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The discovery of ZZ production (red check mark) is an essential prelude to finding or excluding the Higgs boson at the Tevatron particle collider at DOE’s Fermi National Accelerator Laboratory. The discovery is the latest in a series of observations of so-called gauge bosons, or force-carrying particles, by DZero and its sister Tevatron experiment, CDF. The series began (from left) with the study of collisions that produced a single W or Z boson and the already rare production of a W boson plus photon; then Z boson plus photon. In the last couple of years, Tevatron experiments discovered the even rarer production of W pairs, then WZ. Now, DZero has discovered the ZZ production. The ZZ process is the final step before reaching the even rarer process of a Higgs boson decaying into a pair of W bosons.
The DZero collaboration searched for signs for ZZ production in nearly 200 trillion proton-antiproton collisions delivered by the Tevatron. In one analysis, scientists looked for Z bosons decaying into pairs of electrons or muons. Just three events were observed in this mode. But the signature is remarkably distinctive: the predicted background is only two tenths of one event (see graph). (Note: Electrons and muons belong to a class of particles known as leptons.)
One of the three ZZ events recorded by the DZero experiment at Fermilab: Each Z boson decayed into a pair of high-energy muons, yielding four muon tracks in the DZero detector. The green bars indicate the direction associated with each muon.
The Fermilab accelerator complex accelerates protons and antiprotons close to the speed of light. Converting energy into mass, the Tevatron collider can produce particles that are heavier than the protons and antiprotons that are colliding. The Tevatron produces billions of proton-antiproton collisions per second, maximizing the chance for discovery. Two experiments, CDF and DZero, search for new types of particles emerging from the collisions.
The DZero detector is about the size of a 3-story house. The detector surrounds the collision point and records the path, energy and charge of short-lived particles emerging from the collisions. Its subsystems record the "debris" emerging from high-energy proton-antiproton collisions, unveiling the forces governing the subatomic world. Tracing the particle tracks back to the center of the collision, scientists discover what processes take place at the core of proton-antiproton collisions.
Some of the 700 scientists of the DZero collaboration in front of the DZero detector shortly before it began taking data in 2001.