LHC - Purpose

Purpose

Physicists hope that the LHC will help answer some of the fundamental open questions in physics, concerning the basic laws governing the interactions and forces among the elementary objects, the deep structure of space and time, and in particular the interrelation between quantum mechanics and general relativity, where current theories and knowledge are unclear or break down altogether. Data are also needed from high energy particle experiments to suggest which versions of current scientific models are more likely to be correct – in particular to choose between the Standard Model and Higgsless models and to validate their predictions and allow further theoretical development. Many theorists expect new physics beyond the Standard Model to emerge at the TeV energy level, as the Standard Model appears to be unsatisfactory. Issues possibly to be explored by LHC collisions include:

• Are the masses of elementary particles actually generated by the Higgs mechanism via electroweak symmetry breaking? It is expected that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, thereby allowing physicists to consider whether the Standard Model or its Higgsless model alternatives are more likely to be correct.
• Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realised in nature, implying that all known particles have supersymmetric partners?
• Are there extra dimensions, as predicted by various models based on string theory, and can we detect them?
• What is the nature of the dark matter that appears to account for 23% of the mass-energy of the universe?

Other open questions that may be explored using high energy particle collisions:

• It is already known that electromagnetism and the weak nuclear force are different manifestations of a single force called the electroweak force. The LHC may clarify whether the electroweak force and the strong nuclear force are similarly just different manifestations of one universal unified force, as predicted by various Grand Unification Theories.
• Why is the fourth fundamental force (gravity) so many orders of magnitude weaker than the other three fundamental forces? See also Hierarchy problem.
• Are there additional sources of quark flavour mixing, beyond those already predicted within the Standard Model?
• Why are there apparent violations of the symmetry between matter and antimatter? See also CP violation.
• What are the nature and properties of quark-gluon plasma, believed to have existed in the early universe and in certain compact and strange astronomical objects today? This will be investigated by heavy ion collisions in ALICE.