Hints of the last undiscovered particle in the Standard Model of Physics may have been detected by two separate experiments at the world's biggest particle accelerator.
Researchers at the Large Hadron Collider have glimpsed signals that could point to the existence of the Higgs boson — nicknamed the "god particle" by 1988 Nobel Prize winner Leon Lederman — an elusive subatomic particle theorized to impart mass to other particles.
"We saw some tantalizing hints today," said Rolf-Dieter Heuer, director-general of CERN, the European Organization for Nuclear Research, Tuesday, after the results of experiments known as ATLAS and CMS were presented at a public lecture. The ATLAS collaboration includes a number of Canadian scientists.
The Higgs boson and the Standard Model
The Higgs boson is a subatomic particle that plays a key role in the Standard Model of physics that describes our understanding of all elementary particles and how they interact.
The Standard Model includes common particles like electrons and photons along with less familiar ones like muons. Among them, the Higgs boson is the only one that remains undetected in experiments. However, it is extremely important because it allows particles to have mass.
The Higgs boson and its mechanism for imparting mass to other particles were first proposed by British physicist Peter Higgs 40 years ago.
The Large Hadron Collider is expected to help scientists find the Higgs boson, if it exists, because it allows them to observe extremely high energy collisions similar to those during the Big Bang. Those could in turn produce the previously undetected particle.
If a Higgs boson that behaves according to the Standard Model exists, Higgs's theory would be confirmed. However, if the Higgs boson doesn't behave according to predictions or if it is not detected at all, that would open the door to completely new physics outside the Standard Model and our current understanding of nature.
Both experiments, which involve about 5000 scientists altogether, showed signals for a mass between 115 and 130 GeV — the lower end of the range scanned by the scientists — that could indicate the presence of a new particle.
The researchers have now all but ruled out the possibility (with 99 per cent confidence) that the Higgs boson has a mass between 128 and 525 GeV.
However, the researchers warned that not enough data has been collected to make statistically sure that the apparent "Higgs boson" signals they saw are not due to fluctuations in background signals caused by other particles and processes. Heuer warned that the results are preliminary, as they involve very small numbers of signals.
"Please be prudent," he said. "We have not found it yet. We have not excluded it. Stay tuned for next year."
In 2012, scientists at the two experiments expect to collect four times the amount of data they collected in 2011, increasing the number of signals they see and reducing the statistical error significantly.
Fabiola Gianotti, spokeswoman for the ATLAS experiment, said that means the Higgs boson could be unambiguously discovered or ruled out by ATLAS in 2012.
How to hunt for Higgs
Inside the Large Hadron Collider, the ATLAS experiment collides protons, some of the building blocks of atoms, at an energy of 3500 GeV. The collisions are expected to produce Higgs bosons, which have an unknown mass.
Each proton is made up of smaller particles called quarks and gluons that can have a wide range of energies following the collisions. At that point, the particles coming out of the collision hit a detector, which measures their energies. Since they are travelling close to the speed of light, their energies and their masses are roughly equivalent.
Theory predicts how many Higgs bosons should be produced by a collision of a certain energy if it has a certain mass.
"Within 10 per cent, we know how many of them should be and how many our detector should have seen," said William Trischuk, a University of Toronto physicist involved in the ATLAS experiment.
Higgs bosons are extremely short-lived and decay before they can ever be detected. However, when they decay, a number of slightly longer-lived particles are expected to be produced. Those in turn decay into particles that can be detected.
"It's a detective game," said Trischuk, director of the Institute of Particle Physics, which supports and promotes research in the field across Canada.
Based on the particles detected, the researchers need to reconstruct the decay sequence and look for pairs of particles whose combined masses add up to the same number more often than might otherwise be expected.
"That's the harbinger that there's some new particle," Trischuk said. "If there's a Higgs boson there, we would get more at one particular mass than the average or than just random quark collisions."
Those are the types of signals that researchers have seen so far that hint at the existence of the Higgs boson at lower masses or energies.
Several different decay mechanisms — and therefore decay products — are possible.
"There are half a dozen or eight different ways we're looking for the Higgs boson in each of two experiments," Trischuk said. "And in some sense, it's the conspiracy of all of these things working together that gives us the confidence [in the results]."
The Canadian members of ATLAS are all dedicated for looking at one of those particular "channels" of decay. They saw a slight signal, but it was not the biggest one detected.
"It's not the poster child for the tantalizing hint today," Trischuk said.
He said researchers will continue to look at all channels and all energies, even the ones that have so far been "ruled out," to ensure they don't miss anything.
"A year from now, we will have two or three times or maybe more data and we will be able to make unequivocal statements about it."