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Has the Higgs boson been found? Signs point to ... almost

Ezra Klein explains what the Higgs boson is and why physicists are devoting so much effort to the discovery effort, during a moment of geek on "The Rachel Maddow Show" on MSNBC.


Scientists at America's top particle-physics facility, Fermilab, today revealed their almost-final array of almost-strong evidence for the existence of the elusive Higgs boson — in advance of what's expected to be the almost-discovery of the subatomic particle at the Large Hadron Collider, almost half a world away.

The results are based on the full batch of data gathered at Fermilab's Tevatron experiment over the course of more than a decade, and Fermilab said the findings represented the "strongest indication to date for the long-sought Higgs particle" from the separate teams behind the Tevatron's CDF and DZero detectors.

"The Tevatron experiments accomplished the goals that we had set with this data sample," CDF co-spokesperson Rob Roser said in a news release about the revelation. "Our data strongly point toward the existence of the Higgs boson, but it will take results from the experiments at the Large Hadron Collider in Europe to establish a discovery."


Discovery of the Higgs boson is the top objective for the $10 billion Large Hadron Collider, which was started up almost four years ago. Physicists have theorized about the particle and its associated Higgs field for four decades, working it in as a key part of the Standard Model, one of physics' most successful theories. The Higgs field is thought to be the mechanism that imparts mass to some particles while leaving other particles massless. What's more, the Higgs mechanism could serve as a gateway for going beyond the Standard Model and exploring way-out concepts such as supersymmetry and extra dimensions.

No wonder, then, that Nobel-winning Fermilab physicist Leon Lederman dubbed it "the God Particle" almost two decades ago. (Today, most physicists wish he hadn't.)

Fermilab via AFP - Getty Images

The 4-mile-round (6.4-kilometer-round) Tevatron accelerator was shut down last year, but researchers have just released what they say is their final analysis of data from the Tevatron experiments relating to the search for the Higgs boson.

Over the past year, results from Fermilab in Illinois as well as from the LHC on the French-Swiss border have focused in on a "bump" of anomalous data, hinting at an unknown particle with a mass of 125 billion electron volts, or 125 GeV. Two big questions have been hanging in the air: How precisely can the mass be determined? And how sure can the scientists be that what they're seeing is real, rather than merely a fluke in the data?

The Fermilab teams' almost-final answer is that a particle like the Higgs boson can lurk only in the area between 115 and 135 GeV, and they say there's just a 1-in-550 chance that the bump they're seeing is a random fluctuation. Another way of expressing the statistical confidence in the results is to say that it's at the 2.9-sigma level in the bottom-quark decay mode, and 2.5 sigma overall.

That level falls just short of the 3-sigma standard that physicists have been using for "strong evidence" of a subatomic particle's existence, and far short of the 5-sigma standard for "discovery." The 5-sigma level is equivalent to 99.99994 percent confidence. This is why Roser said actually establishing a discovery will have to be up to the LHC.

At least an almost-discovery
As it happens, Europe's CERN particle-physics center has scheduled an announcement about the LHC's search on Wednesday, and for the past couple of weeks, onlookers have been wondering whether this will mark the true 5-sigma discovery of the Higgs boson. Last December, the teams behind the LHC's ATLAS and CMS detectors reported that they saw "tantalizing hints" of the Higgs at 125 GeV, with confidence levels of 3.6 sigma for ATLAS and 2.6 sigma for CMS.

Since that time, the detectors have doubled the amount of data collected, and the energy level for the LHC's collisions has ramped up to four times what was achievable at Fermilab's Tevatron. That has raised expectations that the LHC's results will come close to or even exceed the 5-sigma confidence standard, depending on how Wednesday's announcement is spun.

"What we saw in December suggests that the real fireworks will be on the Fourth of July," said Fermilab physicist Don Lincoln, author of "The Quantum Frontier."

Fermilab

A graphic from Fermilab shows the "bump" that hints at the existence of the Higgs boson in a mass region from 115 to 135 GeV.

Advance indications suggest that the ATLAS and CMS teams both have higher confidence that they're really seeing a particle matching the Higgs boson's description — in the range of 4.5 to 5 sigma, according to Nature. Some of the advance rumblings suggest that the results from the two detectors would have to be combined to get past 5 — but CERN says that particular statistical twist won't figure into this week's announcement.

"Combining the data from two experiments is a complex task, which is why it takes time, and why no combination will be presented on Wednesday," CERN spokesman James Gillies told The Associated Press.

Even if one detector — say, ATLAS — were to get past 5, some physicists might still question the results. After all, the researchers who reported clocking neutrinos at speeds faster than light were pretty sure of their results, too, until they found a flaw in their fiber-optic timing system. But if the findings are as solid as the latest reports suggest, all this hand-wringing over the technical definition of a discovery may be a merely academic matter.

"I agree that any reasonable outside observer would say, ''It looks like a discovery,'" CERN physicist John Ellis told AP. "We've discovered something which is consistent with being a Higgs."

The Large Hadron Collider is continuing to run, and ATLAS and CMS are continuing to collect data. Even if the results being announced this week turn out to be merely an almost-discovery, the matter will certainly be settled by the end of the year, as predicted.

Then what?

After the discovery
Discovering the Higgs boson, or something like it, would just be the start of the real work to be conducted at the LHC: Is the Higgs mechanism totally in sync with what's predicted by the Standard Model? How does particle mass arise in the Higgs field? Are there any anomalous trails that could be followed to new frontiers in physics? This is where the results from Fermilab's Tevatron could come into play again.

Researchers at the LHC and the Tevatron can't detect the Higgs boson directly. Instead, they check a number of pathways by which the particle decays into other particles that can be detected — two photons, for example, or a pair of bottom quarks. To nail down the particle's characteristics completely, observations will have to be analyzed from multiple pathways.

Fermilab scientist Don Lincoln describes the nature of the Higgs boson.

"Being able to see it decaying into photons, and seeing it also in bottom quarks gives us some confidence that it's the Standard Model Higgs — and not some cousin particle that's similar to, but different from what the Standard Model predicts," Fermilab's Lincoln said. "Or, if you want to be terribly perverse, it could be some particle we haven't seen before, but not the Higgs at all."

If the LHC reports a discovery at the 125-GeV mass level, that would provide a new focus for Fermilab. "Once it's established that there's something to look at, we'll be able to retool the analyses to try to work out the question of what it is we're seeing," Lincoln said. Knowing for sure that there's something actually there, at 125 GeV, will allow physicists to fine-tune their analytical tools. 

Fermilab shut down the Tevatron almost a year ago, so no new data can be collected at that collider. But the Tevatron data, when used in combination with the data that will continue to flood from the LHC, could still contribute to solving some of the deepest questions in physics. "The story is not over," Lincoln said.

The big story on July 4
Here are some websites to watch leading up to Wednesday's big reveal:


Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.