Has the Higgs Been Discovered? Physicists Gear Up for Watershed Announcement

Rumors are flying about a December 13 update on the search for the long-sought Higgs boson at Europe's Large Hadron Collider
Part of the LHC in Geneva

CERN/Claudia Marcelloni

The physics buzz reached a frenzy in the past few days over the announcement that the Large Hadron Collider in Geneva is planning to release what is widely expected to be tantalizing—although not conclusive—evidence for the existence of the Higgs boson, the elementary particle hypothesized to be the origin of the mass of all matter.

Many physicists have already swung into action, swapping rumors about the contents of the announcement and proposing grand ideas about what those rumors would mean, if true. "It's impossible to be excited enough," says Gordon Kane, a theoretical physicist at the University of Michigan at Ann Arbor.

The spokespersons of the collaborations using the cathedral-size ATLAS and CMS detectors to search for the Higgs boson and other phenomena at the 27-kilometer-circumference proton accelerator of the Large Hadron Collider (LHC) are scheduled to present updates December 13 based on analyses of the data collected to date. "There won't be a discovery announcement, but it does promise to be interesting," says James Gillies, spokesperson for CERN (European Organization for Nuclear Research), which hosts the LHC.

Joe Lykken, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., and a member of the CMS collaboration, says, "Whatever happens eventually with the Higgs, I think we'll look back on this meeting and say, 'This was the beginning of something.'" (As a CMS member, Lykken says he is not yet sure himself what results ATLAS would unveil; he is bound by his collaboration's rules not to reveal what CMS has in hand.)

[Click here for a lightly edited partial transcript of the interview with Lykken that Davide Castelvecchi conducted for this story.]

The talks were announced last week; true to form, the particle physics rumor mill shifted into high gear, and by the weekend multiple anonymous sources had leaked consistent information, according to several bloggers, including Peter Woit, Lubos Motl and Philip Gibbs. Both experiments are said to have seen evidence of the long-sought Higgs, pointing to a particle mass of around 125 billion electron volts, or 125 GeV. (125 billion electron volts is roughly the mass of 125 hydrogen atoms.)* Such results would not constitute an ironclad discovery quite yet, being below the required "5 sigma," a measure of statistical reliability. But the two experiments are rumored to have seen signals of 2.5 sigma and 3.5 sigma, which together would give a strong hint. (Three sigmas would correspond to a one-in-370 chance of the finding being a statistical quirk, although in particle physics experiments it is not uncommon for 3-sigma results to vanish.)

Previous rounds of data analysis from the LHC as well as from its U.S. predecessor, Fermilab's Tevatron, had narrowed the Higgs mass range down to somewhere between 115 and 140 GeV. But the new announcement would constitute the first time that both LHC experiments had made a precise and consistent estimate of the mass.

Even before the data are out, theoretical physicists around the world are working out the possible implications. Some have pointed out that a value of 125 GeV would be good news for supersymmetry, a theory that predicts that each particle would have a heavier partner known as a superparticle (at least for particles within the framework of the Standard Model of particle physics, the currently accepted description of the subatomic world). "Most supersymmetric models put a Higgs below 140 [GeV] or so," says Matt Strassler of Rutgers University. Supersymmetry has long been a favorite candidate for extending the Standard Model, because it would answer numerous open questions, beginning with the nature of dark matter, the unseen mass that keeps galaxies rotating faster than they otherwise would.

But Kane, a longtime proponent of supersymmetry, makes a more ambitious statement. In a paper posted to the physics preprint site on December 5, he and his collaborators work not from supersymmetry but from an even more radical overhaul of physics: string theory. (String theory is itself an extension of supersymmetry.) Their calculations predict a Higgs mass between 122 and 129 GeV. "If it's in that range it's an incredible success for connecting string theory to the real world," Kane says. He says he is confident that the upcoming LHC announcements, if they pan out as predicted, will constitute evidence for string theory. "I don't think my wife will let us bet our house, but I'll come close," he says.

That Kane and his colleagues released their paper now that the Higgs mass has been—or is about to be—restricted to a particular range, will surely lead some physicists to charge that the new study constitutes not a prediction but a "postdiction." String theory critics have long claimed that the theory has so much flexibility that one can always tweak it to make it predict just about anything.

Moreover, whether string theory can make testable predictions at all has often been the subject of debate. "The trouble is, for all we know, there might be 10,000 other ways of starting with string theory and getting the same Higgs mass, and they may differ in other respects," Lykken cautions.

And when it comes to mass predictions, consistency does not necessarily mean validation, Strassler points out. "If the Higgs turns up at 125 GeV, that would also be consistent with the Standard Model with no supersymmetric particles and no hint of string theory," Strassler says.

For all the excitement, it is still quite possible that any preliminary whiff of the Higgs will later turn out to be a statistical fluke. After all, the CMS and ATLAS detectors cannot directly catch Higgs bosons; those particles would decay into other particles immediately after being created in the LHC's proton collisions. Instead, physicists must analyze the subatomic debris from the decays and reconstruct what happened. Thousands of collisions take place every second, and many of them generate signatures similar to those of the Higgs. "The reason why we don't know whether there's a Higgs yet has mostly to do with the fact that the Higgs boson's decays look like other kinds of physics," Lykken says. "So we need to understand the other kinds of physics enough. It's not just a question of statistics."

Whether next week's announcements pan out, experts say, it is only a matter of time before a final answer is known: Once the experiments have amassed enough data, they either will find the Higgs boson and understand its properties or they will conclusively demonstrate that it does not exist. "It's just a question of when it will happen," Lykken says. "It's not going to be a maybe-yes-maybe-no kind of answer."

*Correction (12/8/11): This sentence was edited after posting. It originally misstated the mass of a proton.

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