Physicists are expected to confirm a major milestone in solving a decades-old puzzle about the nature of matter tonight.
The existence of the Higgs boson particle has been a hypothesis for several decades, yet much of what we know about the universe has been formed around the assumption that it does in fact exist.
The results of what scientists call a potentially "revolutionary" and extremely important experiment regarding its existence will be announced simultaneously in Geneva and Melbourne - where the international High Energy Physics conference is taking place - from 5pm (AEST).
The findings may, or may not, confirm existing theories of the way the universe - and our world - are held together.
The European Organisation for Nuclear Research (CERN) has been leading the search for the Higgs boson, an elusive sub-atomic particle dubbed by some as the "God particle", which is believed to confer mass.
The Higgs has led scientists on a chase since 1964, when British physicist Peter Higgs helped lay the conceptual foundation for it.
If the particle exists, it would vindicate the so-called Standard Model of physics, which identifies the building blocks for matter and the particles that convey fundamental forces.
On the eve of the announcement, rumours are flying about what CERN has in store.
"Whether or not the Higgs has been found, tomorrow will be exciting," Professor Sir Peter Knight, president of Britain's Institute of Physics, told AFP overnight.
"If the Standard Model is confirmed via the discovery of the Higgs boson, or whether we need to abandon and start re-writing the textbooks, it's a historical day in science that we should all be proud of."
A big question concerns the degree of probability scientists need to make a claim.
CERN physicists have said they will not make an announcement until they have proof - from two laboratories working independently at the Large Hadron Collider (LHC) - that the risk of a statistical fluke is vanishingly small.
In scientific parlance, the goal is "five sigma," meaning that there is just a 0.00006 per cent chance that what the two laboratories found is a mathematical quirk.
In a news report, the British science journal Nature said CERN will announce that the two labs saw signals of a new particle with a probability of between 4.5 and 5 sigma.
But CERN will stop short of calling it the Higgs until more is known about what the particle does, Nature said.
"Crucially, they will want to know whether it behaves like a mass-giving Higgs, and more specifically whether it behaves like the Higgs predicted in the Standard Model," the journal said.
Last week, CERN boss Rolf Heuer cautioned about the need for verification.
"It's a bit like spotting a familiar face from far. Sometimes you need closer inspection to find out whether it's really your best friend, or your best friend's twin."
Finding the Higgs
Because the Higgs cannot be seen, its existence - or not - has to be inferred.
This is done by smashing protons together in an underground tunnel, providing a tiny but fierce collision that causes sub-atomic debris to fly into detectors built into the 360-degree walls of a car-sized lab.
The trick then is to sift through the signals from this smash up and look for a pattern that points to the Higgs.
The boson has been so slippery because it is believed to decay almost instantly after it interacts with other particles to endow them with mass.
Over the years, tens of thousands of physicists have been thrown into the search for the Higgs, and billions of dollars have been spent on colliders.
A US machine, the Tevatron, came agonisingly close before it was mothballed in 2011 after 26 years of operations.
Its vanguard role was supplanted by the far bigger LHC, a behemoth that comprises four labs dotted around a ring-shaped tunnel, 27 kilometres long, straddling the Franco-Swiss border.
In a presentation on Monday of data that was analysed after the closure, physicists at Fermilab said they had strong hints that the Higgs exists, but the signal was 2.9 sigma, which falls short of the five-sigma threshold.
According to Nature, the signature occurred at a mass of around 125 gigaelectronvolts, when a Higgs-like particle decayed into two photons, or particles of light.
The Tevatron and the LHC carried out exhaustive experiments to narrow down the mass field and to identify potential Higgs patterns, a task "much worse than [seeking] a needle in a haystack," Fermilab physicist Joe Lykken said.