Higgs Boson Picture Becomes Clearer as LHC Gears Up for Restart

A worker stands inside the ATLAS detector, surrounded by its eight toroidal magnets, just before the installation of the machine's calorimeter. ATLAS, the largest particle detector at Europe's Large Hadron Collider, sits inside an underground cavern as big as a cathedral. Maximilien Brice / CERN file

Physicists have found more evidence that the "Higgs-like particle" they discovered two years ago behaves just as the fabled Higgs boson should — and they've begun ramping up for fresh discoveries from the Large Hadron Collider, the world's biggest particle-smasher.

The fresh findings about the Higgs boson appear in the journal Nature Physics, and are based on a detailed analysis of data from one of the LHC's experiments, the Compact Muon Solenoid or CMS. The results build on previous findings that earned a Nobel Prize last year for the theorists who proposed the Higgs boson's existence decades earlier.

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The Higgs boson is thought to be part of the mechanism that gives rise to the masses of all the fundamental subatomic particles. It's the last piece of one of the scientific world's most successful theories, known as the Standard Model of particle physics, to be nailed down. But to make sure the theory stays nailed down, physicists want to show that the boson behaves as theory suggests it should.

Two years ago, they reported that the Higgs particle could rapidly decay by splitting into pairs of photons, W bosons or Z bosons. The new results suggest that it can also decay directly into a different group of particles called fermions. Fermions include quarks (which combine to create protons, neutrons and many other particles) as well as leptons (such as electrons).

The CMS team focused on the tau lepton, which is 3,500 times more massive than the electron and thus easier to spot. Researchers confirmed that the Higgs boson decayed into such leptons to a confidence level of 3.8 standard deviations. That means there's a 1 in 10,000 chance that the physicists' findings were just a statistical fluke.

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Markus Klute, an assistant professor of physics at the Massachusetts Institute of Technology and leader of the international effort, said the findings suggested that the Higgs boson really does play the role physicists expect it to play.

"This is an enormous breakthrough," Klute said in an MIT news release. "Now we know that particles like electrons get their mass by coupling to the Higgs field, which is really exciting."

However, he and his colleagues aren't yet completely satisfied: They want to raise the confidence level of their observations to 5 standard deviations, which is the traditional definition of a discovery in particle physics.

"Within the current level of precision, there is still room for other models with particles that look like the Standard Model Higgs, so we need to accumulate more data to figure out if there is a deviation," Klute said, "although if we do find a deviation from the Standard Model, it is likely to be a very closely related one."

Special report on the Large Hadron Collider

To get more data, physicists will have to wait for the restart of the LHC, an underground particle accelerator on the French-Swiss border. The LHC was shut down more than a year ago for refurbishment, and it's due to resume operations early next year.

Europe's CERN particle physics lab, which operates the LHC, announced on Monday that the cooldown process has just gotten under way. Eventually, the entire 17-mile-round (27-kilometer-round) accelerator ring will be chilled to a temperature just above absolute zero.

Image: Magnet closure
Workers write their names on the sheathing of the Large Hadron Collider's magnet system to celebrate the closure of the last magnet in the Superconducting Magnets and Circuits Consolidation project, also known as SMACC. CERN

"There is a new buzz about the laboratory and a real sense of anticipation," CERN Director General Rolf Heuer said at the EuroScience Open Forum in Copenhagen, Denmark. "Much work has been carried out on the LHC over the last 18 months or so, and it’s effectively a new machine, poised to set us on the path to new discoveries."

During the upcoming campaign, the machine is expected to come close to its maximum collision energy of 14 trillion electron volts. That could open up a new frontier for particle physics, perhaps bringing evidence of extra dimensions, dark matter particles and supersymmetry.

"The discovery of a Higgs boson was just the beginning of the LHC’s journey," senior CERN physicist Fabiola Gianotti said in Copenhagen. "The increase in energy opens the door to a whole new discovery potential."