A Grand Unified Theory of the weak, electromagnetic and strong color forces can be achieved with the existence of supersymmetric partners to be discovered around a thousand GeV in energy. (A GeV is around the mass of the proton and neutron.) The evolution of coupling strengths of the three forces with increasing energy then leads them to converge to a unified strength at around 10^17 GeV. There are more pages on this blog under particle physics that describe these concepts.
There are more unified particles at the unified mass scale, which are given their masses by a set of heavy Higgs with masses at the same scale. This presents a problem for the light Higgs of the Standard Model, which may be the one that has been discovered. The heavy Higgs couples strongly to the Standard Model Higgs, and this brings its mass up to that of the heavy Higgs, destroying the Standard Model. This can be prevented with supersymmetric particles, which can cancel this coupling and leave the Standard Model Higgs light. They do this since the virtual supersymmetric partners of particles cancel the contribution of the particles with the same coupling and properties.
The inclusion of spin 1/2 particles along with spin 0 and 1 particles into string theory, leads to supersymmetry. This is the connection between the discovery of the light Higgs of the Standard Model and string theory. No supersymmetric particle has yet been found at the Large Hadron Collider, but there are many masses and mixings in the general supersymmetric models, so the most general models are far from being ruled out.
String theory was found to also have a spin 2 particle, which is the graviton, the field particle of gravity. So string theories can describe all four fundamental forces and are called the Theory of Everything.
String theories require ten dimensional spaces to cancel certain infinities. Many models have been examined for extra dimensions to our four, and what effects could show up in experiments if they are microscopic or have excitations in the energy range of the Large Hadron Collider.
Brian Greene’s website: www.briangreene.org
Brian Greene’s book The Elegant Universe (1999) describes superstring theory and extra dimensions.
John Schwarz’ superstring website: www.superstringtheory.com
Another book by a string theorist is Warped Passages, Unraveling the Mysteries of the Universe’s Hidden Dimensions (2005) by Lisa Randall.
