Physics Beyond the Higgs Boson

On the outskirts of Geneva, Switzerland, some 300 feet underground, is the biggest, most complicated machine ever built. With about 1,200 giant superconducting magnets, multiple-ton detectors, a worldwide computing grid and a staff of thousands, the Large Hadron Collider is an international undertaking beyond the means of any single nation. The project’s mission: to see what happens when protons collide at 99.99999 percent of the speed of light in a 17-mile-long circular tunnel.

The LHC started operating in 2008, yet for some physicists it only recently began to live up to its name. “I called it the Little Hadron Collider,” says Maria Spiropulu, an experimental physicist at the California Institute of Technology, “because during its first run, it only operated at half-energy.” That changed in April when the accelerator fired up again after a two-year shutdown dedicated to nearly doubling the energy of its proton beams. Now physicists are anticipating a new era of discoveries from a machine that will dominate experimental particle physics for at least another two decades.

Even before its upgrade, the LHC managed to shake up the physics world. On July 4, 2012, seven months before the machine’s hiatus, physicists announced they had discovered the particle that the $4.9 billion accelerator had been hunting: the Higgs boson. The particle’s existence was first predicted 50 years ago by several physicists working independently, including Peter Higgs at the University of Edinburgh, as a solution to what had been one of the most vexing mysteries in physics: How do particles acquire mass?

For those of us who aren’t physicists, the question barely seems worth asking. Isn’t mass simply a given, a basic feature of matter? No, it turns out. Some properties of particles — the negative electric charge on an electron or the positive charge on a proton, for example — are intrinsic to the particle itself. But not mass.

Mass varies depending on how fast particles move; protons zipping along at close to the speed of light, like those in the LHC, weigh more than protons at rest. And all the fundamental particles in nature have a seemingly random assortment of masses. Why are protons 2,000 times heavier than electrons, yet their charges are simply equal and opposite? Why do photons — particles of light — have no mass at all?

Higgs and company proposed a solution. The universe, they said, is filled with an invisible field, now called the Higgs field, that interacts with all particles via one very special particle: the Higgs boson. Particles that we perceive as being heavy interact strongly with the Higgs field; lighter particles interact more weakly. The discovery of the Higgs boson netted a Nobel Prize in 2013 for Higgs, who is now 86, and François Englert, 82, a physicist at the Free University of Brussels.

With the discovery of the Higgs boson, physicists found the last missing piece of the Standard Model, an overarching theory that describes the universe in terms of a handful of particles and four fundamental forces. “Pretty much everything is explained by the Standard Model,” says Hitoshi Murayama, a theoretical physicist at the University of California, Berkeley. “It’s an incredible success, and we physicists can boast about it.”

Despite their pride in the theory, Murayama and other physicists can hardly wait to punch it full of holes — their field advances only by tearing down the old and replacing it with something new and even more all-encompassing. The LHC itself was conceived as a Standard Model destroyer.

So far, the Standard Model remains practically impregnable. Its predictions continue to match experiments with uncanny precision. Physicists have never found any solid experimental data that contradict the Standard Model, and that’s a problem because they know it can’t be the final word on the nature of the physical universe. “We see some hints that there must be something beyond it,” says Murayama, “but we don’t know exactly what to make of these hints yet.”

One of the most obvious hints of physics beyond the Standard Model is the overwhelming evidence for the existence of dark matter in the cosmos. Unlike all other forms of matter, it doesn’t interact with light, and astronomers don’t know what it’s made of. Dark matter betrays its presence mainly by the gravitational influence it exerts on the motions of galaxies.

Observations indicate that more than 80 percent of all the matter in the universe consists of this unknown stuff. The light-emitting objects that have preoccupied astronomers for ages — all the countless stars and galaxies — are apparently exceptions to the rule of cosmic invisibility.

Many physicists are betting that dark matter particles will turn up in the debris created by the upgraded LHC’s more energetic proton collisions, and searching for them will be one of the project’s prime tasks in the years ahead. But since no one really knows what dark matter is, success is far from certain even from a machine as powerful as the LHC — which, in effect, re-creates the energetic environment of the first thousandth of a billionth of a second of the universe’s existence.

“There’s no guarantee,” says John Ellis, a theoretical physicist at King’s College London. “We just have to crank up the energy as high as we can and make as many collisions as we can and see what we find.”

Perhaps even more significant than what the LHC found during its first five years of operation was what it didn’t find: evidence for physics beyond the Standard Model. Physicists had what they thought would be an ideal extension of the Standard Model, a theory honed over more than 40 years, ready and waiting to be confirmed at the LHC.

Supersymmetry — or SUSY, as physicists call it — predicts that every particle known to physics has a heavier, “super” partner: For every electron, there’s a selectron; for every quark, a squark. SUSY effectively doubles the number of particles in the universe. And although the Higgs field explains howparticles acquire mass, SUSY explains why protons, electrons and other conventional particles have such disparate masses. It also provides a solution to the problem of dark matter in the form of a particle called the neutralino, which, if it exists, would be 100 times heavier than a proton and would hardly interact with normal matter at all — a billion or so could be passing through our bodies every second.

