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Note that the replacement is not so arbitrary, the processes have to obey the conservation of Rparity. Supersymmetry so simplifies the mathematics of quantum field theory and String Theory that it allows theoriests to obtain solutions that would otherwise be far beyond their calculating ability. The general idea is for the unification of all forces of nature including quantum gravity.  
Figure 1512a Superpartner 
Since the graviton has spin 2, while the other gauge bosons have spin 1 and 0, supersymmetry is used to mix them. Starting with the graviton state of spin 2 and acting by supersymmetry generators we get the following chain of states : spin 2 spin 3/2 spin 1 spin 1/2 spin 0. 
in the computation of these masses in an entirely natural way. Hence, the enormous difference between the electroweak scale and GUT scale is an uncontrived feature of supersymmetric models, i.e., supersymmetry provides a natural explanation. This cancelling mechanism becomes more important as it is discovered that the known mass of the Higgs and top quark implies an universe in metastable state (Figure 1512c). It requires an explanation to show why the universe is still here and we are still alive. That's why the failure (up till 2014) of the LHC to find evidence of supersymmetry almost propels a crisis in physics.  
Figure 1512b Supersymmetry _{} 
Figure 1512c State of the Universe _{} 
Another motivation for supersymmetry is its intimate connection with gravity. The supersymmetry considered so far is global. However, if the SUSY generator is local  meaning that it depends on spacetime, and we impose invariance to a theory under such transformation, then the formulation forces the introduction of a gauge field that turns out to have the properties of a graviton. In fact this new type of theory is just Einstein's general relativity within the framework of quantum fields and is thus called supergravity. The problem with supergravity is the divergence. Although it is not as divergent as ordinary gravity, it is still not finite. The infinities cannot be canceled out at the three loops level. Thus such attempt to merge general relativity with quantum mechanics ultimately met with failure, the more promising application is associated with ten dimensional string theories. Supergravity is the low energy limit where the structureless point particle is a good approximation. See "Unitarity Method" for an update. 

Figure 1512d Hierarchy Problem [view large image] 
Supersymmetry also addresses a host of other mysteries in modern physics such as the tremendous concentration of energy in the universe (the cosmological constant problem), the origin of cosmic inflation, matter/antimatter asymmetry, the nature of cold dark matter, and the special forms of the Higgs interactions. 
If supersymmetry were an exact, unbroken symmetry, the superpartners would have the same mass of the ordinary particles. However, no such particles have ever been observed, and supersymmetry, therefore, if it is a true symmetry of particle physics, must be broken. If the breaking of supersymmetry is in such a way that the explanation for the hierarchy problem is still valid, then the mass of the superpartners would be in the order of 10^{3} Gev  just at the mass range accessible to the new generation of accelerators. All current models of supersymmetry breaking predict flavorchanging interactions. These are processes that change quarks or leptons into their other generation  processes not observed in experiments. How to break supersymmetry but prevent flavor changing is a crucial challenge if supersymmetry is to succeed in addressing the hierarchy problem. Figure 1512e depicts a model developed by Lisa Randall. It resolves the flavorchanging problem with two branes sequestered (separated) in a fifth dimension. In the model, the Standard model particles are on one brane, and particles that break supersymmetry are sequestered on the other. Gravitons in the fifth dimension serve as the intermediary particle that carry the effect of supersymmetry breaking to the Standard model particles. Such form of interaction would generate the necessary superpartner masses (in the 250 Gev range), but do not cause quarks or leptons to change to another flavor particles.  
Figure 1512e Supersymmetry Breaking [view large image] 