Physicists have been baffled by the results of an experiment that could change their understanding of how the universe works.
The lengthy test involved sending subatomic particles called muons around a magnetised track at the US Energy Department's Fermilab near Chicago.
As a result the muons "wobbled" in a way scientists could not explain, suggesting they were being affected by a form of matter or energy not yet known to man.
The development has been compared to the discovery of the elusive Higgs boson, known as the "God particle", nearly a decade ago.
Muons were first found in cosmic rays in 1936 and are heavier than electrons, which orbit an atom's centre. They are unstable and exist for only a few microseconds.
Half a century ago scientists developed the Standard Model, which describes how subatomic particles behave.
But when muons were sent around the 50ft long Fermilab track, over eight billion times, they did not react as expected.
Around 200 physicists from seven countries have been working on the project known as "Muon g-2".
Graziano Venanzoni, a spokesman for the programme, said muons had always "made physicists scratch their heads".
He added: "Today is an extraordinary day long awaited, not only by us, but by the whole international physics community."
Renee Fatemi, a University of Kentucky physicist working on the experiment, said: "This is strong evidence that the muon is sensitive to something that is not in our best theory.”
The preliminary results had only a one in 40,000 probability of happening by chance, scientists said.
Matthew McCullough, a theoretical physicist at the European Organization for Nuclear Research (CERN) said it could "take us beyond our current understanding of nature".
It followed results from CERN's Large Hadron Collider last month that found a surprising proportion of particles in the aftermath of high-speed collisions.
According to the Standard Model that experiment was expected to produce an equal number of electrons and muons.
Instead, 15 per cent more electrons were produced.
Both experiments are going to be repeated over the next few years.
In 2011 a finding that neutrinos seemed to be travelling faster than light threatened to upend understanding of the universe.
However, the results turned out to be the result of a loose electrical connection in the experiment.
Alexey Petrov, a particle physicist at Wayne State University in Michigan, said he was "cautiously ecstatic" following the Fermilab findings.
He said: "New particles, new physics might be just beyond our research."