It’s an elegant hypothesis, but for now it is only that. For a generation of physicists who have invested entire careers working on supersymmetry, the first low-energy run of the LHC was a huge disappointment. None of the expected superparticles turned up in the accelerator. There’s no evidence that supersymmetry exists anywhere other than in the minds of theorists.

At least not yet. “It’s reasonable to say that we’re worried,” says Joseph Lykken, a theoretical physicist and deputy director of Fermi National Accelerator Laboratory in Illinois. “But it’s certainly not the case that SUSY is dead. It’s a good example of how the game is supposed to work. If we could figure it all out on the blackboard, we wouldn’t need to build a $5 billion accelerator.”

The challenge for SUSY advocates — a camp that includes nearly all theoretical particle physicists looking beyond the Standard Model — is that the most mathematically straightforward versions of the theory predicted that the LHC should already have found a few of the superparticles in the same collisions that produced the Higgs. Since it didn’t, theorists have fallen back on more complicated, less “natural” versions of supersymmetry. Hard data now constrain the theorists’ imaginations. “We’re straitjacketed,” says Murayama.

One such straitjacket is the mass of the Higgs boson detected at the LHC. Physicists typically describe a particle’s mass in units of energy, using Einstein’s famous equation, e = mc2, which defines the equivalence of mass and energy. The Higgs has a mass energy of 126 billion electron volts. On a human scale, that’s a tiny number — less than a billionth of a billionth of a gram. But on a microscopic scale, it’s huge — heavier than some entire molecules. Unfortunately for the long-term fate of the universe, however, it’s not quite heavy enough.

When theorists plug that 126 billion electron volt mass (along with another known mass, that of the heaviest observed particle, called the top quark) into the equations that describe the Higgs field, they get a disturbing result: a barely stable field. These equations would indicate that, at some point in the very distant future, a quantum fluctuation could spontaneously “burp” a high-energy bubble into existence that would spread across the entire universe at the speed of light, destroying everything in its path. Simple hydrogen atoms would be the only form of matter left in that bleak cosmos.

“You can do a calculation of how long [the cosmic burp] will take to happen, and it’s something like 10100 years,” says Spiropulu. “In my opinion, that tells you something is wrong. It probably means that you have some other physics effect going on that you haven’t accounted for.”

The weird doomsday prediction could be a sign that supersymmetry is correct. If the same Higgs-field calculation is done taking SUSY into account, no cosmicGötterdämmerung results. Supersymmetry predicts there might be as many as five different Higgs bosons, all with different masses, in which case physicists at the LHC might only have discovered the lightest one. The only way to find out will be to produce many more Higgs bosons over the next few years to see if any of them might be super-Higgses.

“What we’re expecting is that this next run of the LHC will give us 10 times as many Higgs bosons as we’ve had so far,” says Ellis. “So there’s a lot riding on precision measurements to see if their properties are what is predicted by supersymmetry or not.”

In a display case at the Lawrence Hall of Science in the foothills above the UC Berkeley campus rests the pint-size progenitor of the Large Hadron Collider. Built of wire and sealing wax in 1930 by a 29-year-old physicist named Ernest Lawrence, the cyclotron, as it came to be called, had an accelerating chamber measuring just 4 inches across — about the size of a saucer. With it, Lawrence kicked protons to energies of 80,000 electron volts. Its price tag? Twenty-five dollars. The invention earned Lawrence — who was nicknamed “the Atom Smasher” — the 1939 Nobel Prize in Physics.

Atom-smashing costs have risen considerably; so have the sizes of the machines that do the smashing.The LHC can reach energies nearly 100 million times higher than Lawrence’s small device, but it took decades and billions of dollars to build, and it required the expertise of thousands of physicists and engineers. As for scale, its circular tunnel spans almost 5½ miles across — about 84,000 times that of the ancestral cyclotron. The giant machine is expected to run until at least 2035, but physicists are already planning for the accelerator that they hope will succeed it.

“Because of the time scales involved in these projects, you can’t afford to make a mistake,” says Lyn Evans, director of the Linear Collider Collaboration, a group of some 2,000 physicists and engineers who are coordinating the efforts to develop the next generation of particle accelerator. “Physicists started planning for the LHC in the early 1980s. It was approved in 1994, and it was 2010 before it was really operating. So the time scales and the decision-making process are really long.”

Unlike the LHC, the next big accelerator probably won’t be circular. While a circular machine has many advantages — protons can be whipped through the LHC’s tunnel multiple times, getting an energy boost on every circuit, and the beams can be crossed repeatedly at many points, creating more collisions — there are trade-offs. The LHC needs enormous magnets to force particles to travel in circles. And when particles accelerate along circular paths, they radiate away energy, so less is available for collisions.

