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An Elusive Particle Makes its Debut

September 2012Perspectives Newsletter

Physicists around the world have been trying to detect the Higgs boson, an elusive subatomic particle, for decades. This summer they finally succeeded, with UW physicists playing a key role in the discovery. The subatomic particle's existence is a major step in understanding the origins of the universe.

"For the UW group, it is particularly exciting because it comes after more than 20 years of dedicated effort," says Henry Lubatti, a UW physics professor.

Higgs boson 1

Scientists at the European Organization for Nuclear Research are dwarfed by the Atlas particle detector, part of the Large Hadron Collider. Media credit: Atlas collaboration

In the very early hours of July 4th, several UW physicists joined a crowd of about 175 people to watch the much-anticipated announcement of the discovery, which came via webcast from the European Center for Nuclear Research (CERN). CERN's Large Hadron Collider began operating in 2008 with the expectation that it would detect the Higgs boson if such a particle existed.

The webcast was emotional for UW physics professor Anna Goussiou. "I jumped up and screamed, 'There it is!' all teary-eyed," she says, recalling the moment the first evidence for the Higgs boson flashed on the screen. "I've been looking for the Higgs non-stop since the year 2000. This is for me the discovery of a lifetime."

Goussiou, along with her students and postdoctoral researchers, made significant contributions to the discovery as they searched collider data for signals of particle decays that could indicate the presence of the Higgs boson. Lubatti and UW physics Professor Gordon Watts searched the data for other signs that the Higgs might be present.

The Higgs boson is considered a key to the "standard model," which physicists use to understand the nature of matter and how the universe is put together. In the standard model, elementary particles such as quarks and electrons acquire mass by interacting with the Higgs field, and the Higgs boson is an excitation of the Higgs field. Observing the Higgs boson helps to confirm that the field exists.

"The most important significance is that the Higgs discovery provides the missing link in the unification of two of the four fundamental forces in nature," Goussiou says. "It brings us a lot closer to understanding the absolute symmetry we believe existed at the very early stage of the universe, right after the Big Bang."

The Large Hadron Collider at CERN was designed and built specifically to make high-energy physics observations, and it has plenty of work left to do.

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This track is an example of simulated data for a Higgs event modeled for the Atlas detector at the Large Hadron Collider at CERN in Switzerland. Media credit: Atlas collaboration

"We are looking to make sure the Higgs works exactly as we expect it to," Watts explains. "Any deviations will mean something interesting, and perhaps a clue to the pieces of the puzzle we are still missing. Longer term, and in parallel, we will need to understand how dark matter and dark energy fit into the puzzle."

The collider works by sending nuclei of hydrogen atoms racing at nearly the speed of light in opposite directions through parallel underground tubes that form a circle about 16.5 miles in circumference along the Swiss-French border. Detectors are positioned to observe what happens when the nuclei collide. The results indicating the presence of the long-sought Higgs boson came from two different detectors, CMS and Atlas.

The UW group played a key role in designing and building the Atlas detector's muon spectrometer, central to the Higgs boson discovery. The forward muon detector contains more than 430 chambers filled with aluminum tubes, whose gold-plated tungsten wire—just half the width of a human hair—detects what happens when subatomic particles collide at nearly the speed of light.

About 30,000 of the tubes were made at the UW between 2000 and 2007, fitted into 80 chambers and shipped to Geneva. Another 60,000 tubes were made with UW methods and specifications at two other U.S. sites. 

The UW group's muon detector development, led by Lubatti and Colin Daly in mechanical engineering, began in 1989 as part of a U.S. project called the Superconducting Super Collider that eventually was cancelled. The group joined the Atlas detector group in 1993 to work on the muon spectrometer.

Others in the UW Physics Department involved in building the muon system include faculty members Joseph Rothberg and Paul Mockett and staff members David Forbush and Matt Twomey. William Kuykendall, a laboratory engineer in mechanical engineering, also participated.

Adapted from a July 6, 2012 article by Vince Stricherz, UW News and Information.