THE search for the elusive Higgs particle has maddened physicists since the particle鈥檚 existence was proposed in the 1960s. And now they know why. A new analysis indicates the particle is heavier than anyone expected.
Finding the Higgs is important because it is the only missing piece in one of the leading theoretical jigsaws of modern physics, the standard model of particle physics. The theory says that all matter is composed of six types of quark 鈥� called up, down, strange, charm, bottom and top 鈥� as well as much lighter particles such as electrons and neutrinos.
There is convincing evidence for the existence of all these particles, but the model also requires the existence of the Higgs particle. The Higgs supposedly drags on other particles as if miring them in molasses, thereby endowing them with mass. Proof of its existence would cement the theory firmly in place, but so far nobody has found convincing evidence for the Higgs.
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And not for lack of trying. During the 1990s, physicists searched for the Higgs using the 鈧�1 billion Large Electron Positron collider (LEP) at CERN, the European particle physics laboratory in Geneva, Switzerland. LEP was able to look for particles at energies up to and beyond the Higgs鈥檚 mass, then predicted to be around 88 gigaelectronvolts (GeV). But in 2001, physicists admitted defeat, saying that a search up to LEP鈥檚 limit of 114 GeV had failed to produce conclusive evidence. Most decided the Higgs must be even heavier.
Now it turns out they were right. A new analysis indicates that the Higgs鈥檚 most likely mass is 117 GeV, just out of LEP鈥檚 reach. And it could weigh as much as 251 GeV (Nature, vol 429, p 638). The revised prediction has caught many by surprise. 鈥淣obody thought it would change by this much,鈥� says Georg Weiglein, a particle physicist at the University of Durham in the UK who has studied the new analysis.
The prediction comes courtesy of the D0 (pronounced 鈥渄ee zero鈥�) experiment at Fermilab in Chicago. D0 collects sightings of the top quark and measures its mass. This data can then be fed into calculations for the Higgs mass. In 2001, student Juan Estrada and his supervisor Gaston Gutierrez had a bright idea about how to improve the accuracy of these calculations. Instead of taking all the measurements of top quark mass and averaging them, they reasoned, why not take into account the fact that some measurements are better quality than others?
鈥淲e weigh each event by how well each event was measured,鈥� Estrada says. The new average, calculated with the help of fellow student Florencia Canelli, gives a top quark mass that鈥檚 more precise, but also higher than previously thought. And that means the Higgs would have to be heavier.
The new results have changed the balance in the race to find the Higgs. After LEP was decommissioned in 2000, the leading contender for finding the particle was the Tevatron accelerator at Fermilab, which is expected to reach the required energies in the next few years. But the new higher-mass estimate means the Tevatron is now less likely to spot it.
The favourite is now CERN鈥檚 Large Hadron Collider (LHC) which is scheduled to start up in 2007 and designed to pick up the search where LEP left off. It will hunt for the Higgs at energies up to 251 GeV and beyond. 鈥淚f the Higgs mass is higher, it鈥檚 most likely to be found at the LHC,鈥� Estrada admits.