U.Va. Physicists Help Hunt Down the Higgs

March 13, 2012 — University of Virginia physicists working with international colleagues to examine data from the former Tevatron collider at the Fermi National Accelerator Laboratory near Chicago have found hints of the Higgs boson, the much-sought-after particle that is thought to give mass to every other particle in existence.

Their findings are consistent with those recently announced from experiments at the Large Hadron Collider in Europe.

The Higgs boson may be the key to understanding the underlying basis of matter. It is the object of intense experimental searches at the Large Hadron Collider and data sifting from years of experiments at the now shuttered Fermilab Tevatron collider.

"After decades of anticipation, the evidence is mounting that we will soon shed light on the most important missing piece in the Standard Model of particle physics," said College of Arts & Sciences physicist Robert Hirosky, who is searching for the Higgs within data from both Fermilab and the Large Hadron Collider.

The Standard Model of Physics is a theory describing the elementary particles believed to make up all matter in the universe. The Higgs boson, hypothesized four decades ago by English physicist Peter Higgs, is the particle that gives mass to all the others. Without it there would be no universe of the type we know.

Only by using high-energy particle colliders such as the Tevatron and the Large Hadron Collider can physicists recreate the energy conditions of the early universe to examine the subatomic particles from which the universe evolved.

While the Large Hadron Collider races protons around an accelerator and collides them to produce subatomic particles, the Tevatron – which ceased operations last September after a decade of experiments – collided protons and antiprotons. Researchers are analyzing trillions of data points from both machines for hints of the Higgs.

If a Higgs boson is created in a high-energy particle collision, it immediately decays into lighter, more stable particles. Physicists retrace the path of these secondary particles to reconstruct the picture of the decaying Higgs boson and to rule out processes that mimic its signal.

Many different decay patterns are possible. Any initial discovery of the Higgs boson will rely on combining evidence for these patterns to produce a statistically significant excess of particles with properties that allow for the mass of the Higgs to be reconstructed.

Recent findings have narrowed the search area for where the Higgs might be hiding. It is believed to have a mass in the range of 115 to 135 electron volts, about 125 times that of the proton. Two independent experiments at the Tevatron surveyed about 500 trillion proton-antiproton collisions and came to outcomes compatible with findings from the Large Hadron Collider – which narrowed the search range.

"We have this very exciting result thanks to the full Tevatron dataset and to a massive analysis effort from dozens of scientists who are reaching unprecedented levels of analysis sensitivity," said U.Va. physicist Robert Group, who is a member of the team searching for the Higgs through data from the Tevatron.

Recent findings indicate that the experiments at the Large Hadron Collider may well be within striking reach of the elusive Higgs. Physicists believe the huge Large Hadron Collider is the machine that ultimately will either prove or disprove the existence of the Higgs.

Though most physicists believe the Higgs exists and will be found, it also is possible that a different explanation for why matter has mass will emerge. If the Higgs were disproved, then physicists would have to go back to the drawing board and develop a new theory for mass and matter.

Hirosky noted that the Standard Model of Physics has so far stood the test of time, except for the missing link – the Higgs.

"We greatly anticipate its discovery – which will allow us to begin study of the mechanism leading to the creation of mass, or, its exclusion and then subsequent hints towards very different ideas on nature's secrets," he said.

Several faculty members, postdocs and graduate students in U.Va.'s High Energy Physics Group have been and are involved with Tevatron and Large Hadron Collider experiments and data analysis, including professors Group, Hirosky, Brad Cox and Christopher Neu; postdoc Yuri Oksuzian; former postdocs Michael Mulhearn and Marc Buehler; and graduate students Hao Liu and Huong Ngyuen. They also have developed components for two large experiments at Fermilab and for a major experiment at the Large Hadron Collider.

– by Fariss Samarrai