Photograph by Toni Albir, European Pressphoto Agency

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British physicist Peter Higgs, for whom the Higgs boson is named.

Photograph by Toni Albir, European Pressphoto Agency

Physicists Increasingly Confident They've Found the Higgs Boson

The Higgs boson is so far consistent with what theory predicts.

Physicists announced this week that they are more confident than ever that the "Higgs-like" particle discovered last year is actually the long-sought Higgs boson—the particle crucial for understanding why objects in the universe have mass.

The probability that last year's data identifying the Higgs boson was a statistical fluke—and that researchers hadn't discovered the long-sought particle—"is now becoming astronomically low," said Tim Barklow, an experimental physicist with the ATLAS Experiment who's based at Stanford University's SLAC National Accelerator Laboratory.

On Thursday, scientists from CERN announced at the annual Moriond Conference in Italy that certain key properties of the particle are so far consistent with what is predicted by the so-called Standard Model of particle physics.

For example, the Higgs boson is postulated to have no rotation, or "spin," and in the Standard Model its parity—a measure of how the particle's mirror image behaves—should be positive.

And indeed, the latest data "point to the new particle having the spin-parity of a Higgs boson as in the Standard Model," Dave Charlton, a spokesperson for CERN's ATLAS project, said in a statement.

ATLAS and the Compact Muon Solenoid (CMS) are the two experiments at CERN's Large Hadron Collider (LHC) that are searching for signs of the Higgs boson. The latest analyses included data from about 500 trillion proton-proton collisions from ATLAS and CMS collected in 2011 and from 1,500 trillion collisions in 2012—or about two and a half times more data than were available at the time of the particle's discovery announcement last July 4.

If It Looks Like a Higgs...

The new evidence is compelling enough that some scientists have thrown caution to the wind and have stopped referring to the new particle as being merely "Higgs-like" and just calling it the Higgs boson.

"The preliminary results with the full 2012 data set are magnificent, and to me it is clear that we are dealing with a Higgs boson," CMS spokesperson Joe Incandela said in a statement.

Incandela added, however, that physicists "still have a long way to go to know what kind of Higgs boson it is."

The Higgs could—as the evidence increasingly suggests—be a single particle that has all of the properties predicted by the Standard Model. Or it might be something more unusual, such as a composite made up of multiple particles.

Howard Gordon, a physicist at Brookhaven National Laboratory in New York and the ATLAS deputy collaboration board chair, called the new data "exciting," but said he would like to see even more precise measurements of the new particle's properties before he is absolutely sure it's the Higgs boson.

CERN physicists are confident to a sigma level of only about three that the spin and parity of the Higgs are zero and positive, respectively, as theory predicts.

Scientists use sigma levels to express their degree of confidence in a result—higher values are better.

Researchers stunned the world last July when they said that the sigma level for the discovery of a new Higgs-like particle was five, meaning that there was only a one-in-a-million chance that the Higgs-like signal they observed was a statistical fluke.

The Higgs' spin and parity are "not as definitive as the fact that we have a new particle," Gordon said.

Also, the decay, or "coupling," paths of the new particle still need to be worked out in more detail. The Standard Model predicts that the Higgs boson will decay into more conventional subatomic particles such as b quarks and leptons.

The latest results indicate that the Higgs particle is sticking to the Standard Model's script, but the confidence levels for the two decay modes also have room for improvement. Its "two [decay] modes still have very large errors associated with them. We don't have those nailed down yet," Gordon said.

Predictable So Far

Although a full picture of the Higgs boson has yet to emerge, some physicists have expressed disappointment that the new particle is so far behaving exactly as theory predicts.

"This is looking very much like a garden-variety [Standard Model] Higgs, which is discouraging for hopes of hints about how to get beyond the Standard Model," Peter Woit, a mathematician at Columbia University in New York, wrote on his blog Not Even Wrong.

And cosmologist Sean Carroll of Caltech tweeted that the new data on the Higgs boson is "looking pretty vanilla."

But SLAC's Barklow said it's too soon to dismiss the Higgs as "boring."

"We are a long way from making that determination," Barklow told National Geographic News.

Brookhaven's Gordon said he is aware that some of his colleagues were hopeful the Higgs would turn out to be exotic, but he pointed out that there are still unanswered questions about the particle that theory can't explain.

For instance, its mass of around 125 gigaelectronvolts (GeV) is lighter than it should be.

"The question of why the Higgs is at 125 GeV is still something that we don't understand," Gordon said, "and there are people trying to figure out why that is."

Work Continues

The Large Hadron Collider is currently in the midst of a scheduled two-year shutdown for maintenance and upgrades that will allow it to smash protons at even higher energies when it comes back online in 2015, so no new Higgs particles are currently being produced.

That doesn't mean Higgs research has ground to a halt. Many physicists are working on ways to improve the analysis of the Higgs data that have already been collected.

"I think there'll be incremental increases [in our understanding of the Higgs] even while the machine is being refurbished and upgraded," Barklow said.