Caps. Always spell out first time, with abbreviation in brackets. Use the abbreviation thereafter.
Experiments at the Large Hadron Collider (LHC) suggest...
Beware of tautologies: Don't use "The LHC machine" or "The LHC accelerator" as this is equivalent to saying "The Large Hadron Collider accelerator..." which doesn't mean anything (The LHC doesn't accelerate accelerators).
Note: The Large Hadron Collider is a large machine which collides hadrons, not a machine which collides large hadrons (there is no such concept as a large hadron).