Use double quotes in text, captions, etc. Single quotes for headlines, and quotes within quotes. See quotes.
Do not use quotation marks to soften headlines.
Higgs boson "found" does not mean anything. The quote marks render the headline ambiguous and confuse the reader. Has it been found or not?
CERN physicists spot tantalizing hints of Higgs boson is more elegant and gives more information, without the need for quotes.