Z Boson + Photon Events and Limits on Anomalous Z-Z-photon and Z-photon-photon Couplings

The D-Zero Experiment - February 13, 1998

From 1992-1995 we observed nearly 6 trillion collisions and identified 39 collisions in which both a Z boson and a photon were produced. We studied the properties of these events and published what we learned.

The Tevatron accelerates protons and anti-protons in opposite directions around a circular path and collides them head-on, converting their energy-of-motion into new particles. Our detector identifies the particles, determines where they went, and how much energy they had.

The Standard Model of electroweak interactions describes two ways for proton/anti-proton collisions to produce events with a Z boson and a photon in them. It also predicts characteristics of the events such as the energy spectrum of the photons and the center-of-mass energy of the Z boson plus the photon. The theory predicts that there are no direct interactions between the Z boson and the photon because they both have neutral charge. A non-Standard Model describes what would happen if there were direct interactions, called anomalous couplings, and predicts that the characteristics of a collection of such Z boson + photon events would differ from the Standard Model prediction. We test this prediction.

We studied three decay modes of the Z boson which we can easily identify. In total we found 17 candidate events where the Z decayed into a muon and anti-muon, 18 candidate events where the Z decayed into an electron and anti-electron (positron), and 4 candidate events (in the first 1/8 of the collisions) where the Z decayed into a neutrino and anti-neutrino. In each case, we found that the number and characteristics of the events were consistent with the predictions of the Standard Model. There were a couple of electron anti-electron events where the photon had ~75 GeV/c energy in the direction perpendicular to the original direction of the protons and anti-protons and where the mass of the Z boson plus the photon was ~200 GeV/c^2 (supposing they originated from the decay of a heavier particle). This is interesting because we know of no particles with a mass that high. However, we estimate that 1 time in 8 repeats of our experiment the Standard Model would produce such a result. Thus, this is a tantalizing prospect but not evidence of a new particle or interaction. The neutrino anti-neutrino measurement is particularly interesting because it is unique to the D0 detector.

We set limits on the maximum value of anomalous couplings that could reproduce the data. They are the most restrictive limits on Z-Z-photon and Z-photon-photon couplings available in the world. We hope to improve the result within the next year when we finish the analysis of the remaining 7/8 of the neutrino anti-neutrino data.

The results were summarized in publications in journals which are read by physicists around the world. If you are interested in seeing what these papers are like, see:

For further information contact:
Dr. Tom Diehl (diehl@FNAL.Gov)
Dr. Gerald Guglielmo (gug@FNAL.Gov)
Dr. Greg Landsberg (landsberg@FNAL.Gov)

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Last modified: Fri Feb 13 08:55:31 CST 1998