The leading candidate for succession is a project called the International Linear Collider. The ILC would hurl electrons and their antimatter counterparts, positrons, from opposite ends of a straight, 19-mile-long tunnel, generating collisions at the machine’s center. By using electron and positron beams instead of heavier protons, the ILC will allow physicists to probe particle properties with much greater precision than they can at the LHC.

“The LHC is a like a collider of cherry pies,” says Berkeley’s Murayama. “Cherry pies are easy to throw, and they smash together rather easily, but they produce a huge splash, and all the goo comes out of the pie.” In the case of the LHC’s collisions, the “goo” consists of the protons’ components — quarks and gluons. The sheer messiness of proton-proton collisions makes it difficult to detect new particles, or to make accurate measurements of known particles.

Electrons and positrons, on the other hand, don’t have any components, so the collisions are cleaner. At the LHC, maybe one in a billion proton-proton collisions yields a Higgs boson. Physicists estimate that a Higgs should pop up roughly every hundred electron-positron collisions at the ILC. The challenge then will be aiming the electron beams accurately enough to ensure enough collisions occur.

“In circular colliders, you have collisions happening many times as the beam circulates inside the ring,” says Murayama. “But a linear collider gives you only one shot, and to get decent data, you have to squeeze the beam down to an incredibly small size so the probability of a collision between an electron and positron becomes high.”

The ILC’s collision point will be will be less than 10 nanometers — about a hundred atoms wide. “You have to operate these tiny objects coming at the speed of light and make sure they meet! I’m still surprised that people think they can do this,” says Murayama.

But the project’s supporters say the only thing the ILC needs now is funding, as well as a country willing to host the project — Japan is currently the front-runner. “The ILC is shovel-ready,” says consortium director Evans. Like the LHC, it will take more than a decade to build. Cost estimates range from $10 billion to $25 billion.

The LHC is giving physicists their first look at an entirely new high-energy realm, but so far, aside from the Higgs boson, nothing new has turned up.

“For me, the next run of the LHC will be very important,” says Evans. “If SUSY is not found, then the justification for a linear collider will be even stronger.” It’s possible, he says, that the LHC may fail to find evidence for new physics — the energies required may simply lie beyond the reach of any machine we could conceivably build.

Even so, exacting measurements of the Higgs boson at the ILC might still allow physicists to tease out fine differences between the predictions of SUSY — or perhaps some other theory, should SUSY fizzle — and those of the Standard Model.

“We would be able to measure the properties of the Higgs boson with extreme precision,” says Evans, “and try to crack the Standard Model in that way.”

As physicists continue their efforts to unravel the warp and woof of the physical world, they will need increasingly powerful machines. A generation from now, their greatest challenge may not come in the form of some unknowable mystery of nature, but rather from the inability — or unwillingness — of society to fund ever larger and more expensive accelerators.

“Unless we do something dramatically different, we will reach the end of the line, whether that’s 10 years from now, 20, or 30,” says Wim Leemans, director of accelerator technology and applied physics at Lawrence Berkeley National Laboratory. “The machines will become too large and too costly to build unless we come up with radically new technology.”

Leemans has been working on a fundamentally new type of accelerator that may allow physicists to scale back the size of their particle-smashing behemoths without sacrificing power.

His device relies on extremely short, powerful bursts of laser beams. The laser fires quadrillion-watt pulses into a thin, 9-centimeter-long sapphire tube filled with hydrogen gas. As the laser shoots through the gas, it strips electrons away from the hydrogen atoms. The laser acts sort of like a motorboat, dragging those particles in its wake, says Leemans. 

He and his colleagues have managed to accelerate electrons to about 4 billion electron volts in a device that is less than a meter long (not including the room-filling laser).

“We’re aiming at 10 billion electron volts,” says Leemans. “We think we can do that in half a meter.” That would put his tabletop device nearly on a level with the Stanford Linear Accelerator, which pushes particles to 50 billion electron volts in a 2-mile-long tunnel.

Still, Leemans says he has a long way to go before matching the trillion electron-volt energies of a machine like the LHC. Scaling up his system will require coordinating the firing of multiple lasers with nearly quadrillionth-of-a-second timing.

If he succeeds, how small could a laser-driven accelerator be that could perform at the frontiers of physics? “That’s a difficult question,” he says. “We don’t know the precise answer. Our straw-man designs say we should be able to build a linear accelerator well under a kilometer long.”

Such a machine is still decades away. But something like it will be essential if advances in particle physics are to continue after the LHC shuts down in 2035. By then, perhaps, physicists will have solved the mysteries of dark matter; maybe supersymmetry will be on firm footing. Or maybe the Standard Model will continue to reign supreme, and physicists will still be looking for their theory of everything.

“We’ve come a long way in understanding the fundamental nature of the universe,” says Joseph Incandela, a physicist at the University of California, Santa Barbara, and the leader of one of the teams that discovered the Higgs boson. “Heck, this whole idea, if you think about it, goes back 2,600 years. So we, the human race, have to be patient. The breakthroughs have been so quick and so stunning in the last century and a half or so that we’ve become kind of spoiled. It could be that we have to leave some things for future generations.”