Physicists at the Large Hadron Collider (LHC), the giant particle-physics experiment near Geneva, Switzerland, have searched for many possible subatomic particles and novel phenomena. They have tried to recreate dark matter, reveal extra dimensions of and collapse matter into microscopic black holes.
But the possibility of an electrically neutral particle that is four times heavier than the top quark — the current heaviest — and that could decay into pairs of photons has apparently never crossed anybody’s mind. No theorist has ever predicted that such a particle should exist. No experiment has ever been designed to look for one.
So when, on 15 December last year, two separate teams at the LHC independently reported hints of such a particle (see Naturehttp://doi.org/bc4t; 2015), the reaction of many experts was similar to that of US physicist Isidor Isaac Rabi when the muon, a heavier relative of the electron, was discovered in 1936: “Who ordered that?”
If the particle exists, the implications would be enormous. Precisely because it is so unexpected, it could be the most important discovery in particle physics since quarks — the elementary constituents of protons and neutrons — were confirmed to exist in the 1970s. Perhaps it would be the biggest deal since the muon itself.