Rolf Heuer, director of the European Centre for Nuclear Research (CERN) in Meyrin, Switzerland, near the border with France, made the announcement early Wednesday, saying that researchers "have now found the missing cornerstone of particle physics."
He described the discovery as a boson, a broader class of subatomic particle, but stopped short of confirming that it's a Higgs boson — an extremely fine distinction.
"As a layman, I think we did it," he said. "We have observed a new particle that is consistent with a Higgs boson."
The Higgs boson has been labelled the "God particle" in the mainstream media because of the fundamental questions it could answer about matter and the creation of the universe, and although most physicists avoid using the term, they do agree that the Higgs boson plays a key role in what is known as the Standard Model of particle physics, which describes the particles from which everything in the universe is made and how they interact.
Theory 1st proposed in 1960s
The Standard Model includes common subatomic particles like electrons and protons along with less familiar ones like muons.
The Higgs boson is the only one that remains undetected in experiments because it lives for only a tiny fraction of a second before decaying into other subatomic particles, such as photons, muons or leptons. The only way to measure it is to measure the products of its decay.
The reason physicists have been so determined to find the Higgs boson is because it is believed to impart mass to all the other fundamental particles.
"It's important to realize it's not responsible for most of the mass we are made of, but it's involved in the mass of fundamental particles," explained Itay Yavin, assistant professor of physics at McMaster University in Hamilton, in a briefing about the discovery prepared by the Science Media Centre.
"It plays an important role, and it's needed to make the Standard Model consistent, but it's not much more important than other particles — it's the last ingredient in this beautiful puzzle called the Standard Model."
The existence of a Higgs boson was first proposed by British physicist Peter Higgs and his colleagues at Edinburgh University in 1964 as a way of explaining how particles gained mass, a property that at first did not seem to fit into the Standard Model of how electrons and protons interact.
Higgs proposed that particles gain mass by interacting with a medium, or Higgs field, that exists everywhere in space and is made up of unseen particles called bosons.
Higgs's theory does not assign a specific mass to the boson itself but gives a range of values for the potential masses it could have. Based on these values, physicists can determine the amount of secondary particles that the boson decays into that they should expect to find when observing high-speed particle collisions.
This is the "footprint" of the Higgs boson that scientists said they found when analysing trillions of high-speed proton-proton collisions within CERN'sLarge Hadron Collider.
The CERN physicists conducted two separate but parallel experiments over 2011 and 2012 using two particle-collision detectors called ATLAS and CMS that are part of the collider.
The scientists said data from both detectors gave "strong indications for the presence of a new particle, which could be the Higgs boson, in the mass region around 126 gigaelectronvolts [about 130 times the mass of a proton]."
TRIUMF physicists involved in hunt
Wednesday's announcement follows decades of work and billions of dollars spent on a project that has involved scientists in dozens of countries, including more than 100 in Canada.
To mark the occasion in Canada, a few dozen physicists and other experts at TRIUMF, a particle physics lab in Vancouver that has been involved with the hunt for the Higgs boson, gathered overnight to celebrate.
"It's a big day for CERN, a big day for international science, and a big day for science in Canada," said Oliver Stelzer-Chilton, a physicist at the TRIUMF lab.
"But now, we have to come to the bottom of it," he said. "What is nature telling us that this new particle is?"
"At this point, we can say it's consistent with the properties that we expect from a Higgs boson, but we basically have to measure those properties to be sure."
Although CERN said the discovery met the high experimental certainty threshhold known as 5 sigma, indicating that there is only the tiniest chance — about one in three million — that the results are down to pure chance, it will take years to confirm that the particle is, indeed, a Higgs boson.
"The chance of either of these experiments being wrong is less than one in a million, and for both of them to be wrong is more like one in a trillion," University of Toronto physics professor Pekka Sinervo, who was invovled in analysing the Higgs boson data, told the Science Media Centre.
"We can safely conclude something new is there. … All the evidence suggests it's the Higgs boson, but the results released today just aren't strong enough to conclude that it is the Higgs."
Sinervo said he expects the CERN research teams to have two to three times more data to analyse by the end of the year.
"Will we be able to conclude that it is the Higgs by the end of the year? It depends what you mean by 'conclude,' but we'll at least have some strong data," he said.
"It's been an extraordinarily long haul. Some of us have been involved in this since the early 80s. That's a 30-year journey. Personally, I've been involved in this chase for 25 years. The results today are one of the stronger forms of delayed gratification."
Hints of Higgs seen previously
Physicists working at the Large Hadron Collider had earlier already found what they believe were hints pointing to the existence of the Higgs boson, but until now, it has remained a theoretical particle.
The ATLAS and CMS teams were tasked with running separate experiments that would each aim to spot traces of the boson independently.
The ATLAS collaboration includes researchers from around the world, including Canada, who have been working to analyze the vast amounts of collision data generated at CERN.
One-tenth of the data generated by the ATLAS experiment is sent to the TRIUMF supercomputer in Vancouver, where it is then analyzed by experts at a few Canadian universities.
On Wednesday, the leaders of the two CERN teams — Joe Incandela, head of CMS, with 2,100 scientists, and Fabiola Gianotti, head of ATLAS, with 3,000 scientists — each presented what was essentially extremely strong evidence of a new particle.
The announcement was timed to coincide with the first day of the International Conference on High Energy Physics in Melbourne.
The researchers stressed that the results are preliminary and that a full analysis of the data they gathered over 2011 and 2012 won't be published until the end of July.
"Positive identification of the new particle's characteristics will take considerable time and data," the CERN statement said. "But whatever form the Higgs particle takes, our knowledge of the fundamental structure of matter is about to take a major step forward."
CERN described the particle as a possible "bridge to understanding the 96 per cent of our universe that remains obscure," a reference to the fact that the matter that we can see represents only about four per cent of all matter in the universe.