When the W boson was discovered in the 1980s, nobody spoke of sigmas. How many sigmas was it at that time?
Look at figure 1.3 in this lecture.
The number of Z bosons, about 22, over an extrapolated background of 0, makes it a five sigma.
The W is more complicated, since it is detected by the Jacobean peak (search for Jacobean) of the seen electron, fig. 1.4, but still it is well over 5 sigma.
Actually when a phenomenon is way out of the possible background, even one event is significant beyond the statistics of one. Take the lambda baryon. Even if you see only one, there is not doubt of its existence. A pair production of a proton and a negative pion is not something that can be swept under the rug of statistics (except if it is a measurement error, which is a different story).