GENEVA, Switzerland - Scientists working at the world's biggest atom smasher plan to announce Wednesday that they have gathered enough evidence to show that the long-sought particle that may answer fundamental questions about the universe almost certainly does exist.

But after decades of work and billions of dollars spent, researchers at the European Organization for Nuclear Research (CERN) aren't quite ready to say they've "discovered" the particle.

A number of Philadelphia-area scientists have spent years on the hunt, including contingents from Princeton and Rutgers Universities. Twenty-seven University of Pennsylvania scientists have been involved with the project, looking through one of two windows into the world exposed by the collider. That window, named Atlas, is a doughnut-shaped detector, 80 feet in diameter, designed to detect exotic particles like the Higgs. Another team is using a similarly large and elaborate detector called CMS.

The teams compete with each other so they can provide independent confirmation of each other's work.

Experts familiar with the research at CERN's vast complex on the Swiss-French border say that the massive data they have obtained will essentially show the footprint of the key particle known as the Higgs boson - all but proving it exists. But the findings don't allow them to say it has actually been glimpsed.

It appears to be a fine distinction.

Senior CERN scientists say the two independent teams of physicists who plan to present their work at CERN's vast complex on the Swiss-French border on Wednesday are about as close as you can get to a discovery without actually calling it one.

"I agree that any reasonable outside observer would say, 'It looks like a discovery,' " British theoretical physicist John Ellis, a professor at King's College London who has worked at CERN since the 1970s, told the Associated Press. "We've discovered something which is consistent with being a Higgs."

The evidence for the Higgs is statistical, constructed out of a lot of tracks in the detectors, any given one of which may not be a signature of the Higgs.

Penn professor Brig Williams explained that there are three major patterns of debris that are likely to be left behind by the Higgs.

One possible Higgs track entails two high-energy photons. The other two involve pairs of exotic particles called W and Z, which themselves spontaneously disappear and become more commonplace particles that speed through the detectors. The problem is that any of these signals could show up without a Higgs ever materializing.

Last December CERN researchers had collected enough potential Higgs candidates to reduce their odds of a false sighting to one in a thousand. That sounds good, said Williams, but back then, they were hunting for an unexpected signal over a large range of energy, so the odds were not so remote that something weird would crop up.

Now they're looking for something more specific, since they have narrowed down the mass of the possible Higgs. Seeing a signal in a specific range would give them much more confidence in their statistics. What they really want is less than a one-in-a-million chance they have seen a statistical fluke.

The discovery of the Higgs boson won't change people's lives, but will help explain the underpinnings of the universe. It would confirm the standard model of physics that explains why fundamental particles have mass. Those particles are the building blocks of the universe.

Though an impenetrable concept to many, the Higgs boson has until now been a concept intended to explain a riddle: How were subatomic particles, such as electrons, protons and neutrons, themselves formed? What gives them their mass?

The answer came in a theory first proposed by physicist Peter Higgs and others in the 1960s. It envisioned an energy field where particles interact with a key particle, the Higgs boson.

Scientists with access to the new CERN data say it shows with a high degree of certainty that the Higgs boson may already have been glimpsed, and that by unofficially combining the separate results from the two teams, called Atlas and CMS, it can be argued that a discovery is near at hand.

CERN spokesman James Gillies said Monday, however, that he would be "very cautious" about unofficial combinations of Atlas and CMS data. "Combining the data from two experiments is a complex task, which is why it takes time, and why no combination will be presented on Wednesday," he told the AP.

Sean M. Carroll, a California Institute of Technology physicist flying to Geneva for the announcement, said that if both Atlas and CMS had independently reached these high thresholds on the Higgs boson, then "only the most curmudgeonly will not believe that they have found it."