A hint of the Higgs

University of Illinois scientists have played a leading role in the search for the Higgs boson for many years. New measurements announced March 7 by scientists from the CDF and DZero collaborations at the Department of Energy’s Fermi National Accelerator Laboratory indicate that the elusive Higgs boson may nearly be cornered.

After analyzing the full data set from the Tevatron accelerator, which completed its last run in September 2011, the two independent experiments see hints of a Higgs boson. 

Physicists from the CDF and DZero collaborations found excesses in their data that might be interpreted as coming from a Higgs boson with a mass in the region of 115 to 135 GeV. (For comparison, a proton has a mass of 1 GeV.) In this range, the new result has a 2% probability of being due to a background fluctuation. This new result also excludes the possibility of the Higgs having a mass in the range from 147 to 179 GeV.

This result sits well within the stringent constraints established by earlier direct and indirect measurements made by CERN’s Large Hadron Collider, the Tevatron, and other accelerators, which place the mass of the Higgs boson within the range of 115 to 127 GeV. These findings are also consistent with the December 2011 announcement of excesses seen in that range by LHC experiments, which searched for the Higgs in different decay patterns. None of the hints announced so far from the Tevatron or LHC experiments, however, are strong enough to claim evidence for the Higgs boson.

“The end game is approaching in the hunt for the Higgs boson," said Jim Siegrist, DOE Associate Director of Science for High Energy Physics. “This is an important milestone for the Tevatron experiments, and demonstrates the continuing importance of independent measurements in the quest to understand the building blocks of nature.”

Physicists from the CDF and DZero experiments made the announcement at the annual conference on Electroweak Interactions and Unified Theories known as Rencontres de Moriond in Italy. This is the latest result in a decade-long search by teams of physicists at the Tevatron.

“I am thrilled with the pace of progress in the hunt for the Higgs boson. CDF and DZero scientists from around the world have pulled out all the stops to reach this very nice and important contribution to the Higgs boson search,” said Fermilab Director Pier Oddone. “The two collaborations independently combed through hundreds of trillions of proton-antiproton collisions recorded by their experiments to arrive at this exciting result.”

University of Illinois physicists played a significant role in this effort. Professor Kevin Pitts led a team of scientists that constructed the CDF trigger system, which allowed the experiment to identify the interesting events in a morass of mundane background. Pitts was recently the Physics Coordinator for the experiment, overseeing the effort to sift through thousands of terabytes of data to look for the fleeting evidence of the Higgs boson. Benjamin Carls recently completed his Ph.D. thesis at Illinois searching for the Higgs boson decaying into two massive particles. Carls’ work is incorporated into the results presented today.

Higgs bosons, if they exist, are short-lived and can decay in many different ways. Just as a vending machine might return the same amount of change using different combinations of coins, the Higgs can decay into different combinations of particles.

U of I Professor Scott Willenbrock and his collaborators were the first to propose the production and decay process that would be relevant to the Higgs search at the Tevatron. "At that time, nobody was even talking about a Higgs search at the Tevatron; everyone was focused on the next big particle accelerator. We realized that the Tevatron had a chance," said Willenbrock, a theoretical physicist. "This exciting result would not have been possible without the hard work of many dedicated individuals."

“This has been a long quest, and we are very proud of the progress that we've made,” said Pitts. “We’ve been working on this experiment for many years. The Higgs is so elusive that we just now have the sensitivity to begin to say anything. Taken in conjunction with the LHC data, this really seems to indicate that we are on the crest of saying that we’ve found it. It’s particularly exciting here at Illinois, because we have scientists working on Tevatron data and scientists working on LHC data.”

University of Illinois Physicists Steve Errede, Debbie Errede, Tony Liss, and Mark Neubauer, along with a number of students and postdoctoral researchers, are members of the ATLAS experiment operating at the Large Hadron Collider in Europe.

Only high-energy particle colliders such as the Tevatron and LHC can recreate the energy conditions found in the universe shortly after the big bang. According to the standard model, the theory that explains and predicts how nature’s building blocks behave and interact with each other, the Higgs boson gives mass to other particles. “There are only two experimental facilities in the world that can hope to answer how particles in the universe became massive. It’s really neat that physicists at the University of Illinois are leading members in both efforts,” said Pitts.

If a Higgs boson is created in a high-energy particle collision, it immediately decays into lighter more stable particles before even the world’s best detectors and fastest computers can snap a picture of it. To find the Higgs boson, physicists retraced the path of these secondary particles and ruled out processes that mimic its signal.

The experiments at the Tevatron and the LHC offer a complementary search strategy for the Higgs boson. The Tevatron was a proton/anti-proton collider, with a maximum center of mass energy of 2 TeV, whereas the LHC is a proton/proton collider that will ultimately reach 14 TeV. Because the two accelerators collide different pairs of particles at different energies and produce different types of backgrounds, the search strategies are different.

At the Tevatron, for example, the most powerful method is to search the CDF and DZero datasets to look for a Higgs boson that decays into a pair of bottom quarks if the Higgs boson mass is approximately 115 GeV to 130 GeV. It is crucial to observe the Higgs boson in several types of decay modes because the standard model predicts different branching ratios for different decay modes. If these ratios are observed, then this is experimental confirmation of both the standard model and the Higgs.

This new updated analysis uses 10 inverse femtobarns of data from both CDF and DZero, the full data set collected from 10 years of the Tevatron’s collider program. Ten inverse femtobarns of data represent about 500 trillion proton-antiproton collisions. Data analysis will continue at both experiments.

“This result represents years of work from hundreds of scientists around the world,” said Giovanni Punzi, CDF co-spokesperson and physicist at the University of Pisa. “But we are not done yet – together with our LHC colleagues, we expect 2012 to be the year we know whether the Higgs exists or not, and assuming it is discovered, we will have first indications that it behaves as predicted by the standard model.”
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Writer/Contact: Kevin Pitts, Department of Physics, 217/333-3946.

Celia M. Elliott, Department of Physics, 217/244-7725.

Photos: Courtesy Fermilab Visual Media Services

If you have any questions about the College of Engineering, or other story ideas, contact Rick Kubetz, editor, Engineering Communications Office, University of Illinois at Urbana-Champaign, 217/244-7716.