Wisconsin team narrows search for Higgs boson

With time running out for Europe’s largest particle accelerator, a team of Wisconsin physicists may be tantalizingly close to being among the first to see the Higgs boson, the subatomic particle that is responsible for endowing all matter with mass.

At a meeting Sept. 5, a collaboration of scientists representing ALEPH, one of four large experiments at the European Laboratory for Particle Physics‘ (CERN) Large Electron Positron (LEP) collider, presented evidence of what may be the first observation of the Higgs boson, a particle so crucial to the current understanding of Nature that it is sometimes referred to as the “God particle.”

“The discovery of the Higgs boson would mark a profound point in the history of science,” says Sau Lan Wu, Enrico Fermi Professor of Physics at UW–Madison and the head of the UW–Madison’s High Energy Physics group working on the ALEPH detector.

“The Higgs boson is perhaps the most important and unique elementary particle,” Wu explains. “There is literally no other particle like it, and without it our understanding of the behavior of matter and energy at the most fundamental levels breaks down.”

Wu’s group is at the forefront of the endeavor with the ALEPH experiment. She and members of her group are among those who observed a number of Higgs boson candidates with a mass of roughly 114 GeV (or 122 times heavier than a proton).

“If these candidates are proved to be signs of the Higgs boson, we finally have a complete picture of the behavior of matter and energy at the most fundamental levels currently experimentally accessible to us,” Wu says.

The Higgs boson is named after Peter Higgs, a Scottish theorist who suggested its existence in the 1960s.

To explore the world of subatomic particles, scientists must orchestrate collisions between ordinary particles such as protons or electrons by accelerating them to great speeds and detecting the product of these collisions. The work is done in a large particle accelerator, LEP at CERN, located in a 17-mile underground tunnel near Geneva, Switzerland.

Complicating the hunt for the Higgs boson, however, is the planned closing of the LEP accelerator Oct. 1 to make way for a new accelerator known as the Large Hadron Collider. But with strong hints of the Higgs boson emerging from ALEPH and possibly a hint from one other detector DELPHI, CERN officials have extended the life of LEP by a month, giving scientists an opportunity to collect more data in the hope of firming up observations of Higgs candidates.

At the center of the current excitement at CERN are three collisions — among many thousands — seen by the ALEPH group. The three events bear all the characteristic signs of the appearance of the Higgs boson, Wu says.

Wu’s group has been searching for the elusive particle for several years by combing mountains of data collected during collisions in the ALEPH detector. Production of the Higgs boson is extremely rare and there are other, more mundane processes that might mimic its production.

While not conclusive, the new evidence is compelling.

“The statistical interpretation is exceedingly complex,” Wu says, and picking those events out against background noise can be a labor of Hercules.

“However, in our case, the interesting Higgs candidates are clustered around a mass of 114 GeV, roughly 122 times the mass of a proton. In this region, we expect few background events, but the possibility of an upward statistical fluctuation from these background events cannot yet be ruled out.”

In this, the final year of LEP operations, the collision energy of LEP was pushed to the highest level yet achieved, thanks to the excellent work of the accelerator physicists at CERN, bringing new opportunities for discovery, Wu says.

“An immediate turn-around time was required for analyzing the ALEPH data and extracting results during this critical year,” says Wisconsin researcher Stephen Armstrong. To accomplish this, an automated search system known as “BEHOLD!” was developed by Armstrong and graduate student Jason Nielsen. This acted as an early warning system to alert ALEPH physicists that the Higgs boson may have been detected.

“Although the results are exciting and compelling, more data are required for an unambiguous declaration of discovery,” adds Wisconsin researcher Peter McNamara.

It is possible that by the end of the year, a combination of data from the four LEP experiments may be able to confirm if the Higgs boson has at last been found.

Wu is no stranger to groundbreaking discoveries. As a young postdoctoral researcher, she worked in the group that first discovered the charm quark, another basic subatomic particle, in 1974. As an assistant professor at Wisconsin, Wu was the leading figure in the discovery of another fundamental particle in 1979, the gluon, responsible for the strong or “color” force which binds together the quarks to form protons and neutrons.

She shared the 1995 European Physical Society Prize for the gluon discovery. Wu also holds a Vilas Professorship with the UW–Madison and is a fellow of the American Academy of Arts and Sciences.