On July 4, the European Organization for Nuclear Research, CERN, announced the discovery of a particle thought to be the elusive Higgs boson, nicknamed the “God particle,” responsible for imbuing the universe with mass.
 
The discovery was made possible by CERN’S Large Hadron Collider (LHC) in Switzerland, the $10 billion particle accelerator built to aid researchers in their quest to answer several fundamental questions of physics. Proving the existence of the Higgs boson was its most public goal, doggedly pursued by a collaboration of thousands of physicists from around the world.  

“It couldn’t be more exciting,” says Brig Williams, professor of physics in Penn’s School of Arts and Sciences. “The machine has been running extremely well. We've already taken more data since April than we had all of last year.”

Williams and fellow Penn physics professors Joseph Kroll, Eliot Lipelles, and Evelyn Thompson, along with their students, have played a large role in the design and construction of ATLAS, one of the two main particle detectors at the LHC.

The LHC works by accelerating protons in opposite directions around a 17-mile-long circular track. When the beams are crossed inside the detectors, the protons collide in a huge explosion of energy that can create new particles. Only a machine as powerful as the LHC is thought to be able to create a particle as energetic as the Higgs boson.  

The problem inherent in studying this particle is that as soon as a Higgs is created, it immediately breaks apart into different particles that fly off in different directions. Penn’s chief contributions to ATLAS is the development of the electronics that help capture the proton collision outcomes in the ATLAS’ Inner Detector, and the creation of a process that helps researchers record only the most noteworthy of the millions of collisions that occur every second.

Following a collision, researchers trace the energy and trajectory of the shrapnel of the shattered particle as it passes through the detectors. This allows them to work backwards, piecing together the size of the original particle. One Penn team has been working at observing a process that would turn a Higgs particle into two photons, while another Penn team is looking at more exotic particles known as “W’ bosons.

There are many other phenomena in particle physics that could produce the patterns associated with the Higgs, Williams says. Therefore scientists must collect mountains of evidence that all indicate the Higgs has one particular mass. CERN announced last December that researchers found hints of the Higgs between 125 and 126 gigaelectron volts (GeV), but more data was needed to declare with confidence the discovery of the Higgs.

The July 4 announcement places a “5 sigma” likelihood of the Higgs particle being between 125 and 126 GeV. That is a mathematical way of saying that the odds of the discovery being a coincidence are 1 in 1.7 million.

“The outstanding performance of the LHC and ATLAS and the huge efforts of many people have brought us to this exciting stage,” says ATLAS experiment spokesperson Fabiola Gianotti, “but a little more time is needed to prepare these results for publication.”

“There is a tradition in particle physics not to make a formal claim of something until you’ve submitted the paper,” says Williams. “We expect to do that at the end of July.”