field. Supersymmetry itself has theoretical problems. Account for neutrino oscillation and mass. Make new nontrivial predictions, such as cold dark matter candidates. Explain why there exists only the observed variety of particle species, and give a model with reasons for producing only these observed particles (since the prediction of non-observed particles is a problem with many current models, such as supersymmetry). Background Before the Standard Model was developed in the 1970s (the key elements of the Standard Model known as quarks were proposed by Murray Gell-Mann and George Zweig in 1964), physicists observed hundreds of different kinds of particles in particle accelerators. These were organized into relationships on their physical properties in a largely ad-hoc system of hierarchies, not entirely unlike the way taxonomy grouped animals based on their physical features. Not surprisingly, the huge number of particles was referred to as the "particle zoo". The Standard Model, which is now the prevailing model of particle physics, dramatically simplified this picture by showing that most of the observed particles were mesons, which are combinations of two quarks, or baryons which are combinations of three quarks, plus a handful of other particles. The particles being seen in the ever-more-powerful accelerators were, according to the theory, typically nothing more than combinations of these quarks. Comparisons of quarks, leptons, and bosons Within the Standard Model, there are several classes of particles. One of these, the quarks, has six types, of which there are three varieties in each (dubbed "colors", red, green, and blue, giving rise to quantum chromodynamics). Additionally, there are six different types of what are known as leptons. Of these six leptons, there are three charged particles: the electron, muon, and tau. The neutrinos comprise the other three leptons, and each neutrino pairs with one the three charged leptons. In the Standard Model, there are also bosons, including the photons and gluons; W, W, and Z bosons; and the Higgs boson; and an open space left for the graviton. Almost all of these particles come in "left-handed" and "right-handed" versions (see chirality). The quarks, leptons, and W boson all have antiparticles with opposite electric charge (or in the case of the neutrinos, opposite weak isospin). Unresolved problems with the Standard Model The Standard Model also has a number of problems which have not been entirely solved. In particular, no successful theory of gravitation based on a particle theory has yet been