Looking for exotic Z boson decays

The diphoton mass distribution of the Drell-Yan and non-resonant (γγ,γ-jet and jet-jet) backgrounds is compared to the data for the region 60 < mγγ < 120 GeV/c2, which includes part of the fit region along with the signal region.

The Z boson, one of the particles responsible for weak interactions, has been studied extensively since its discovery at CERN in 1983. We now know that the Z boson decays into quark-antiquark pairs about 70 percent of the time, into neutrino-antineutrino pairs about 20 percent of the time and into a pair of charged leptons about 10 percent of the time. In addition to these dominant decay modes, the observation of rare Z boson decays, which occur less than 1 percent of the time, would greatly contribute to the understanding of Z boson properties and would test predictions made by the Standard Model.

In a 2012 paper, CDF reported finding 138,172 Z‘s that decayed into electron-positron pairs (leptons) using approximately one-fifth of the Tevatron Run II data; this sample is large enough that we can look for rare decay modes of Z‘s at the Tevatron. Scientists can furthermore test the Standard Model by studying particular Z boson decays not predicted by the theory but still allowed by alternative theories.

To this end, the CDF experiment looks for three different exotic decay modes using the full Tevatron data set. The rare mode Z → π0γ is predicted by the Standard Model to occur less than 1 time in a billion. The modes Z → γγ and Z → π0π0 are forbidden because of quantum mechanics, but we nevertheless search for them in order to rule them out experimentally. In seeking out these three decay modes, we look for a signature that contains two energetic photon-like objects, which appear as isolated electromagnetic showers in our calorimeter.

We reconstruct the invariant mass of these two photon-like particles — two photons, a photon and a neutral pion, or two neutral pions — and plot the data (see top figure). If there had been a decay of one of these particles, the result of our experiment would be a very narrow peak of width 3.5 GeV in a small region around the Z mass (91 GeV/c2).

We observe no such peaks in our data and thus are able to set limits on the Z decay modes. The observed (expected) upper bounds at the 95 percent credibility level are 2.01 (2.34) × 10-5 for π0γ, 1.46 (1.72) × 10-5 for γγ and 1.52 (1.76) × 10-5 for π0π0. These are currently the first such results from a hadron collider and the most stringent limits to date.

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edited by Andy Beretvas

Top row: Karen Bland (Baylor U.). Second row, from left: Ray Culbertson (Fermilab) and Jay Dittmann (Baylor U.). Third row, from left: Craig Group (U. Virginia and Fermilab) and Costas Vellidis (Fermilab).