The 7,000-ton ATLAS detector was one of the two key instruments the Large Hadron Collider that found the Higgs.
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"My God!" Gianotti exclaimed, jumping up in her chair after she was brought the readouts proving that the Higgs had been found. Maybe it was just an exclamation, but the empiricist nonetheless took care to correct herself at the press conference later. "Thanks, nature!" she called out. But it was too late; the cat was out of the bag. She and her colleagues were grappling with something bigger than mere physics, something that defies the mathematical and brushes up--at least fleetingly--against the spiritual.
Keeping the Cosmos Sane
Despite its bland name, the standard model of particle physics describes some pretty elegant stuff. Completed in the 1970s after decades of work by physicists all over the world, the theory describes three of the great engines that run the universe: the weak nuclear force, the strong force and electromagnetism.
The weak force is carried by two particles--the W and Z bosons--and, as its name suggests, bonds matter loosely and over very short distances. Its tenuous grip on things is what leads to radioactive decay and, much more happily, initiates the hydrogen fusion that keeps the lights burning in stars like the sun. The strong force is a more robust thing: it causes protons and neutrons to come together in the nucleus of an atom. Carried by gluons, it is also the force that binds the quarks that make up protons. Electromagnetism is the force behind such phenomena as light and other everyday waves from radio to X-rays.
Neat, simple, almost intuitive. Except for one thing: all the particles at play in the model--except photons, which transmit light--have mass. And mass needs something to coax it into existence. Enter the Higgs boson. As Higgs and his collaborators explained things, the universe is filled with an energy field through which energetic particles must move the way an airplane has to push its way through a stiff headwind. Higgs bosons suffuse the field and are drawn to the particles; the more energetic particles attract more bosons, the less energetic ones attract fewer. This clustering gives the particles the solidity we associate with matter--and it does something else too. "The Higgs boson has two functions," says Gianotti. "One is to give mass. The other is to prevent the standard model from going bananas."
Bananas, in this case, means the standard model would fall apart. Avoiding that mess was a half-century job, but the pace picked up dramatically in the past two years thanks to work conducted by the LHC and the recently shuttered Tevatron collider outside Chicago. In both facilities, physicists didn't study the proton collisions themselves so much as the quantum debris in the form of other particles that results from them. The goal was to find some that weigh in at 125 billion GeV (or electron volts), the mass predicted for the Higgs.
Lots of bumps appeared in the data at or around that target weight, but the Tevatron was never powerful enough to pin things down firmly, and the LHC, which went to work in 2008, has come online slowly over the years and did not achieve enough propulsive oomph to prove the Higgs case until 2011. Even then, it took trillions of proton crack-ups to produce enough readings to get to what physicists call the five-sigma level of certainty--and what everyone else calls the eureka moment.