It's been dubbed the "God particle," although if it could speak for itself, it might be a bit more modest about its pedigree.
The particle's formal tag is the Higgs boson, and tomorrow at the European Center for Nuclear Research (CERN) scientists are expected to report seeing hints of the long-sought Higgs – the particle linked to a mechanism that gives other subatomic particles their mass.
For more than a century, physicists have painstakingly built the so-called standard model of physics, which catalogs a clutch of fundamental particles, describes their traits, and explains their interactions. The Higgs boson is a relative latecomer to the menagerie and represents the Standard Model's last undetected fundamental particle.
Detection of the Higgs would be "very significant," says Gail Hanson, a physicist at the University of California at Riverside who currently is at CERN as a member of a US team taking part in the international hunt.
"This is the one thing that hasn't been found that we need in the standard model" if it is to underpin a "theory of everything," she adds.
However, information emerging tomorrow is far more likely to be tantalizing than conclusive, she cautions.
Indeed, she adds, "It would be interesting if we didn't find it."
The six, working in three independent groups, were trying to solve a riddle involving a group of fundamental particles called bosons. Bosons are particles associated with the four fundamental forces of nature, rather than with the constituents of atoms. Electromagnetism is one of these fundamental forces. Its associated boson is the photon. Another fundamental force, which governs radioactive decay, is the so-called weak force. It has two bosons associated with it.
In the 196Os, theorists showed that electromagnetism and the weak force were low-energy manifestations of a combined electroweak force that existed shortly after the big bang, the sudden release of energy that formed the universe some 13.7 billion years ago.
Just as the electromagnetic field has its photon, calculations suggested that the electroweak force had two bosons associated with it. The calculations also suggested that like the photon, this new duo of particles should have no mass.
Yet when physicists found the three hypothesized particles via experiments at particle accelerators, they had very large masses. Either the initial calculations predicting the particles were wrong, or something was missing from the picture.
Enter Higgs. The six physicists tackling this mass mismatch crunched numbers and posited that a mass-imparting field permeates space. It became known as the Higgs field.
When a particle encounters the field, it's like a rock star arriving at a party, suggests Howard Gordon, a senior physicist at the Brookhaven National Laboratory.
As the star arrives, people are milling around – the Higgs field. Few recognize the rock star until the star starts moving through the group.
As he heads toward the hors d'oeuvres table, "he becomes very massive" as people migrate to him for a chat, says Dr. Gordon, more so than they would a garage-band guitarist.
Similarly, different particles couple to the Higgs field with different intensities. The stronger the coupling, the more mass the Higgs field imparts to the particles, Gordon says.
"The theory of this Higgs mechanism implies that there must be a particle associated with it" – the Higgs boson, specifically – says Gordon, deputy program manager for the US collaboration on ATLAS, one of two general-purpose, underground detectors at the Large Hadron Collider (LHC) at CERN. These detectors measure the collision debris generated by the collider, a circular particle race track 17 miles in circumference that straddles the French-Swiss border.
From the debris, physicists can identify the more-massive particles the collisions fleetingly create.
Successive experiments at particle accelerators less powerful than the LHC have suggested a range of masses within which the Higgs boson should be found.
Detectors at the LHC have been designed to probe that range.
The effort often has been likened to looking for a needle in a haystack. The analysis emerging tomorrow is said to come from more than 380 trillion collisions between protons. From those collisions come perhaps 100,000 Higgs bosons.
Or something that looks like them but are not.
If one of the two large detectors spots something vaguely resembling a Higgs signature, "it's encouraging," writes Pauline Gagnon, a physicists and senior research scientist at the University of Indiana in Bloomington, in her blog on the website Quantum Diaries.
But if both detectors see similar signatures and they yield the same mass estimates for the Higgs boson, it won't be enough to claim discovery, but it will be "time to call your mother," Ms. Gagnon adds.