D0 snares last rare boson pair

The D0 collaboration at Fermilab has announced the observation of pairs of Z bosons produced in proton–antiproton collisions. This is the final and rarest state in the series of gauge boson pairs observed and studied by D0 and the CDF experiment at the Tevatron: Wγ, Zγ, WW, WZ and ZZ. Earlier this year CDF published evidence for ZZ production, but the D0 results presented on 25 July showed for the first time sufficient significance to rank as an observation.

D0 observed ZZ production in 2.7 fb^–1 of data with a combination of two analyses that look for Z decays into different final states. One analysis looked for a Z decaying into two electrons or two muons, the other for a Z decaying into neutrinos. The neutrino signature is challenging experimentally, but worthwhile to pursue because it occurs relatively frequently, although even this decay signature is predicted to occur less than once every 10¹² collisions. The process of both Zs decaying to either electrons or muons is an even rarer process. In this analysis, three candidate events were observed with an expected background of less than 0.2 events. The statistical significance of the combined analysis is 5.7 σ, which firmly establishes the discovery of ZZ production at the Tevatron.

D0 measured a cross section for ZZ production of 1.5±0.6 pb, which is in excellent agreement with the prediction of the Standard Model. This is important as Z bosons in the Standard Model do not couple directly to one other. A higher rate would have implied anomalous self-couplings.

The observation of ZZ is connected with the search for the Higgs boson in several ways. The next rarest diboson production processes after ZZ are those involving Higgs bosons; seeing ZZ is an essential step in demonstrating the ability of an experiment to see the Higgs. Pairs of Z bosons also constitute one of the backgrounds to Higgs searches. At small values of the Higgs mass, ZZ can mimic the signature for a Higgs boson produced in association with a Z boson. At large values of the Higgs mass, the Higgs can decay into WW or ZZ. In more ways than one, ZZ observation is an essential prelude to finding, or excluding, the Higgs boson at the Tevatron.