The Standard Model (pictured right) is one of the great successes of 20th century physics. But even without including gravity, we know that the Standard Model cannot be the whole story. There must be a deeper conceptual structure lurking behind it. The Standard Model signals its own incompleteness in several ways. Here are three.
The hierarchy problem is the mystery of why gravity is so much weaker than the other forces. This is related to the mass of the Higgs boson, which has now been measured as 125 GeV (around 130 times the mass of a proton) at the Large Hadron Collider (LHC) at CERN.
Worryingly, the theoretical value of this mass calculated from the Standard Model is enormously larger than this experimental result, unless parameters in the Standard Model are fine-tuned to a level of one part in 1030. This is an almost farcical demand for precision, leading many to believe that there must be a better solution. Ideas such as supersymmetry can help, but the problem is not yet settled.
Issues also surround the structure of fermion masses. The matter particles in the Standard Model group into exactly three generations, with each progressively heavier than the one before. There is no fourth generation. The explanation for these facts is not known, but must necessarily involve physics beyond that of the Standard Model.
Finally there’s the strong CP problem. The Standard Model contains a constant angle with no apparent preferred value. This angle affects how electric charge appears to be distributed in the neutron. Measuring this charge distribution reveals that the angle is zero to within approximately one-billionth of a degree. It seems unlikely that the angle would be so close to zero by pure chance; there should be a deeper explanation.
These three examples illustrate that the Standard Model is not complete by itself, even when we neglect gravity. We need to find some deeper underlying structure. String phenomenology is the vibrant branch of research which tries to determine whether string theory can provide a more profound framework for particle physics.
In particular researchers aim to construct string models that reproduce the Standard Model, and thereby attempt to account for its unexplained properties. Moreover some aspire to predict new phenomena beyond Standard Model physics.
Theorists proceed by building extra-dimensional models, possibly involving D-branes, where six of the dimensions are curled up. The precise shape of the extra dimensions affects the four-dimensional physics predicted. Questions about masses of particles or the presence of supersymmetry turn into queries about the geometry of the extra dimensions.
The importance of extra-dimensional structure means string models can relate apparently disparate areas of physics; a single geometric feature can determine several different properties of the universe. Hence unraveling one problem can lead to a prediction in a completely separate discipline!