Published July 3, 2012
Just hours from now, physicists could announce they’re 99.9936 percent sure the Higgs boson exists, according to experts on the hunt for the so-called God particle.
The combined findings of two teams at the proton-smashing Large Hadron Collider (LHC) should help explain why objects in our universe have mass—and in so doing, why galaxies, planets, and even humans have any right to exist.
“There are very few people even in ATLAS and CMS who know what’s going to be presented on Wednesday. It’s kept that secret,” said particle physicist David Evans, leader of the U.K. team that works on the LHC’s ALICE experiment.
The rumor mill, though, got a late-breaking boost Tuesday, when, according to Science News, an official CERN video clip surfaced briefly, publicly, and—presumably—prematurely.
In the video CMS spokesperson Joe Incandela reportedly says, “We’ve observed a new particle … we have quite strong evidence that there’s something there” in the expected mass range for the Higgs boson. “This is the most massive such particle that exists, if we confirm all of this—which I think we will.”
On Higgs Hunt, It’s “Easy to Fool Yourself”
Results in particle physics are ranked on a scale from zero to five “sigma.” Last December the ATLAS and CMS teams said their data showed a two-sigma likelihood that the Higgs particle has a mass of about 125 gigaelectron volts (GeV)—about 125 times the mass of a proton, a positively charged particle in an atom’s nucleus.
“For the first time there was a case where we expected to [rule out] the Higgs, and we weren’t able to do so,” said Tim Barklow, an experimental physicist with the ATLAS Experiment who’s based at Stanford University’s SLAC National Accelerator Laboratory.
A two-sigma finding translates to about a 95 percent chance that results are not due to a statistical fluke.
While that might seem impressive, it falls short of the stringent five-sigma level that high-energy physicists traditionally require for an official discovery. Five sigma means there’s a less than one in a million probability that a finding is due to chance.
“We make these rules and impose them on ourselves because, when you are exploring on the frontier, it is easy to fool yourself,” said Michael Peskin, a theoretical physicist also at SLAC.
God Particle Discovery a Safe Bet?
Based on CERN’s June announcement that the teams now have more than double the data they had last year, ALICE’s Evans thinks it’s likely the groups could announce they’ve reached a four-sigma level of certainty—that is, they are 99.9936 percent confident—that the Higgs particle exists in the 125 GeV mass range.
While that result still won’t be enough to count as an official discovery, “it would be strong enough that most physicists would be quite comfortable betting a month’s salary that an announcement of the discovery of the Higgs will come by the end of the year,” said Evans, who’s based at the U.K.’s University of Birmingham.
Alternatively, the two groups would also have enough data to determine if last December’s results were a statistical fluke.
A negative result would be equally exciting for physicists, Evans added, because it would hint at completely new physics, forcing a rethink of the laws of the universe.
Higgs Holds It All Together?
The Higgs boson is one of the final puzzle pieces required for a complete understanding of the standard model of physics—the so-far successful theory that explains how fundamental particles interact with the elementary forces of nature.
The so-called God particle was proposed in the 1960s by physicist Peter Higgs to explain why some particles, such as quarks—building blocks of protons, among other things—and electrons have mass while others, such as the light-carrying photon particle, do not.
Higgs’s idea was that the universe is bathed in an invisible field similar to a magnetic field. Every particle feels this field—now known as the Higgs field—but to varying degrees.
If a particle can move through this field with little or no interaction, there will be no drag, and that particle will have little or no mass. Alternatively, if a particle interacts significantly with the Higgs field, it will have a higher mass.
The idea of the Higgs field requires the acceptance of a related particle: the Higgs boson.
According to the standard model, if the Higgs field didn’t exist, the universe would be a very different place, said SLAC’s Peskin, who isn’t involved in the LHC experiments.
“It would be very difficult to form atoms,” Peskin said. “So our orderly world, where matter is made of atoms, and electrons form chemical bonds—we wouldn’t have that if we did not have the Higgs field.”
In other words: no galaxies, no stars, no planets, no life on Earth.
“Nature Is Really Nasty” to God Particle Seekers
Buried beneath the French-Swiss border, the Large Hadron Collider is essentially a 7-mile-long (27-kilometer-long) oval tunnel. Inside, counter-rotating beams of protons are boosted to nearly the speed of light using an electric field before being steered into collisions.
Exotic fundamental particles—some of which likely haven’t existed since the early moments after the big bang—are created in the high-energy crashes. But the odd particles hang around for only fractions of a second before decaying into other particles.
Theory predicts that the Higgs boson’s existence is too fleeting to be recorded by LHC instruments, but physicists think they can confirm its creation if they can spot the particles it decays into.
If, based on these observations, the Higgs does turn out to have a mass of around 125 GeV, as previous evidence suggested, the result would help explain why the God particle has avoided detection for so long.
This mass is just high enough to be out of reach of earlier, lower-energy particle accelerators, such as the LHC’s predecessor, the Large Electron-Positron Collider, which could probe to only about 115 GeV.
At the same time 125 GeV is not so massive that it produces decay products so unusual that their detection would be clear proof of the Higgs’s existence.
In reality the Higgs appears to transform into relatively commonplace decay products such as quarks, which are produced by the millions at the LHC.
“It just so happens that nature is really nasty to us, and the range that we’ve narrowed [the Higgs] down to is the range that makes it most difficult to find,” ALICE’s Evans said.
To detect the Higgs’s signal amid this high background noise, scientists must calculate very precisely what the distribution of a particular decay particle for a collision will be at a given energy, and how many extras of that particle they’d expect to see if a Higgs boson has been created.
Additionally, to ensure that a signal is not a statistical fluke, LHC physicists require lots of collisions—the atom smasher can produce about 800 million per second—to generate enough Higgs-creating collisions.
“You need the total number of collisions to be large so you can see this rare event happening,” explained ATLAS physicist Michael Tuts of Columbia University.
Going for the Gold
While the search for the Higgs was a primary motivation for the construction of the LHC, activity at the world’s largest atom smasher won’t stop if the Higgs boson is confirmed.
For one thing, lingering questions will require years of follow-up work, such as what the God particle’s “decay channels” are—that is, what particles the Higgs transforms into as it sheds energy.
The answer to that question will allow physicists to determine whether the particle they have discovered is the one predicted from theory or something more exotic, Tuts added.
Something the public often forgets, too, is that ATLAS and CMS make up only two of the LHC’s four major experiments, Evans said. The other two—LHCb and Evan’s own ALICE— are investigating other physics arcana, such as why the universe contains so little antimatter. (See “Antimatter Atoms Trapped for First Time—’A Big Deal.'”)
“If you want to compare it to the Olympics, finding the Higgs would be like winning just one gold medal,” Evans said.
“I’m sure most countries would like to win more than one gold medal. And I think CERN is going to deliver a lot more gold medals over the years.”