The ultra-shy Higgs boson may have finally shown itself at the LHC. Both of the main detectors, ATLAS and CMS, have uncovered hints of a lightweight Higgs. If it pans out, the only remaining hole in the standard model would be filled.
Even more exciting, a Higgs of this mass, about 125 gigaelectronvolts, would also blast a path to uncharted terrain. Such a featherweight would need at least one new type of particle to stabilise it. "It's very exciting," says CMS spokesman Guido Tonelli. "This could be the first ring in a chain of discoveries."
As the leading theory for how particles and forces interact, the standard model has been spectacularly successful since it was proposed in the 1960s. But it works only on the assumption that the Higgs boson actually exists. The particle is the calling card of an unseen entity called the Higgs field, which is thought to give all particles their mass. The trouble is the standard model cannot predict what the Higgs itself weighs.
So physicists have been hunting for the simplest version of the Higgs at various particle colliders for years. Experiments have steadily ruled it out at a range of masses, except for a narrow window between 115 and 141 GeV.
Now physicists at the Large Hadron Collider at CERN, near Geneva in Switzerland, have probed that range in more detail than ever before. Today, Tonelli and Fabiola Gianotti, head of the ATLAS detector, separately presented results from more than 300 trillion high-speed particle collisions made in the last year. "This is the first time we're really exploring the entire [mass] region with the right sensitivity ? the one that will allow you, if there is something there, to start seeing something," says Tonelli.
Similar mass
The ATLAS data restricts the Higgs to within 115 and 131 GeV; CMS rules out a Higgs heavier than 127 GeV.
Most excitingly, ATLAS saw a tantalising hint of the Higgs at 126 GeV; CMS saw one at 124 GeV. It is the first time both experiments have seen a signal at nearly the same mass. "We're very competitive, but once I see they're coming with results, I'm happy," Tonelli says. "Their results are important for us. They're obtained in a completely independent manner."
The Higgs is expected to appear fleetingly in the wreckage of high-speed proton collisions at the LHC, but cannot be seen directly. Instead physicists look for the shower of lighter particles and photons that result from the decay of Higgs bosons of various masses. Because garden-variety particles also produce the same decay products, Higgs hunters look out for suspicious excesses of these products in their detectors.
Tentative signal
Although both teams see an excess around the same mass, there is not yet enough data to claim a discovery. The ATLAS signal has a statistical significance at 126 GeV of 2.3 sigma, meaning that the result has around a 2 per cent chance of being down to a random fluctuation; the comparable excess at CMS has a significance of just 1.9 sigma. To claim a discovery you need a 5 sigma signal, meaning there is less than 1 in a million chance of the result being a fluke. "There's clearly not enough to conclude anything at this stage," Gianotti says. "It could be something interesting, or just a fluctuation."
Even a hint of a 125-GeV Higgs has some theorists sighing with relief. Although the standard model can't predict the particle's mass directly, it does predict how other particles interact with the Higgs ? in particular, the W and Z bosons that are responsible for the weak nuclear force.
Earlier experiments found that the W and Z bosons weigh 80.4 and 91.2 GeV, respectively. Because of the way those particles interact, the Higgs mass probably comes out somewhere between about 115 and 130 GeV. A Higgs at 125 GeV or so "is just what the doctor ordered," says Nobel laureate Frank Wilczek of the Massachusetts Institute of Technology.
New physics
That mass also paves the way for physics beyond the standard model. Thanks to subtle quantum mechanical effects, a lightweight Higgs needs a heavier companion particle "acting as a sort of bodyguard", Tonelli says. Otherwise, the quantum vacuum from which particles appear would be unstable, and the universe would long ago have disintegrated. If the Higgs is lightweight, the fact that we are here today suggests there is at least one extra particle beyond the standard model.
Wilczek thinks that's great news. It leaves the door open for one of the most mathematically beautiful extensions of the standard model. Supersymmetry or SUSY for short, suggests that every known particle has an as yet-undetected partner and promises to resolve a lot of the standard model's shortcomings. It can unite the strong and weak nuclear forces with the electromagnetic force and offers a candidate for dark matter.
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