"Superspace" is the coordinate space of a theory exhibiting supersymmetry. In such a formulation, along with ordinary space dimensions x, y, z, ..., there are also "anticommuting" dimensions whose coordinates are labeled in Grassmann numbers rather than real numbers. The ordinary space dimensions correspond to bosonic degrees of freedom, the anticommuting dimensions to fermionic degrees of freedom.
See also supermanifold (although the definition of a superspace as a supermanifold here does not agree with the definition used in that article).
Rm|n is the Z2-graded vector space with Rm as the even subspace and Rn as the odd subspace. The same definition applies to Cm|n.
The word "superspace" was first used by John Wheeler in an unrelated sense to describe the configuration space of general relativity; for example, this usage may be seen in his 1973 textbook Gravitation.
The smallest superspace is a point which contains neither bosonic nor fermionic directions. Other trivial examples include the n-dimensional real plane Rn, which is a vector space extending in n real, bosonic directions and no fermionic directions. The vector space R0|n, which is the n-dimensional real Grassmann algebra. The space R1|1 of one even and one odd direction is known as the space of dual numbers, introduced by William Clifford in 1873.
The superspace of supersymmetric quantum mechanics
Supersymmetric quantum mechanics with N supercharges is often formulated in the superspace R1|2N, which contains one real direction t identified with time and N complex Grassmann directions which are spanned by Θi and Θ*i, where i runs for 1 to N.
Consider the special case N = 1. The superspace R1|2 is a 3-dimensional vector space. A given coordinate therefore may be written as a triple (t, Θ, Θ*). The coordinates form a Lie superalgebra, in which the gradation degree of t is even and that of Θ and Θ* is odd. This means that a bracket may be defined between any two elements of this vector space, and that this bracket reduces to the commutator on two even coordinates and on one even and one odd coordinate while it is an anticommutator on two odd coordinates. This superspace is an abelian Lie superalgebra, which means that all of the forementioned brackets vanish
where is the commutator of a and b and is the anticommutator of a and b.
One may define functions from this vectorspace to itself, which are called superfields. The above algebraic relations imply that, if we expand our superfield as a power series in Θ and Θ* then we will only find terms at the zeroeth and first orders, because Θ2 = Θ*2 = 0. Therefore superfields may be written as arbitrary functions of t multiplied by the zeroeth and first order terms in the two Grassmann coordinates
One may then define derivatives in the Grassmann directions, which take the first order term in the expansion of a superfield to the zeroeth order term and annihilate the zeroeth order term. One can choose sign conventions such that the derivatives satisfy the anticommutation relations
These derivatives may be assembled into supercharges
whose anticommutators identify them as the fermionic generators of a supersymmetry algebra
where i times the time derivative is the Hamiltonian operator in quantum mechanics. Both Q and its adjoint anticommute with themselves. The supersymmetry variation with supersymmetry parameter ε of a superfield Φ is defined to be
We can evaluate this variation using the action of Q on the superfields
Similarly one may define covariant derivatives on superspace
which anticommute with the supercharges and satisfy a wrong sign supersymmetry algebra
The fact that the covariant derivatives anticommute with the supercharges means the supersymmetry transformation of a covariant derivative of a superfield is equal to the covariant derivative of the same supersymmetry transformation of the same superfield. Thus, generalizing the covariant derivative in bosonic geometry which constructs tensors from tensors, the superspace covariant derivative constructs superfields from superfields.
Four-dimensional N = 1 superspace
Perhaps the most popular superspace in physics is d=4 N=1 super Minkowski space R4|4, which is the direct sum of four real bosonic dimensions and four real Grassmann dimensions. In supersymmetric quantum field theories one is interested in superspaces which furnish representations of a Lie superalgebra called a supersymmetry algebra. The bosonic part of the supersymmetry algebra is the Poincaré algebra, while the fermionic part is constructed using spinors of Grassmann numbers.
For this reason, in physical applications one considers an action of the supersymmetry algebra on the four fermionic directions of R4|4 such that they transform as a spinor under the Poincaré subalgebra. In four dimensions there are three distinct irreducible 4-component spinors. There is the Majorana spinor, the left-handed Weyl spinor and the right-handed Weyl spinor. The CPT theorem implies that in a unitary, Poincaré invariant theory, which is a theory in which the S-matrix is a unitary matrix and the same Poincaré generators act on the asymptotic in-states as on the asymptotic out-states, the supersymmetry algebra must contain an equal number of left-handed and right-handed Weyl spinors. However, since each Weyl spinor has four components, this means that if one includes any Weyl spinors one must have 8 fermionic directions. Such a theory is said to have extended supersymmetry, and such models have received a lot of attention. For example, supersymmetric gauge theories with eight supercharges and fundamental matter have been solved by Nathan Seiberg and Edward Witten, see Seiberg–Witten gauge theory. However in this subsection we are considering the superspace with four fermionic components and so no Weyl spinors are consistent with the CPT theorem.
Note: There are many sign conventions in use and this is only one of them.
This leaves us with one possibility, the four fermionic directions transform as a Majorana spinor θα. We can also form a conjugate spinor
where C is the charge conjugation matrix, which is defined by the property that when it conjugates a gamma matrix, the gamma matrix is negated and transposed. The first equality is the definition of θ while the second is a consequence of the Majorana spinor condition θ* = iγ0Cθ. The conjugate spinor plays a role similar to that of θ* in the superspace R1|2, except that the Majorana condition, as manifested in the above equation, imposes that θ and θ* are not independent.
In particular we may construct the supercharges
which satisfy the supersymmetry algebra
where is the 4-momentum operator. Again the covariant derivative is defined like the supercharge but with the second term negated and it anticommutes with the supercharges. Thus the covariant derivative of a supermultiplet is another supermultiplet.