Studies of WW and WZ Production and Limits on Anomalous W-W-gamma and W-W-Z Couplings
The D-Zero Experiment - October 17, 1999
From 1992-1995 we searched proton-antiproton collisions at the Fermilab Tevatron for occurances of two special kinds of events involving W and Z bosons, the carriers of the weak nuclear force. The first kind of event contained two W bosons, where one W decayed to a muon and a neutrino, and the other to two jets. The second kind of event contained a W boson plus a Z boson, where the W decayed to a muon and a neutrino or an electron and a neutrino, and the Z boson decayed to an electron and a positron. We found 224 WW candidates and 1 WZ candidate. From the number of observed events, we can infer the strength of the electroweak coupling between the W boson and the photon and between the W and Z bosons. These measurements can be compared with the predictions of our current understanding of sub-nuclear forces, called the "Standard Model".
In 6 trillion proton-antiproton collisions, we found 224 events with a muon, a neutrino, and two or more jets, which looked like they could have originated from the production and decay of two W bosons. After the fact, we can use the Standard Model to "predict" that we should have seen 229 events (a number statistically similar to what was actually observed) and that only 4 to 5 of the observed events were really from WW production. The rest are from less-interesting processes which mimic the more-interesting WW production. In short, the Standard Model "prediction" turned out to be very accurate.
In the same 6 trillion proton-antiproton collisions, we found only one collision which satisfied our criteria for an event that contained both a W boson and a Z boson. The low number of events is not surprising, for the Standard Model had predicted that the most probable outcome was one candidate. The theory also predicted that it was twice as likely to be a process that mimicked a WZ event as it was to be an actual WZ. In fact, because the rate of events was expected to be low, we had decided to use rather modest standards for identifying WZ candidates, trading a lower signal-to-fake probability for more total signal. After further study it was realized that the WZ event could have passed more stringent identification tests. The kinematic properties (direction and energies of particles) of the event are described in our paper. It is the first WZ candidate ever described in a physics journal. There is a link to a projected view of the WZ candidate below.
Based on the observed number of WW and WZ events, and using the results of our previous measurements, we concluded that the electroweak coupling strengths are within 39% of, and agree within our ability to measure them, the Standard Model predictions.
The results were summarized in a journal called Physical Review D. For papers to be published in this journal, it must have passed a through critical review by a particle physicist who is an expert on this particular topic but who is not involved in producing the result. This journal is read by physicists around the world. If you are interested in seeing what the paper is like, see the second link below.
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