On 10 September 2008, the proton beams were successfully circulated in the main ring of the LHC for the first time.On 19 September 2008, the operations were halted due to a serious fault between two superconducting bending magnets. Due to the time required to repair the resulting damage and to add additional safety features, the LHC is scheduled to be operational in mid-November 2009.
It is anticipated that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, the last unobserved particle among those predicted by the Standard Model. Experimentally verifying the existence of the Higgs boson would shed light on the mechanism of electroweak symmetry breaking, through which the particles of the Standard Model are thought to acquire their mass. In addition to the Higgs boson, new particles predicted by possible extensions of the Standard Model might be produced at the LHC. More generally, physicists hope that the LHC will help answer key questions such as:
* Is the Higgs mechanism for generating elementary particle masses in the Standard Model indeed realised in nature? If so, how many Higgs bosons are there, and what are their masses?
* Are electromagnetism, the strong nuclear force and the weak nuclear force just different manifestations of a single unified force, as predicted by various Grand Unification Theories?
* Why is gravity so many orders of magnitude weaker than the other three fundamental forces? See also Hierarchy problem.
* Is Supersymmetry realised in nature, implying that the known Standard Model particles have supersymmetric partners?
* Are there additional sources of quark flavour violation 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 is the nature of dark matter and dark energy?
* Are there extra dimensions, as predicted by various models inspired by string theory, and can we detect them?
Of the discoveries the LHC might make, the possibility of the discovery of the Higgs particle and supersymmetric partners have been keenly awaited by physicists for over 30 years, although neither of these can be considered certainties. Of the Higgs Stephen Hawking said in a BBC interview that "I think it will be much more exciting if we don't find the Higgs. That will show something is wrong, and we need to think again. I have a bet of one hundred dollars that we won't find the Higgs." Of supersymmetry it has been said "If the LHC does find supersymmetry, this would be one of the greatest achievements in the history of theoretical physics", which Hawking says "would be a key confirmation of string theory" and adds that "Whatever the LHC finds, or fails to find, the results will tell us a lot about the structure of the universe."
The expectation that the Higgs boson will be discovered at the LHC is reinforced by the impressive agreement between the precise measurements of particle processes at the LEP and the Tevatron and the predictions of the Standard Model (formulated under the assumption that the Higgs boson exists). Moreover, there are strong theoretical reasons leading physicists to expect that the LHC will discover new phenomena beyond those predicted by the Standard Model. Referring to the so-called hierarchy problem, namely the fact that the Higgs boson mass is subject to quantum corrections which - barring extremely precise cancellations - would make it so large as to undermine the internal consistency of the Standard Model, Chris Quigg writes: "Physicists have learned to be suspicious of immensely precise cancellations that are not mandated by deeper principles. Accordingly, in common with many of my colleagues, I think it highly likely that both the Higgs boson and other new phenomena will be found with the LHC." He then goes on presenting supersymmetry as a leading candidate for physics beyond the Standard Model, together with composite-Higgs models and large extra dimensions.
The LHC physics program is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the program. While lighter ions are considered as well, the baseline scheme deals with lead ions (see A Large Ion Collider Experiment). This will allow an advancement in the experimental program currently in progress at the Relativistic Heavy Ion Collider (RHIC). The aim of the heavy-ion program is to provide a window on a state of matter known as Quark–gluon plasma, which characterized the early stage of the life of the Universe.