CERN physicists find hint of Higgs boson
The Large Hadron Collider detected events that could pin down the mass of the elusive subatomic particle, but there's still plenty of uncertainty.
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Researchers at the CERN particle accelerator have found "intriguing hints" of the Higgs boson, a moment of major progress in years of previously unfruitful searching for the elusive subatomic particle.
The search for the Higgs boson is the top priority of CERN's massive and expensive Large Hadron Collider near Geneva, Switzerland. Its Atlas experiment showed a statistically suspicious increase in activity that indicates the Higgs could be pinned down with a mass of 126 giga-electron-volts, and showing some important agreement, its independent CMS experiment found a possible result nearby at 124GeV.
"We observe an excess of events around mass of about 126 GeV," CERN physicist and Atlas leader Fabiola Gianotti said in slides presented today at a CERN seminar to physicists who applauded her results. That equates to about 212 quintillionths of a gram; by comparison, a proton is more than 100 times lighter with a mass of 0.938GeV.
Her small sentence carries big import for physics. That's because the Higgs boson, thought by some to endow other particles with mass, is a key missing ingredient in physicists' understanding of what makes the universe tick. It's predicted by the Standard Model of particle physics, but no one has been able to confirm its existence or nature.
"The Higgs could be the first link in a chain of discovery. This is what we hope," said Guido Tonelli of the Universita degli Studi di Pisa and leader of the CMS project, in a news conference after the seminar. Another year of continued data gathering should be enough to provide a conclusive answer on this particular matter, the physicists said.
Gianotti called the findings "beautiful results" at the seminar, but stopped well short of declaring victory because there's not enough data for statistical certainty. "It's too early to draw definite conclusions...We believe we have built a solid foundation on the exciting months to come."
Finding the Higgs boson is essentially a matter of checking for a variety of events--or their absence. The LHC's detectors have been gradually ruling out ranges of possible mass for the Higgs boson.
"The window for the Higgs mass gets smaller and smaller," and today we saw "intriguing hints" of its possible nature, said CERN Director General Rolf Heuer. "We have not found it yet. We have not excluded it yet. Stay tuned for next year."
The Higgs boson isn't observed directly, but rather is detected by extremely rare side effects of collisions between protons smashing into each other. To increase the likelihood of collisions, the LHC operators have been gradually increasing the beam intensity.
Gianotti also said the CMS results predict with a 95 percent confidence level that the Higgs boson has a mass between 115.5GeV and 131GeV.
Another experiment, the Compact Muon Solenoid (CMS), also helped narrow down the possible mass of the Higgs boson. Its results showed with a 95 percent confidence level that the particle can't be between 127GeV and 600GeV, Tonelli said.
The CMS experiment also found "a modest excess of events" that could be evidence of the Higgs boson between 115GeV and 127GeV, Tonelli said in a presentation at the seminar. "The excess is most compatible with a Standard Model Higgs hypothesis in the vicinity of 124GeV and below, but the statistical significance is not large enough to say anything conclusive."
One of the big mysteries that physicists hope to plumb with the Higgs is an idea called supersymmetry. The Standard Model predicts a wide range of particles, of which the Higgs is the last to be pinned down. But with supersymmetry, each of the conventional elementary particles in the standard model, including the Higgs, has a companion. If there's only one Higgs boson, it's part of the Standard Model. But with supersymmetry, there have to be at least five Higgs bosons. Supersymmetry would double the number of particles to resolve physics problems in a similar way that the prediction--and later discovery--of antimatter did decades ago.
If the Higgs boson weighs about 125GeV, it would match many physicists' general expectations--but also carry some importance. That's because it's at the light end of the range of possibilities, and physicists believe a particle that light needs another particle from the sypersymmetry collection to anchor it.
"Our expectation is that you have something heavy. It could be something related to SUSY," Tonelli said, referring to the nickname for supersymmetry theory. "Or maybe not," he added.
When it comes to mass, physicists liken the Higgs boson to groupies at a party. Heavy particles interact strongly with Higgs bosons, equivalent to a lot of people swarming a celebrity and making it harder for the famous person to start moving and, once moving, harder to stop. Particles with little mass are those that interact weakly with Higgs bosons, making them more fleet-footed.
"A heavier particle is nothing more that one than has more interactions with the Higgs particle as it passes through the vacuum," said Lawrence Sulak, chairman of Boston University's physics department.
If the Higgs boson is precisely measured in the next year, the LHC can be used to look further down the same pathway, Tonelli added, possibly finding supersymmetric particles--"if they are in the energy range of the LHC."
Such particles would likely be vastly heavier--many thousands, perhaps millions, of GeVs, he said.
That would be quite a coup: supersymmetric particles are a possible explanation for dark matter, material that in the universe outweighs the ordinary matter of which we're made but that generally interacts with ordinary matter only through gravitational pull.
To find harder particles, CERN plans an LHC upgrade that will let protons be smashed together at twice today's energy level. "Hopefully we'll explore a large region of masses," Tonelli said. And then, the supersymmetry work can begin in earnest. "A lot of parameters are still open, a lot of SUSY models are still open and are waiting to be excluded or confirmed," he said.
The LHC is a huge, phenomenally complex instrument built in a circular subterranean tunnel 27 kilometers in circumference. It can accelerate protons fast enough that, when they collide, they reproduce energy levels found only in the earliest moments of the universe after the Big Bang.
Updated at 7:26 a.m. PT and 10:44 a.m. PT with further detail.
