The SM is not a complete theory since it does not take into account gravity. Physicists have not yet been able to come up with a quantum field theory for gravity, but other theories are being developed to explain gravity such as string theories. A big problem with the SM was that it could not predict the mass of the particles. Specifically the neutrinos were assumed to be massless but have been experimentally proven to have mass. One of the most popular topics of the SM that has yet to be proven experimentally is the Higgs Boson. This particle has been used theoretically to calculate the mass of some of the other members of the SM. At this time it is believed by some that the LHC will be able to go into the energy regime needed to detect the Higgs Boson.
The Higgs Boson is the exchange particle in the Higgs Boson field, which extends the Standard Model to explain how particles acquire the properties associated with mass. As an example, consider the electroweak force. This force unifies electromagnetism with the weak nuclear force. The exchange particle for the electromagnetic force is the photon, which is known to be massless.31 The exchange particles for the weak force, W and Z bosons, on the other hand have masses 80 times greater than a proton. The question is, if these particles are part of the same electroweak force, why is there such a difference in their masses?32 It is hoped that the Higgs Boson will help to answer questions of this nature.
At present, we know the existence of four fundamental forces: the Strong, Weak, Electromagnetic and Gravitational force. We have been able to determine the exchange particles mediating the first three forces; however the exchange particle for gravity, the so called graviton, still remains a mystery. Gravity plays a role in the formation of the cosmos, so finding the particle that mediates this force is of somewhat great importance. However, it is impossible to detect a Graviton with any physically reasonable detector. Even if one was to build a perfect detector the size of Jupiter, in orbit around a neutron start, we would only detect one graviton every ten years!33 Fortunately, there are other ways of "detecting" Gravitons, by detecting gravitational waves.
It is hoped that the LHC will produce evidence for the Higgs Boson. If the standard model is the correct theory for particle physics, it is expected that a Higgs Boson will be detected every couple hours. In addition to this, experiments at the LHC may give insight about the nature of gravity and if the four fundamental forces are all just different manifestations of one single force. The LHC also hopes that the energies generated from collisions will give rise to new, never before seen particles which may in turn help with the unification of the four fundamental forces.