The rarest of the diboson processes – ZZ pairs

This figure plots the ZZ reconstructed invariant masses for both simulated and observed events.

When neutrinos are present, ZZ kinematic properties are analyzed with an artificial neural network to distinguish the signal from the other background processes.

The appearance of a single W or Z boson in the data from the Tevatron was a common occurrence. The W or Z boson is a physical particle that emerges when the weak and electromagnetic forces come together after a proton-antiproton collision. Out of all of the diboson processes, ZZ production is the most rare – for every 20,000 W bosons, only one ZZ pair is produced.

In these analyses, the CDF experimenters looked for ZZ production. This is one of the channels in which the Higgs boson would be produced, although at a significantly smaller rate than that expected in the WW channel.

The Z boson decays mostly to pairs of quarks, manifesting as jets of particles in the detector. It makes looking for Z bosons messy because of the plethora of quarks available in a proton-antiproton collision. So, the scientists focused on dibosons that decayed to a pair of leptons, which is more rare than the common quark decays. It makes for a much cleaner analysis with more manageable backgrounds.

The first analysis required both Z bosons to decay to pairs of electrons or muons. The second analysis allowed for one of the two Z bosons to decay to a pair of neutrinos. The neutrinos cannot be directly detected in the detector; their presence is inferred by an imbalance in energy.

The combination of these two analyses leads to a considerably improved measurement over previous CDF results. The ZZ cross section is measured to be 1.64+0.44-0.38 picobarns, which is the most accurate Tevatron measurement to date. It’s also in good agreement with the Standard Model value of 1.4 ± 0.1 picobarns.

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—Edited by Andy Beretvas

These physicists were responsible for this analysis.