Limits on Anomalous W-photon Couplings from W + photon events at the Fermilab Tevatron
The DØ Collaboration

Feb. 1, 1997

Working with a data sample in which about a hundred thousand W bosons are detected, the physicists of the DØ ("D-Zero") Collaboration at Fermilab have made new measurements of the coupling of the photon to the W boson and have used the results to set limits on possible anomalies in this coupling. The Standard Model of particle physics is a non-Abelian theory and therefore predicts that the gauge bosons interact with each other, allowing coupling vertices such as W-W-Z and W-W-photon. The value of the couplings may be modified by radiative corrections involving particles predicted by extensions to the Standard Model such as Supersymmetry. Models based on substructure of the gauge bosons would also lead to non Standard Model or "anomalous" couplings. Making experimental measurements of the couplings is therefore an important test of the Standard Model and its extensions.

The latest measurement reported by the DØ Collaboration in a paper submitted to Physical Review Letters is the most precise experimental test of the W-W-photon coupling to date. DØ physicists used events in which a high energy photon is produced in association with a W boson (indicated by its decay to an electron or muon and a neutrino) to test the coupling. Within the experimental error, the measurement was in agreement with the Standard Model predictions. In particular, the non-Abelian nature of the W-W-photon coupling was revealed. The measurement ruled out (at the 95% confidence level) the possibility that the photon couples only to the electric charge of the W and not to the SU(2) weak component of the W charge.

The DØ measurement also allows limits to be set on possible anomalous couplings (i.e. deviations from the Standard Model predictions). Limits on the anomalies in the two dimensionless couplings (lambda and kappa) were set at < 0.3 and < 0.94 respectively. These are the tightest direct limits on the anomalous W-W-photon couplings available to date.

A copy of the paper ("Limits on Anomalous WWgamma Couplings from p pbar --> Wgamma + X Events at sqrt(s) = 1.8 TeV"), in preprint form, is available from hep-ex/9612002.

For further information contact Dr. John Ellison, Univ. of California, Riverside, email:

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Figures from the "Limits on Anomalous W-photon Couplings from W + photon events at the Fermilab Tevatron",
The DØ Collaboration.
Figure 1 (postscript) Figure 2 (postscript)

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