But until Tuesday, the only person likely to have known the whole picture was the director of CERN, the European laboratory that houses the $8 billion collider.
Williams and around 30 physicists working with him at Penn knew half the story because they have been looking through one of two windows into the world exposed by the collider. That window, named Atlas, is a doughnut-shaped detector, 80 feet in diameter, designed to detect exotic particles like the Higgs. Another team is using a similarly large and elaborate detector called CMS. Neither team has been privy to the results of the other, but now both will show their hands.
Williams, who Friday was getting ready to fly to CERN, near Geneva, said both teams recently reported results to the director, and whatever he heard persuaded him to hold a news briefing Tuesday.
Hanging over the heads of the Penn physicists is the worry that somehow the other team, operating CMS, will present stronger, more statistically significant evidence for a Higgs particle.
The physicists say one team could pull ahead through luck alone, if, say, they were blessed with more Higgs particles being made and then leaving behind more definitive tracks. This would represent the first particle to be found in over 16 years, and no one wants to be on the second team to find it.
The collider works by accelerating beams of protons, which are components of the atomic nucleus, to near the speed of light and steering them into head-on collisions.
Those collisions, which happen by the millions per second, produce concentrations of energy unlike anything that has existed since the origin of the universe.
The energy can spontaneously condense into exotic bits of matter through the relationship defined by the famous Einstein equation E=mc2. Since no one has ever concentrated this much energy before, the physicists hope not only to see the Higgs particle but some surprises too.
The Higgs is named after the English physicist Peter Higgs, who proposed a theory in 1964 to explain a mystery: why the constituents of matter have mass but the light particle, or photon, is massless. Scientists had gotten used to the idea that electric and magnetic fields could pervade space, and Higgs proposed another field, bearing his name, through which particles could acquire mass.
In the 1970s, the Higgs field was incorporated into a Nobel-winning theory that united electromagnetism with one of the forces involved in nuclear reactions. Now the Higgs idea is woven into the so-called Standard Model, which describes the constituents of matter and the forces that act on them.
For years, the Higgs particle has remained the only missing piece in the Standard Model. In the late 1980s, physicists planned on finding it with a 50-mile-long, mega-accelerator called the Superconducting Supercollider. Tunnels were begun near Waxahachie, Texas, before it was killed by Congress in 1993.
Luckily, CERN was building a rival machine. Many U.S. scientists joined what became an international collaboration on the collider, which costs $1 billion a year to run.
Each of the two detectors required several thousand people to build and operate. Penn scientists built a piece of the Atlas detector's heart. Called the transition radiation tracker, it uses 300,000 gas-filled tubes the size of drinking straws. As particles move through the straws, their paths are photographed.
Neither detector actually sees the Higgs. That's because it lasts less than a billionth of a second before exploding into a shower of other particles that fly out in different directions. It is from pictures of the debris that the scientists try to assess whether a Higgs was there, but even then, there are always mundane events that can produce the same exact spray of subatomic debris.
So proof must be statistical, and to claim discovery, either team would have to leave less than a one in a million chance of statistical error. One in a thousand or even ten thousand isn't good enough.
But they are expecting the Higgs, since previous experiments have narrowed down the energy that should be required to produce it.
The Higgs could take different forms, said Penn physicist Mark Trodden. It could be the expected "standard model" Higgs, or a Higgs that has some unexpected properties, which would be more exciting since it suggests a new mystery.
"The most exciting thing that can happen is something will go wrong with the Standard Model," said Rutgers physicist Matt Strassler. "That would be fantastic."
Contact staff writer Faye Flam at 215-854-4977 or firstname.lastname@example.org.