proposal, TLEP, calls for a new tunnel with a 50-mile circumference. It would reach energies around 400 billion electronvolts, and the tunnel could later house a powerful LHC-type proton collider.

“TLEP is my favorite project,” says LEP3 co-author and CERN physicist Frank Zimmermann. “I think it’s still cheaper than the linear collider. We can study the Higgs production with five times higher luminosity and do so at several collision points. It offers a long-term potential, since the tunnel could later be used for a higher energy proton–proton collider, approaching 100 trillion electronvolt center-of-mass energy.”

The unruly muon

Farther on the horizon is the concept of a circular muon–muon collider. Similar to the electron, the muon is an elementary particle with a negative charge. More than 200 times heavier than an electron, muons have an advantage: They lose much less energy while accelerating around a circular ring. 

“That allows you to build a high-energy circular collider,” says US Muon Accelerator Program Director Mark Palmer. “The muon collider is particularly interesting if you think about going to much higher energies, energies up at the TeV and multi-TeV scale. That’s really the thing that drove the interest in muon accelerators.”

But muons are also much harder to produce than electrons. First, protons are accelerated into a liquid metal target, triggering a torrent of pions, which subsequently decay into muons. Then the muons have to be arranged into a neat beam, which is accelerated again. As if that’s not enough, muons also have a finite lifetime. If put into a ring for storage, they can circle perhaps 1000 times before decaying into other particles. 

Despite the obstacles, muons are appealing because they would enable unmatched precision in measurements of a particle like the Higgs. The muons could be collided at energies between 126 billion and 6 trillion electronvolts in rings with a circumference between 1000 feet and 3.75 miles. This means a muon collider could be the smallest and most flexible option for a Higgs factory.

Like ILC and CLIC, a muon collider could be multipurpose. If the high-energy version is built, it could be tuned to study later LHC discoveries. Muons are also closely tied to neutrino research, so a muon accelerator complex could also support the operation of a high-precision neutrino factory. Neutrinos are poorly understood particles that are far more prevalent in the universe than electrons, protons and neutrons. They resemble an electron, but are roughly a million times lighter and carry no electric charge. Neutrinos rarely interact, so producing huge quantities is the only way to study them.

Palmer says the Muon Accelerator Program is pushing to have an understanding of the technology necessary to build a muon collider within six years. The cost can be estimated after the design and technology issues are understood.

“The major goal is to try to bring the critical threads of that research and development together and push aggressively to a conclusion,” Palmer says. “My mandate is to change the perception that this may not converge within the lifetime of current researchers.” 

Producing Higgs from light

Scientists have also proposed a machine that collides gamma rays, a high-energy form of light. Gamma rays are appealing because they can convert directly into Higgs bosons.

“If you shine lasers onto electron beams, you can convert a large fraction of them into energetic photons, and you can make the photons collide,” says Ellis, who co-authored a proposal for a gamma–gamma collider called SAPPHiRE.

This type of gamma–gamma collider would accelerate electrons in a circle before hitting them with a laser beam to create the high-energy photons required for collisions. The proposed SAPPHiRE collider would be about 5.5 miles in circumference and would accelerate particles to 80 billion electronvolts—making it the lowest-energy Higgs factory. It is also expected to be less expensive to build, although a significant amount of research and development would need to be completed.

Photon–photon colliders of this type have been considered as add-ons to electron–electron colliders for about 30 years. In 2001, Ellis and his colleagues proposed another gamma–gamma collider, called the CLIC Higgs experiment, which would adapt CLIC to collide gamma rays at an energy much lower than the 3 trillion electronvolts planned for CLIC.

“Maybe [SAPPHiRE] could be a cheap way toward a Higgs factory,” Ellis says. “It of course could be built at CERN, but, as we wrote in our paper, it could be built elsewhere. It could be built at Fermilab. It could be built in Japan.”

LHC not through

It will be more than a decade before any Higgs factory produces its first Higgs. In the meantime, physicists plan to exploit the LHC to its maximum potential. CMS physicist and Higgs group co-convener Albert de Roeck says the LHC has already performed extremely well finding the new boson.

“We do already know much more than what people were projecting,” de Roeck says. “They would not have thought we would have a discovery by now.”

The LHC will go into a long shutdown next year for upgrades and essential repairs. When it turns back on in 2015 at energies above 13 trillion electronvolts, physicists will focus on pinning down the details of the Higgs-like boson, including its exact mass and spin. They also expect to measure precisely how the boson interacts with other particles. The results will reveal if the particle matches the Higgs boson predicted by the Standard Model Higgs or if it’s something more exotic.

At the very least, the LHC will point to where a potential Higgs factory should look.

“My first advice would be to maximize the machine we have right now. This is our best hope to get a fast answer to Higgs questions,” de Roeck says. “That, plus maybe other things the LHC can exclude or find, should drive us to what we really want next.”

In the meantime, Ellis calls up a quote from the infamous Chairman Mao Tse Tung: “Let a hundred flowers bloom.”

“Whatever the LHC has just discovered, whether it’s the Higgs or not, it opens up a whole new era in particle physics,” Ellis says. “What one should do is take a little pause for thought and see what possibilities are out there and then make a considered decision.”