Today’s exciting press release from Tevatron on the Higgs boson keeps its head cool and say that physicists saw a “hint” of the Higgs boson because the signal is barely above the two-sigma level. In an effort to explain the reasons for this, the press release says:
Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas. A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas.
I wonder: what's the difference between “claim[ing] evidence of a new particle” and “a discovery” (of said new particle)? Is this the common vocabulary for these levels (three and five sigmas) of signal?
The reason I ask is that I can understand the need for having different vocabulary to talk about discoveries with different levels of certitude, and I could see, say, the need for using “discovering a particle” and “confirming the discovery of a particle”, but “evidence” and “discovery” seem semantically very close to me.
Edit — to be clearer, the question is: how universal are the choices of terms (“evidence”, “discovery”) and associated incertitudes?