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At Long Last, Physicists Discover Famed Higgs Boson

4 July 2012 6:39 am
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CMS and ATLAS collaborations

Twin peaks. Both the CMS (top) and the ATLAS (bottom) detectors see evidence of the Higgs boson decaying into a pair of photons in the form of a peak in a so-called mass plot. The agreement of the two peaks and other data clinch the discovery of the Higgs.

MEYRIN, SWITZERLAND—The long wait is over. Today, physicists working with the world's largest atom smasher here at the European particle physics laboratory, CERN, reported that they have discovered the long-sought Higgs boson—the last missing bit in their standard model of fundamental particles and forces and the key to their explanation of how all the other fundamental particles get their masses.

The discovery fulfills a 48-year-old prediction and marks a signal intellectual achievement. But even as physicists celebrate, the discovery raises worries among some that there may remain no new physics that can be discovered with the atom smasher. For particle physics, the discovery of the Higgs could be the end of the road.

The data come from the Large Hadron Collider (LHC), a 27-kilometer-long subterranean particle accelerator that circles below the French-Swiss border near Geneva. The LHC smashes protons together at enormous energy to blast into existence new and fleeting subatomic particles. The protons collide within two huge particle detectors, called ATLAS and CMS, that are hunting the Higgs. Any Higgs bosons created in them should decay into particular combinations of particles that are visible to the detectors. Today, the ATLAS and CMS teams both presented their latest data for the Higgs search at a special seminar at CERN. And both teams see essentially definitive signals.

For example, the roughly 3000 researchers working on the CMS detector see clear signs of the Higgs decaying into two photons. From the energies of the two photons, physicists can infer the mass of their supposed parent particle. And when CMS researchers make a plot of the masses of the inferred particles, they see a clear peak atop a background produced by random photon pairs (see figure). That peak signals the presence of a Higgs-like particle with a mass of 125 giga-electron volts (GeV), or about 133 times the mass of the proton, as Joseph Incandela, a physicist at the University of California, Santa Barbara, and spokesperson for the CMS team, told a packed auditorium at CERN.

CMS researchers also see evidence of the Higgs decaying into a pair of particles called W bosons or a pair of particles called Z bosons, Incandela reported. Those massive particles convey the weak nuclear force in exactly the same way that photons convey the electromagnetic force. Including decays into still more combinations of familiar particles, the data leave little doubt that the Higgs-like particle is there. The chances that random statistical fluctuations could produce such signals are just a hair higher than the arbitrary 1-in-3.5-million level that particle physicists hew to for an official discovery—the so-called 5-sigma standard.

The ATLAS team sees a similar peak in the mass plot for Higgses decaying into photon pairs, reported Fabiola Gianotti, a physicist at CERN and spokesperson for the ATLAS experiment. And ATLAS researchers also see the Higgs decaying into Z bosons and other combinations of particles. Taken together, ATLAS's signals just meet the 5-sigma standard of discovery, Gianotti reported, earning immediate applause.

At the end of the seminar, physicists greeted the results with a long standing ovation and whoops of approval. "I think we've got it," said CERN Director General Rolf Dieter Heuer. "You agree?" Peter Higgs, the 83-year-old theorist from the University of Edinburgh in the United Kingdom who predicted the boson's existence in 1964 and who was on hand for the event, said, "I'd like to thank the experimenters. I didn't think I'd see this in my lifetime."

Physicists still need to test whether the observed particle has precisely the properties that the standard model predicts it should, stresses John Ellis, a theorist at King's College London. For example, researchers must compare the relative rates at which Higgses decay into different combinations of particles, as those rates are predicted by theory. But the fact that the particle was discovered through the predicted decays in the first place suggests it cannot be too wildly different from the standard model Higgs, Ellis acknowledges.

If it really is the Higgs boson, then the discovery will fulfill a prediction made decades ago by Higgs, although others developed some of the same basic ideas at roughly the same time. That would be just the latest in a handful of profound predictions made by particle physicists. For example, in 1970, theorists predicted the existence of a particle called the charm quark; two experimenters independently discovered the particle in 1974, for which they received the Nobel Prize in physics 2 years later. In 1968, theorists predicted the existence of the W and Z bosons; in 1983, those particles were also discovered. In that case the theory behind the prediction won the Nobel Prize in 1979 and the discovery won it in 1984.

The prediction and discovery of the Higgs boson are as important as those of the W and Z, says Stuart Raby, a theorist at Ohio State University, Columbus. "It's certainly on a par," he says. "It's certainly as fundamental."

Now that the standard model of particle physics is complete, a key question is whether new particles lie within the reach of the LHC or any higher-energy atom smasher that might come after it. Physicists say that conceptual holes in the standard model strongly suggest that the theory is incomplete. For example, in the standard model, interactions between the Higgs and the other particles ought to force the mass of the Higgs to skyrocket to a value a trillion times larger. Yet that doesn't happen. So most physicists suspect there are new particles out there that somehow counteract ballooning of the Higgs mass.

But will such particles have low enough masses to be discovered with any conceivable humanmade atom smasher? "There's absolutely no guarantee," says Steven Weinberg, a theorist at the University of Texas, Austin, who shared the 1979 Nobel Prize for the theory behind the W and the Z. "My nightmare, and it's not just me, but a lot of us [in particle physics], is that the LHC discovers the Higgs boson and nothing else," Weinberg says. "That would be like closing a door."

But most physicists say they are optimistic that, now that the LHC has discovered the Higgs, other more surprising discoveries will follow. For now, the discovery of the Higgs is a dream come true.

*The story has revised to correct the year in which the Nobel Prize in physics honored the theory behind the W and Z bosons from 1972 to 1979.

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