10 July 2012
Many physicists here spent the night huddled in the hall so that they could secure a prized seat. By 8:00 a.m., the fire brigade was turning away bleary-eyed scientists who had queued for hours. The lucky few who made it inside the lecture theatre at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, witnessed the end of an epic quest in high-energy physics — and the start of a new campaign.
With the announcement on 4 July that they had found the Higgs boson, physicists unveiled the final piece of the standard model of particle physics: a theoretical framework that describes with pinpoint accuracy all fundamental particles and forces except gravity. Discovering the Higgs boson had been billed as the main goal of the Large Hadron Collider (LHC), a US$6-billion, 27-kilometre-circumference proton collider that, along with its four building-sized detectors, took thousands of physicists decades to assemble.
The discovery has given the machine a new mission: to pin down the properties of the Higgs boson. Researchers will also be scouring the data for hints of something beyond the standard model — a still-more comprehensive theory that could lead physicists towards a unified understanding of the Universe.
The greatest particle-physics discovery in a generation appeared as no more than a modest bump on a gently sloping plot (see ‘Bump of destiny’). Yet it drew a burst of applause as the two main experimental groups seeking the particle flashed their data onto the screen last week. The bump was the clear signal of a Higgs particle at a mass of around 125 gigaelectronvolts (mass and energy are used interchangeably in particle physics). Both the ATLAS and the Compact Muon Solenoid (CMS) detector groups reported that the significance of their signal was around five standard deviations — meaning that if the Higgs particle did not exist, there would be less than a one-in-a-million chance of getting these data by chance.
Theorist Peter Higgs himself was present for the announcement, as were three of the other physicists who first predicted the boson back in 1964: Gerald Guralnik, François Englert and Carl Hagen. “It’s really an incredible thing that it’s happened in my lifetime,” the 83-year-old Higgs told the crowd, choking back tears.
The Higgs boson is an expression of the Higgs field — the mechanism ultimately responsible for the mass of known particles (see ‘What is the Higgs?’). Evidence for the Higgs boson had been mounting for decades, says Tom Kibble, a retired physicist at Imperial College London, and another of the theorists responsible for the original prediction. The boson and field are needed in calculations to unify the electromagnetic and weak nuclear forces into a single ‘electroweak’ force, which in turn predicts the properties of other particles. Those predictions have matched measurements to high accuracy, he says. Nevertheless, Kibble says, “there are quite a few things that we don’t know about it that have to be confirmed”.
The key quantity is the particle’s spin — a defining quantum-mechanical property. According to theory, the Higgs boson’s spin must be zero. Otherwise, says Kibble, the masses of fundamental particles could change according to their orientations in space. “Spin zero is key,” he says. If the particle turns out to have a non-zero spin, it would be a shocking discovery — and would mean that the particle would be something other than the Higgs.