After more than 10 years of gathering and analyzing data produced by the U.S. Department of Energy’s Tevatron collider, scientists from the CDF and DZero collaborations have found their strongest indication to date for the long-sought Higgs particle.
Squeezing the last bit of information out of 500 trillion collisions produced by the Tevatron for each experiment since March 2001, the final analysis of the data does not settle the question of whether the Higgs particle exists, but gets closer to an answer. The Tevatron scientists, including six University of Chicago faculty members, unveiled their latest results on July 2, two days before the highly anticipated announcement of the latest Higgs-search results from the Large Hadron Collider in Europe.
“Our data strongly point toward the existence of the Higgs boson, but it will take results from the experiments at the Large Hadron Collider in Europe to establish a discovery,” said Fermilab’s Rob Roser, co-spokesperson for the CDF experiment at DOE’s Fermi National Accelerator Laboratory.
Scientists of the CDF and DZero collider experiments at the Tevatron received a round of rousing applause from hundreds of colleagues when they presented their results at a scientific seminar at Fermilab. The LHC results will be announced at a scientific seminar at 2 a.m. CDT on July 4 at the CERN particle physics laboratory in Geneva, Switzerland.
The Tevatron results indicate that the Higgs particle, if it exists, has a mass between 115 and 135 GeV/c2, or about 130 times the mass of the proton.
Tevatron scientists found that the observed Higgs signal in the combined CDF and DZero data in the bottom-quark decay mode has a statistical significance of 2.9 sigma. This means there is only a 1-in-550 chance that the signal is due to a statistical fluctuation.
“It’s the best measurement in the bottom-anti-bottom mode of the Higgs, which is actually the dominant mode for a 125 GeV Higgs,” said Jonathan Rosner, a professor emeritus in physics at UChicago and a member of the CDF collaboration.
Rosner noted that particle physics effects at 2.9 sigma come and go, which leads many scientists to regard them skeptically. “We’ve been fooled before. But if CERN is claiming a signal at a certain mass, then Fermilab definitely has something to contribute.”
According to the theoretical framework known as the Standard Model of Particles, Higgs bosons 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. LHC experiments can most easily observe the existence of a Higgs particle by searching for its decay into two energetic photons. Tevatron experiments most easily see the decay of a Higgs particle into a pair of bottom quarks.
The Tevatron was shut down in September 2011. Rosner noted that with another year of running time, the Tevatron could have produced a statistically significant Higgs signal. “Just barely, but still it wouldn’t have been something that would qualify for a ‘discovery.’”
The Higgs particle is named after Scottish physicist Peter Higgs, who in the 1960s helped develop the theoretical model that explains why some particles have mass and others don’t, a major step toward understanding the origin of mass.
The model predicts the existence of a new particle, which has eluded experimental detection ever since. Only high-energy particle colliders such as the LHC, which produced its first collisions in November 2009, and the Tevatron have the chance to produce the Higgs particle. About 1,700 scientists from U.S. institutions, including Fermilab, Argonne National Laboratory and UChicago, are working on the LHC experiments.
“During its life, the Tevatron must have produced thousands of Higgs particles, if they actually exist,” said Luciano Ristori, co-spokesperson of the CDF experiment and physicist at Fermilab and the Italian Istituto Nazionale di Fisica Nucleare (INFN). “We have developed sophisticated simulation and analysis programs to identify Higgs-like patterns. Still, it is easier to look for a friend’s face in a sports stadium filled with 100,000 people than to search for a Higgs-like event among trillions of collisions.”
The final Tevatron results corroborate the Higgs search results that scientists from the Tevatron and the LHC presented at physics conferences in March 2012.
The Tevatron is one of eight particle accelerators and storage rings on the Fermilab site. The largest operational accelerator at Fermilab now is the 2-mile-circumference Main Injector, which provides particles for the laboratory’s neutrino and muon research programs.
The CDF and DZero collaborations submitted their joint Higgs search results to the electronic preprint archive arXiv.org. The paper also is available at:
http://tevnphwg.fnal.gov/results/SM_Higgs_Summer_12/
CDF (Collider Detector at Fermilab) is an international experiment of 430 physicists from 58 institutions in 15 countries. DZero is an international experiment conducted by 446 physicists from 82 institutions in 18 countries. Funding for the CDF and DZero experiments comes from DOE’s Office of Science, the National Science Foundation, and various international funding agencies.
—Prof. Jacqueline Stewart
