33.6 Guts: the unification of forces  (Page 3/12)

It is beyond the scope of this text to go into more detail on the types of quark and gluon interactions that underlie the observable particles, but the theory ( quantum chromodynamics    or QCD) is very self-consistent. So successful have QCD and the electroweak theory been that, taken together, they are called the Standard Model . Advances in knowledge are expected to modify, but not overthrow, the Standard Model of particle physics and forces.

How can forces be unified? They are definitely distinct under most circumstances, for example, being carried by different particles and having greatly different strengths. But experiments show that at extremely small distances, the strengths of the forces begin to become more similar. In fact, electroweak theory’s prediction of the $W^{+}$ , $W^{-}$ , and $Z^0$ carrier particles was based on the strengths of the two forces being identical at extremely small distances as seen in [link] . As discussed in case of the creation of virtual particles for extremely short times, the small distances or short ranges correspond to the large masses of the carrier particles and the correspondingly large energies needed to create them. Thus, the energy scale on the horizontal axis of [link] corresponds to smaller and smaller distances, with 100 GeV corresponding to approximately, ${10}^{-18}\text{m}$ for example. At that distance, the strengths of the EM and weak forces are the same. To test physics at that distance, energies of about 100 GeV must be put into the system, and that is sufficient to create and release the $W^{+}$ , $W^{-}$ , and $Z^0$ carrier particles. At those and higher energies, the masses of the carrier particles becomes less and less relevant, and the $Z^0$ in particular resembles the massless, chargeless, spin 1 photon. In fact, there is enough energy when things are pushed to even smaller distances to transform the, and $Z^0$ into massless carrier particles more similar to photons and gluons. These have not been observed experimentally, but there is a prediction of an associated particle called the Higgs boson    . The mass of this particle is not predicted with nearly the certainty with which the mass of the $W^{+},W^{-},$ and $Z^0$ particles were predicted, but it was hoped that the Higgs boson could be observed at the now-canceled Superconducting Super Collider (SSC). Ongoing experiments at the Large Hadron Collider at CERN have presented some evidence for a Higgs boson with a mass of 125 GeV, and there is a possibility of a direct discovery during 2012. The existence of this more massive particle would give validity to the theory that the carrier particles are identical under certain circumstances.