A headline is a short statement (60 characters max, including spaces) which should entice readers to read on. It should grab their attention and interest, and give them an idea of what to expect in the story. Headlines should be understandable on their own. Use active verbs where possible ("Experiment opens at CERN"; "LHCb discovers new particle") unless the impact is stronger in the passive ("Higgs boson found").
Puns and references to popular culture are welcome, but be careful – many of those reading CERN content in English will not be mothertongue English speakers (indeed, at the time of writing, most web traffic to the CERN's English webpages is from Germany). Remember also that puns may be lost in translation to French.
As always, the most important thing is to think of the readers and remember that we are writing headlines for their benefit, not for our own amusement or to show how clever we are.