What is the Higgs/antimatter/dark matter?

These questions are implicitly answered in the descriptions of the different LHC detectors. ATLAS and CMS are looking for any new physics, which could include the Higgs boson or dark matter, and LHCb is specifically investigating antimatter.

Will the LHC destroy the world?

Some people predicted that novel entities which may be produced in the LHC could destroy the Earth, or even the Universe. The mechanisms for this could be production of a black hole which will suck the Earth into it; production of a so-called ‘strangelet’ which would convert all matter it touched into further strangelets; or tunnelling to a new vacuum state. There are two compelling angles from which to allay these fears. Firstly, theoretically: for example, the very same physics which predicts formation of micro black holes predicts that they will evaporate very rapidly in a shower of new particles. Secondly, experimentally: the Earth and indeed the entire visible Universe are under constant bombardment from high-energy cosmic rays with energies far in excess of those in the LHC, and yet we’re still here. Nothing can ever be declared entirely safe—but, philosophical misgivings aside, the scientific case for the LHC’s safety is robust.

What happened in the LHC accident in 2009?

The understanding of cryogens and superconductors gained from the show gives the audience enough information for a basic overview of the accident which put the LHC offline for several months shortly after first beam in 2009. It was caused by a superconducting magnet ‘quench’, where a small part of the superconductor rises above its transition temperature and goes normal. This means it suddenly has electrical resistance, which means the enormous current passing through it causes resistive heating, which heats up adjacent parts of the superconductor and heats them above their transition temperature too. This chain reaction then vapourised a large volume of liquid helium—some seven tonnes!—whose rapid expansion as a gas caused an explosion.