A press release from CERN cautiously announces the discovery of what may be the long-sought Higgs boson, with a spokesperson being quoted as saying, “The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found.”
Note that they are not saying it definitely is the Higgs. The release goes on to say, “The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the long-sought Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic?”
This is usually the way with scientific discoveries. It is never a clean event. The data is collected over a long time, the analyses can take months if not years, and the initial claims are almost always tentative and await corroborative evidence. If that evidence comes about, and that process could also take years, then the community will retrospectively label this day as the day on which the Higgs was discovered, even though the process took so long and cannot be pinned to any single event.
While popular writing on science often emphasizes the unexpected discovery, of finding something while looking for something else (like in the case of X-rays or the cosmic microwave background), that is actually quite rare. Most discoveries are those which were predicted accurately and were not surprises at all. That should not take away from the fact that they are tremendous achievements.