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Higgs vs. hype: a mini-guide

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

Updated 9:50 a.m. ET Dec. 13:

Physicists have revealed what they've found so far in their quest for the Higgs boson at Europe's Large Hadron Collider on Tuesday, after days of buildup that put the "God particle" on a par with Obi-Wan Kenobi and the Force. But the Higgs boson isn't a religious experience, and it won't help you destroy the Death Star. So what is the Higgs? And what do scientists know about it? Here's a small guide to the Large Hadron Collider's latest:

Why it's important: For decades, physicists have used a theory known as the Standard Model to explain the interactions of subatomic particles, and the theory works beautifully. It's guided our way through the world of nuclear power, television, microwave ovens and lasers. One problem: The theory needed something extra to explain why some particles have mass and some don't. Back in the 1960s, physicist Peter Higgs and his colleagues proposed the existence of a mysterious energy field that interacts with some particles more than others, resulting in varying values for particle mass. That field is known as the Higgs field, and it's associated with a particle called the Higgs boson.

Today, the Higgs boson is the last fundamental piece missing from the Standard Model. Finding it is the most commonly cited reason for building the $10 billion LHC. If the characteristics of the Higgs particle (or particles) match what's predicted by the current formulation of the Standard Model, that would bring a sense of completion to particle physics. If the Higgs isn't found, that might force physicists to tweak or even discard the Standard Model. "I find it difficult to imagine how the theory works without it," Peter Higgs recently told the London monthly Prospect. If a non-Standard Higgs is detected, that could totally change the way we see the universe. In the far future, we might even find a way to take advantage of the Higgs field, just as earlier physicists took advantage of the electromagnetic field, radioactivity or quantum effects.

Where they're at: The quest for the Higgs is being conducted using two detectors at the LHC, which is housed at Europe's CERN particle physics center on the French-Swiss border. The collider has been built inside a 17-mile-round (27-kilometer-round) underground tunnel where two beams of protons are smashed together at 99.999999 percent of the speed of light.

The detectors, known as ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid), are placed at key points on the collider ring. They're built somewhat differently, and they serve as a system of checks and balances to make sure one team can confirm what the other team is seeing. The LHC is the only collider on earth that can achieve the energies required to probe the Higgs boson's potential hiding places. (However, higher energies have been observed in cosmic ray collisions high above Earth's surface.)


This graphic shows a typical candidate event in the search for the Higgs boson, including two high-energy photons whose energy (depicted by red towers) has been measured in the CMS electromagnetic calorimeter. The yellow lines are the measured tracks of other particles produced in the collision.

What they've learned: The ATLAS and CMS teams shared their results in a series of public presentations at CERN, beginning at 8 a.m. ET Tuesday. Aidan Randle-Condle has been liveblogging the event at the Quantum Diaries blog. You'll find a less geeky liveblog at The Guardian. Canada's Perimeter Institute for Theoretical Physics is presenting a webcast discussion after the announcement, at 12:30 p.m. ET.

Here are the key numbers: The CMS team said that if it exists, the Higgs boson would have to have a mass somewhere between 115 billion and 127 electron volts (that's 115-127 GeV for short). ATLAS reported a range of 116-130 GeV. Both teams saw "tantalizing hints" of a detection around the 124-125 GeV level, but nothing that could yet be called a discovery. That's because the confidence values are no higher than 3.6 sigma for ATLAS, and 2.6 sigma for CMS.

Wait ... what's a sigma? Those numbers measure how likely it is that the effect seen amid the billions of collisions at the LHC is real rather than a statistical fluke. Suppose you have a machine that flips coins to check whether they've been stamped correctly with heads and tails, rather than two heads. You have to decide when to stop the conveyor belt to remove a coin with two heads, based purely on the machine's report. If the machine flips five heads in a row, you have more than 2 sigma confidence that there are heads on both sides of the coin. If it flips 10 heads in a row, the confidence goes up to more than 3 sigma. If it flips 20 heads in a row, you have a 5-sigma observation. (You could just have someone look at both sides of the coin, but you get the idea.)

In scientific observations, a level of 3 sigma constitutes "evidence" that an observed effect is real, and not just a fluke. You have to go up to 5 sigma to declare a "discovery." Thus, the observations hint at where the Higgs boson might be found, but this can't yet be called a discovery. In its news release, CERN used a different analogy to describe the confidence level, using dice rather than coins: "Taken individually, none of these excesses is any more statistically significant than rolling a die and coming up with two sixes in a row."

Fermilab's Don Lincoln explains the latest results in the search for the Higgs boson.

What's next? However the results are spun, more data will be required to nail down a confirmed detection of the Higgs. The proton beams have been shut down for CERN's holiday break, but they'll be started up again next year. The results so far have raised hopes that confirmation of the Higgs' existence (or its non-existence) will come by the end of 2012. After next year's round of experiments, the LHC will be shut down until 2014 for a major upgrade. It won't ramp up to its full power of 7 trillion electron volts per beam until after the upgrade. There'll be a long wait to get to the deepest mysteries of particle physics — but based on the latest results, there's renewed hope for the Higgs.

More on the Higgs boson and the LHC:
