Consider a one-dimensional, non-relativistic particle with a two state internal degree of freedom called "spin". Let b and its Hermitian adjointb† signify operators which transform a "spin up" particle into a "spin down" particle and vice versa, respectively. Furthermore, take b and b† to be normalized such that the anticommutator equals 1, and take that b2 equals 0. Let p represent the momentum of the particle and x represent its position with =i, where we use natural units so that. Let W represent an arbitrary differentiable function of x and define the supersymmetric operators Q1 and Q2 as Note that Q1 and Q2 seem self-adjoint. Let the Hamiltonian be where W' signifies the derivative of W. Also note that =0. Under these circumstances, the above system is a toy model of N=2 supersymmetry. The spin down and spin up states are often referred to as the "bosonic" and "fermionic" states, respectively, in an analogy to quantum field theory. With these definitions, Q1 and Q2 map "bosonic" states into "fermionic" states and vice versa. Restricting to the bosonic or fermionic sectors gives two partner potentialsdetermined by
In supersymmetric quantum field theories with four spacetime dimensions, which might have some connection to nature, it turns out that scalar fields arise as the lowest component of a chiral superfield, which tends to automatically be complex valued. We may identify the complex conjugate of a chiral superfield as an anti-chiral superfield. There are two possible ways to obtain an action from a set of superfields:
Integrate a superfield on the whole superspace spanned by and,
or
Integrate a chiral superfield on the chiral half of a superspace, spanned by and, not on.
The second option tells us that an arbitrary holomorphic function of a set of chiral superfields can show up as a term in a Lagrangian which is invariant under supersymmetry. In this context, holomorphic means that the function can only depend on the chiral superfields, not their complex conjugates. We may call such a function W, the superpotential. The fact that W is holomorphic in the chiral superfields helps explain why supersymmetric theories are relatively tractable, as it allows one to use powerful mathematical tools from complex analysis. Indeed, it is known that W receives no perturbative corrections, a result referred to as the perturbative non-renormalization theorem. Note that non-perturbative processes may correct this, for example through contributions to the beta functions due to instantons.
