Long-sought boson completes standard model of physics

By Alexandra Witze

Web edition: December 13, 2012
Print edition: December 29, 2012; Vol.182 #13 (p. 16)

2012 SCIENCE NEWS TOP 25: 1

It’s hard enough to muster a standing-room-only crowd for a physics talk, let alone an overnight queue. But on the night of July 3, scientists sacrificed sleep to line up outside the main auditorium at CERN, the particle physics laboratory near Geneva. Their goal: get a seat to hear Joe Incandela. It wasn’t the laconic, gray-suited scientist they had lined up for, though. Incandela, a particle physicist at the University of California, Santa Barbara, was expected to be the first to unveil the biggest physics news in years.

At 9 o’clock the next morning, with the auditorium packed, Incandela launched into a flood of charts and graphs. Blips in the data represented what happened when proton beams slammed into one another in CERN’s mammoth particle collider. Buried in this data was one blip representing a subatomic celebrity that scientists had been hunting for years — the Higgs boson.

Incandela didn’t disappoint. “We’re seeing something; it’s relatively significant,” he told the anxious onlookers.

He clicked to the next slide. The blip grew bigger. There it was: the Higgs. The room erupted in applause.

The next speaker, CERN’s Fabiola Gianotti, only strengthened the case when she unveiled her team’s evidence.

In many ways, that moment at CERN was the culmination of decades of scientific questing. Finding the Higgs meant that physicists had finally succeeded in explaining why the universe looks the way it does (SN: 7/28/12, pp. 5, 26 & 28). Their framework of the universe at the subatomic scale was complete.

“When it comes to discovering the ultimate workings of reality, the easy part is now officially over,” says Sean Carroll, a theoretical physicist at Caltech. “We’ve put the finishing touches on a complete theory of the matter we see around us in our everyday lives.”

Now that most scientists agree the Higgs is here, they can begin to map uncharted realms, from the possibility of extra dimensions of space and time to massive, secretive particles that shadow those already known.

Ultimately, the Higgs particle is important because it helps explain mass. It and the closely related Higgs field are the reason the universe didn’t remain a sea of massless particles after the Big Bang.

Just nanoseconds after the cosmos was born, a field permeating all of space switched on. This was the Higgs field (named, like the particle, after University of Edinburgh physicist Peter Higgs, one of several scientists who dreamed up the idea in the 1960s). Suddenly some of the particles zipping around hit the Higgs field and slowed down, like marbles rolling through honey. That slowdown endowed them with mass. Once they had mass and could properly stick together, particles could combine and congeal into the atoms and molecules that make up everything from stars to people.

Only some of the particles slowed down; others, like photons, can buzz right through the Higgs field and thus still have no mass.

No wonder physicists have been hunting the Higgs for so long. But scientists can’t spot the Higgs field directly; they can only deduce its existence by detecting the Higgs boson. A boson is a type of particle often closely linked to a force, and the Higgs boson emerges from the Higgs field (illustrated at left).

And the only way to observe a Higgs boson today, nearly 14 billion years after the Big Bang, is to create one in high-energy smashups at particle accelerators. For decades, nobody had a powerful enough machine to generate the energies required.

Accelerators such as the Tevatron at Fermilab, outside Chicago, took a shot at it. Einstein’s equation E=mc² says that energy and mass are interchangeable. Smash two particles together at high enough energies, say nearly the speed of light, and an even more massive particle can pop into existence.

The Tevatron helped narrow the range of masses the Higgs might have, but it took CERN’s bigger Large Hadron Collider to shake the Higgs loose. Out of every trillion collisions between protons, perhaps one created the rare, unstable Higgs — which quickly decayed to other kinds of particles.

Peering into sprays of debris from 500 trillion proton collisions, two detectors independently spotted signatures of Higgs decay. Working backward from the particle debris, scientists calculated that the Higgs has the mass of about 133 protons.

To convince themselves they were seeing true Higgs decays, the CERN physicists set themselves a rigorous statistical standard. They required a level of certainty known as five sigma, which holds that there is a 1-in-3.5-million chance that a statistical fluke could have created a signal of the observed magnitude or greater. At the time of the CERN announcement, both Higgs detectors independently achieved the five-sigma level; the statistical strength has only increased since, with both detectors now between six and seven sigma.

Still, current theories predict a very specific set of behaviors for the Higgs, and it’s not yet clear whether the particle found at CERN meets those. The discovery may yet turn out to be a close cousin, rather than an identical twin, to the particle that Peter Higgs predicted. Scientists have been posting papers almost daily at arXiv.org, an online forum for new research not yet in journals, exploring the consequences of what a non–standard model Higgs might mean — from limiting the scope of other theories to raising the possibility of brand-new particles never before dreamed of.

If so, physicists will need new theories to explore what’s going on. That may take years: Their best tool, the LHC, is scheduled to shut down in early 2013 for an upgrade that could take up to two years. It will come back with more than 50 percent more energy.

That ultimate energy should be enough for physicists to distinguish between the several Higgs possibilities. There may even turn out to be not one but many kinds of Higgs particles, each with a different mass.

An LHC running at full bore may also be able to answer one of the biggest puzzles about the collider so far: why it hasn’t spotted any evidence of supersymmetry, a theory that holds that all the particles in the ordinary world have a heavy partner lurking nearby. Supersymmetry could explain why the mass of the Higgs boson isn’t infinite, as standard model math would have it. Supersymmetry could also mean that the Higgs itself has its own massive superpartner particle. Powering up to higher energies will let the LHC probe the possibilities of this shadowy otherworld, if it exists.

“This isn’t the end of the story,” says Fermilab physicist Rob Roser, “but the beginning of a new chapter in science.”

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