Northern Illinois University

Northern Today

Gerald Blazey
Gerald Blazey

David Hedin
David Hedin

NIU scientists among Fermilab collaboration
announcing major discovery of ‘Z bosons’

DZero experiment observes ‘prelude to the Higgs’

August 4, 2008

NIU can take more than a little pride in Fermilab’s announcement last week of a major discovery.

NIU physicists are among the scientists participating in Fermilab’s DZero collaboration, which announced the observation of pairs of Z bosons – force-carrying particles produced in proton-antiproton collisions at the Tevatron, the world’s highest-energy particle accelerator.

The properties of the ZZ diboson make its discovery an essential prelude to finding or excluding the Higgs boson at the Tevatron. The Higgs boson is considered by some to be the holy grail of particle physics.

“Discovery of ZZ production is an interesting result in its own right,” said NIU Distinguished Research Professor Gerald Blazey, a former co-spokesperson for the DZero collaboration.

Blazey is currently serving in an Intergovernmental Personnel Assignment with the U.S. Department of Energy Office of Science, where he is participating in program planning for the Office of High Energy Physics and is program manager for International Linear Collider research and development.

“Extraction of the signal shows that the Tevatron experiments and collider are well prepared for the increasingly intense hunt for the Higgs particle,” Blazey said.

Blazey and NIU Distinguished Research Professor David Hedin, also a longtime member of DZero, both took part in DZero’s internal review of the ZZ diboson results process.

“NIU was very much involved in aspects of DZero leading to this result,” Hedin said. “The Zs in the event either decay to 2 muons, to 2 electrons or to 2 neutrinos. NIU helped build the muon detector, including writing the reconstruction software. And Mike Eads, a 2005 graduate of NIU, is now co-coordinator of the group which verifies the muon identification tools used by the people that produced the result.”

The Standard Model of particle physics – the best explanation scientists have of the origins of the universe – predicts the existence of the Higgs boson. Its detection would confirm the existence of the Higgs field, which is thought to permeate the universe. When particles interact with this field, they gain mass. Without mass, all particles would travel at the speed of light, never sticking together, and only these tiny mass-less particles would populate the universe.

The observation of the ZZ connects to the search for the Higgs boson in several ways.

The process of producing the ZZ is very rare and hence difficult to detect. The rarest diboson processes after ZZ are those involving the Higgs boson, so seeing ZZ is an essential step in demonstrating the ability of the experimenters to see the Higgs.

The signature for pairs of Z bosons can also mimic the Higgs signature for large values of the Higgs mass. For lower Higgs masses, the production of a Z boson and a Higgs boson together, a ZH, makes a major contribution to Higgs search sensitivity, and the ZZ shares important characteristics and signatures with ZH.

The ZZ is the latest in a series of observations of pairs of the so-called gauge bosons, or force-carrying particles, by DZero and its sister Tevatron experiment, CDF. Earlier this year, CDF found evidence for ZZ production; the new DZero results for the first time showed sufficient significance, well above five standard deviations, to rank as a discovery of ZZ production. 

DZero searched for ZZ production in nearly 200 trillion proton-antiproton collisions delivered by the Tevatron. 

DZero is an international experiment conducted by about 600 physicists from 90 institutions in 18 countries. Funding for the DZero experiment comes from the Department of Energy’s Office of Science, the National Science Foundation and a number of international funding agencies.