A nearly 50-year bid to explain a riddle of fundamental matter faces a key moment on Wednesday when physicists unveil fresh data in their search for a particle called the Higgs boson.
Expectations are high that the European Organisation for Nuclear Research (CERN) has something dramatic to say about the Higgs, which exists so far only in theory.
The particle is deemed to be so pervasive, so important -- and also so elusive -- that it has been jokingly called the "God particle."
At 0700 GMT, chief scientists at CERN are to start briefing fellow physicists at the Large Hadron Collider (LHC) as well as VIP guests who include Peter Higgs, the British researcher who in 1964 laid much of the conceptual groundwork for the boson.
The webcast seminar, explaining the results from two labs working independently, will be followed by a press conference.
Finding the Higgs would vindicate an intellectual framework for the nature of the cosmos called the Standard Model.
Devised in the 1970s, the Standard Model identifies the building blocks for matter and the particles that convey fundamental forces.
It is considered a hugely successful theory but has several gaps, the biggest of which is why some particles have mass and others do not.
The hypothesis is that Higgs bosons exist in a treacly, invisible, ubiquitous field created by the Big Bang some 13.7 billion years ago.
When some particles encounter the Higgs, they slow down and acquire mass. Others, such as particles of light, encounter no obstacle.
"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," Peter Knight, president of Britain's Institute of Physics, told AFP on Tuesday.
CERN's data has been kept closely under wraps, stoking frenzied speculation about what, if anything, will be announced.
But a video from the CERN lab, apparently posted mistakenly on the eve of the announcement, reveals that a new subatomic particle has been observed in the relevant range.
"We've observed a new particle," Joe Incandela, a spokesman for CERN, says in the video that appeared on the Science News website before being picked up elsewhere.
"We have quite strong evidence that there's something there... To ascertain its properties is still going to take us a bit of time," Incandela says.
The agency says it will not make any claim of discovery until it has proof that the risk of a statistical fluke is vanishingly small.
In scientific parlance, the goal is "five sigma," meaning there is just a 0.00006% chance that the results are a mathematical quirk.
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.
Physicists then sift through the smashup and look for a pattern that points to the Higgs.
The task has been arduous because there are trillions of signals, occurring among particles at different ranges of mass.
Over the years, tens of thousands of physicists and billions of dollars have been thrown into the search for the Higgs, gradually narrowing down the mass range where it might exist.
The campaign stepped up several gears in 2008 with the completion of the LHC, a giant collider costing six billion Swiss francs (five billion euros, $6.27 billion dollars).