Physicists say they have all but proven that the “God particle” exists. They have a footprint and a shadow, and the only thing left is to see for themselves the elusive subatomic particle believed to give all matter in the universe size and shape.

Scientists at the world's biggest atom smasher plan to announce Wednesday that they have nearly confirmed the primary plank of a theory that could restructure the understanding of why matter has mass, which combines with gravity to give an object weight.

The idea is much like gravity and Isaac Newton's discovery: It was there all the time before Newton explained it. But now scientists know what it is and can put that knowledge to further use.

The focus of the excitement is the Higgs boson, a subatomic particle long sought by physicists.

Researchers at the European Organization for Nuclear Research, or CERN, say that they have compiled vast amounts of data that show the footprint and shadow of the particle, even though it has never actually been glimpsed.

But two independent teams of physicists are cautious after decades of work and billions of dollars spent. They don't plan to use the word “discovery.” They say they will come as close as possible to a “eureka” announcement without overstating their findings.

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

CERN's atom smasher, the $10 billion Large Hadron Collider on the Swiss-French border, has been creating high-energy collisions of protons to investigate dark matter, antimatter and the creation of the universe, which many theorize occurred in a massive explosion known as the Big Bang.

The phrase “God particle,” coined by Nobel Prize-winning physicist Leon Lederman, is used by laymen, not physicists, more as an explanation for how the subatomic universe works than how it all started. (AP)

Rob Roser, who leads the search for the Higgs boson at the Fermilab in Chicago, said: “Particle physicists have a very high standard for what it takes to be a discovery,” and he thinks it is a hair's breadth away. Roser compared the results that scientists will announce Wednesday to finding the fossilized imprint of a dinosaur: “You see the footprints and the shadow of the object, but you don't actually see it.”

Fermilab, whose competing atom smasher reported its final results Monday after shutting down last year, said its data doesn't settle the question of the Higgs boson, but it came tantalizingly close.

“It's a real cliffhanger,” said Gregorio Bernardi, a physicist at the University of Paris who helped lead one of the main experiments at Fermilab. He cited “strong indications of the production and decay of Higgs bosons” in some of their observations.

Fermilab theorist Joseph Lykken said the Higgs boson “gets at the center, for some physicists, of why the universe is here in the first place.”

Though an impenetrable concept to many, the Higgs boson has until now been just that _ 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 Scottish physicist Peter Higgs and others in the 1960s. It envisioned an energy field where particles interact with a key particle, the Higgs boson.

The idea is that other particles attract Higgs bosons and the more they attract, the bigger their mass will be. Some liken the effect to a ubiquitous Higgs snowfield that affects other particles traveling through it depending on whether they are wearing, metaphorically speaking, skis, snowshoes or just shoes.

<관련 한글 기사>

신의 입자 '힉스', 그 정체 밝혀지나...

힉스 입자(Higgs Boson )가 정말 있는지에 대한 연구가 한 걸음 진전했지만, '발견'을 선언하기에는 아직 이르다고 미국 연구진이 발표했다.

미 페르미연구소 연구진은 2일(현지시간) 힉스 입자가 118~132기가전자볼트(GeV )의 질량을 가지는 것으로 추정된다고 밝혔다.

힉스 입자는 우주 탄생에 대한 대폭발 이론을 구성하는 표준 모형(standard mod el)에서 폭발 직후 물질에 어떻게 질량을 부여했는지를 규명할 단서로 '신의 입자'라고도 불린다.

GeV는 원자보다 더 작은 아(亞)원자 세계의 질량을 측정하는 표준 단위로, 1GeV 는 대략 양성자 1개의 질량과 비슷하다.

연구진은 지난해 가동을 멈춘 입자가속기 '테바트론'에서 얻어진 실험 자료를 분석해 이번 결과를 얻었다고 설명했다.

이들은 입자를 빛의 속도에 육박하도록 움직인 다음 충돌시키는 과정에서, 만약 에 힉스 입자가 존재한다면 물질의 기본 요소인 소립자들 중 특정한 쿼크가 나타날 것이라고 가정하고 충돌 실험을 반복해 왔다.

그 결과 힉스 입자의 존재를 증명하는 것으로 보이는 경우가 약 1천번 나타났다.

그러나 연구진은 이 분석 결과 역시 힉스 입자를 '발견'했다고 선언하기에는 불 충분하다는 입장이다.

페르미연구소의 롭 로저 연구원은 "이번 결과는 힉스 입자의 존재를 강력하게 시사한다"며 "현 시점에서 가장 훌륭한 답변"이라고 의미를 부여했다.

연구진은 유럽입자물리연구소(CERN)에서 강입자가속기(LHC)를 이용해 얻은 실험 결과를 발표하면 페르미연구소의 성과가 힉스 입자의 존재 여부를 증명할 수 있는 수준인지에 대한 판단이 이뤄질 것이라고 예상했다.

페르미연구소와 달리 CERN에서는 충돌로 인해 높은 에너지 수준의 광자(光子) 쌍이 발견되면 힉스 입자의 존재를 증명할 수 있다는 가설 아래 연구를 진행하고 있 다.

CERN 연구진들 또한 힉스 입자의 흔적을 보여줄 수 있는 방대한 양의 자료를 축적해 왔다고 말해 왔다.

이 연구소는 오는 4일 호주에서 열리는 물리학 학회를 통해 실험 결과를 발표할 계획이다.