By Paul Rincon
BBC News Online science staff
A scientist says one of the most sought after particles in physics - the Higgs boson - may have been found, but the evidence is still relatively weak.
Once produced, the Higgs boson would decay very quickly
Peter Renton, of the University of Oxford, says the particle may have been detected by researchers at an atom-smashing facility in Switzerland.
The Higgs boson explains why all other particles have mass and is fundamental to a complete understanding of matter.
Dr Renton's assessment of the Higgs hunt is published in Nature magazine.
His paper in the journal reviews the current state of play.
"There's certainly evidence for something, whether it's the Higgs boson is questionable," Dr Renton, a particle physicist at Oxford, told BBC News Online.
"It's compatible with the Higgs boson certainly, but only a direct observation would show that."
If correct, Dr Renton's assessment would place the elusive particle's mass at about 115 gigaelectronvolts.
This comes from a signal obtained at the large electron positron collider (LEP) in Geneva, Switzerland, which has now been dismantled to make way for its replacement - the large hadron collider (LHC).
However, there is a 9% probability that the signal could be background "noise".
Before the LEP accelerator was decommissioned, physicists used it to send particles called electrons and positrons careering in opposite directions around its circular pipe, which had a circumference of about 27km.
When these particles collided, they created bursts of high energy. Such collisions themselves are too small to study but new, heavier particles can appear amongst the debris.
The Higgs boson is thought to be highly unstable and, once produced, should quickly decay.
Dr Renton cites indirect evidence taken from observations of the behaviour of other particles in colliders that agrees with the figure of 115 gigaelectronvolts for the mass of the Higgs boson.
"It's controversial. The data is possibly indicative, but it needs confirmation," said Bryan Webber, professor of theoretical physics at the University of Cambridge.
The LEP's huge ring was used to study the particles in our universe
"Its mass is right at the maximum energy they could run the [LEP] at. But the indirect indications are that the Higgs boson should be close to that value."
Physicists have observed 16 particles that make up all matter under the Standard Model of fundamental particles and interactions.
But the sums do not quite add up for the Standard Model to be true if these particles are considered alone. If only 16 particles existed, they would have no mass - contradicting what we know to be true in nature.
Another particle has to give them this mass. Enter the Higgs boson, first proposed by University of Edinburgh physicist Peter Higgs and colleagues in the late 1960s.
Their theory was that all particles acquire their mass through interactions with an all-pervading field, called the Higgs field, which is carried by the Higgs boson.
The Higgs' importance to the Standard Model has led some to dub it the "God particle".
Dr Renton said he hoped that once the large hadron collider was up and running in 2007, the Higgs boson would be detected within a year or two.
The LHC is a more energetic accelerator which will allow a much higher mass range to be explored. It will also be capable of producing much more intense particle beams which means that data can be aggregated much faster.
It is also possible the Fermi National Accelerator Laboratory outside Chicago, US, could make the discovery.
Researchers there are hopeful they can secure enough data to prove the Higgs' existence before the LHC comes online.