US-based physicists reported finding strong hints of the Higgs boson, the elusive "God particle" believed to give objects mass, but said European data is needed to confirm any potential discovery.
If physicists can confirm the existence of the Higgs boson, the last missing piece in the standard model of physics, the announcement would rank among the most important scientific breakthroughs of the last century.
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The final findings from Fermi National Accelerator Laboratory (Fermilab) in the midwestern US state of Illinois will be followed by the announcement of more definitive results from a potent European atom-smasher on Wednesday.
"Our data strongly point toward the existence of the Higgs boson, but it will take results from the experiments at the Large Hadron Collider in Europe to establish a discovery," said Fermilab spokesman Rob Roser.
The results come from 10 years of data from the Tevatron, a powerful atom-smasher that began its collider work in 1985 and closed down last year.
"During its life, the Tevatron must have produced thousands of Higgs particles, if they actually exist, and it's up to us to try to find them in the data we have collected," said Luciano Ristori, a physicist at Fermilab and Italy's National Institute for Nuclear Physics, or INFN.
"We have developed sophisticated simulation and analysis programs to identify Higgs-like patterns. Still, it is easier to look for a friend's face in a sports stadium filled with 100,000 people than to search for a Higgs-like event among trillions of collisions."
Difficult to pin down
The Higgs boson, named after Scottish physicist Peter Higgs, was first described in the 1960s and has been notoriously difficult to pin down.
"The Higgs boson is special," Fermilab theoretical physicist Joe Lykken told reporters, adding that the tough-to-find elementary particle "gets at why the universe is here in the first place."
Lykken said it can be thought of almost like an energy field that gives mass to objects. But it decays almost immediately into other particles.
Furthermore, just one in a trillion collisions in an atom-smasher experiment will produce a Higgs boson.
"This is much worse than a needle in a haystack," Lykken said, adding that he and many other physicists are eagerly anticipating the European results.
"We think we are getting very, very close to where we want to be, and by the end of the week we may be much closer."
The Tevatron results show that the Higgs particle, if it exists, has a mass between 115 and 135 gigaelectronvolts (GeV/c2), or about 130 times the mass of the proton.
Based on two experiments, known as CDF and DZero which include nearly 1,000 physicists from more than a dozen different countries, the team found that there is only a one-in-550 chance that the signal is a mere statistical fluke.
However, the statistical significance of the signal measures 2.9 sigma, and is not strong enough to meet the five sigma threshold required to say whether or not the particle has been discovered.
"We achieved a critical step in the search for the Higgs boson," said Dmitri Denisov, DZero spokesman and physicist at Fermilab.
"While 5-sigma significance is required for a discovery, it seems unlikely that the Tevatron collisions mimicked a Higgs signal. Nobody expected the Tevatron to get this far when it was built in the 1980s."
A more powerful machine at the European Center for Nuclear Research in December 2011 announced "tantalizing hints" that the sought-after particle was hiding inside a narrow range of mass.
CERN's Large Hadron Collider -- the world's largest atom-smasher, located along the French-Swiss border -- showed a likely range for the Higgs boson between 115 to 127 gigaelectronvolts.
US-based experiments echoed those findings in March 2012, though in a slightly larger range.
Now, the scientific community is eagerly anticipating the European results, expected at 0700 GMT on Wednesday from the CERN particle physics laboratory in Geneva, Switzerland.
"It is a real cliffhanger," said DZero spokesman Gregorio Bernardi, physicist at the Laboratory of Nuclear and High Energy Physics at the University of Paris VI and VII. "We are very excited about it."