Supercommutator

In mathematics, a Lie superalgebra is a generalisation of a Lie algebra to include a Z₂ grading. Lie superalgebras are important in theoretical physics where they are used to describe the mathematics of supersymmetry. In most of these theories, the ''even'' elements of the superalgebra correspond to bosons and ''odd'' elements to fermions (but this is not always true; for example, the BRST supersymmetry is the other way around).

Definition

Formally, a Lie superalgebra is a nonassociative Z₂-graded algebra, or ''superalgebra'', over a commutative ring (typically R or C) whose product �, Â· called the Lie superbracket or supercommutator, satisfies the two conditions (analogs of the usual Lie algebra axioms, with grading):

Super skew-symmetry:

: ,y-(-1)^ ,x\

The super Jacobi identity:

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where ''x'', ''y'', and ''z'' are pure in the Z₂-grading. Here, ">''x'', denotes the degree of ''x'' (either 0 or 1). The degree of ,yis the sum of degree of x and y modulo 2.

One also sometimes adds the axioms ''x''"> = 0 (if 2 is invertible this follows automatically) and $x,x$x0 for , ''x'',  = 1 (if 3 is invertible this follows automatically). When the ground ring is the integers or the Lie superalgebra is a free module, these conditions are equivalent to the condition that the Poincaré–Birkhoff–Witt theorem holds (and, in general, they are necessary conditions for the theorem to hold).

Just as for Lie algebras, the universal enveloping algebra of the Lie superalgebra can be given a Hopf algebra structure.

A graded Lie algebra (say, graded by Z or N) that is anticommutative and Jacobi in the graded sense also has a $Z_2$ grading (which is called "rolling up" the algebra into odd and even parts), but is not referred to as "super". See note at graded Lie algebra for discussion.

Properties

Let $\mathfrak g = \mathfrak g_0 \oplus \mathfrak g_1$ be a Lie superalgebra. By inspecting the Jacobi identity, one sees that there are eight cases depending on whether arguments are even or odd. These fall into four classes, indexed by the number of odd elements:

# No odd elements. The statement is just that $\mathfrak g_0$ is an ordinary Lie algebra.
# One odd element. Then $\mathfrak g_1$ is a $\mathfrak g_0$-module for the action $\mathrm_a: b \rightarrow$, b \quad a \in \mathfrak g_0, \quad b, , b\in \mathfrak g_1.
# Two odd elements. The Jacobi identity says that the bracket $\mathfrak g_1 \otimes \mathfrak g_1 \rightarrow \mathfrak g_0$ is a ''symmetric'' $\mathfrak g_1$-map.
# Three odd elements. For all $b \in \mathfrak g_1$, ,[b,b = 0.

Thus the even subalgebra $\mathfrak g_0$ of a Lie superalgebra forms a (normal) Lie algebra as all the signs disappear, and the superbracket becomes a normal Lie bracket, while $\mathfrak g_1$ is a linear representation of $\mathfrak g_0$, and there exists a representation of a Lie algebra">linear representation of $\mathfrak g_0$, and there exists a symmetric $\mathfrak g_0$-equivariant">symmetric">representation of a Lie algebra">linear representation of $\mathfrak g_0$, and there exists a symmetric $\mathfrak g_0$-equivariant linear map $\:\mathfrak g_1\otimes \mathfrak g_1\rightarrow \mathfrak g_0$ such that,

:$[\left\,z]+[\left\,x]+[\left\,y]=0, \quad x,y, z \in \mathfrak g_1.$

Conditions (1)–(3) are linear and can all be understood in terms of ordinary Lie algebras. Condition (4) is nonlinear, and is the most difficult one to verify when constructing a Lie superalgebra starting from an ordinary Lie algebra ($\mathfrak g_0$) and a representation ($\mathfrak g_1$).

Involution

A ^∗ Lie superalgebra is a complex Lie superalgebra equipped with an involutive antilinear map from itself to itself which respects the Z₂ grading and satisfies
'x'',''y''sup>* =  'y''^*,''x''^*for all ''x'' and ''y'' in the Lie superalgebra. (Some authors prefer the convention 'x'',''y''sup>* = (−1)^{, ''x'', , ''y'',} 'y''^*,''x''^* changing * to −* switches between the two conventions.) Its universal enveloping algebra would be an ordinary ^*-algebra.

Examples

Given any associative superalgebra $A$ one can define the supercommutator on homogeneous elements by

: ,y= xy - (-1)^yx\

and then extending by linearity to all elements. The algebra $A$ together with the supercommutator then becomes a Lie superalgebra. The simplest example of this procedure is perhaps when $A$ is the space of all linear functions $\mathbf (V)$ of a super vector space $V$ to itself. When $V = \mathbb K^$, this space is denoted by $M^$ or $M(p, q)$. With the Lie bracket per above, the space is denoted $\mathfrak (p, q)$.

The Whitehead product on homotopy groups gives many examples of Lie superalgebras over the integers.

The super-Poincaré algebra generates the isometries of flat superspace.

Classification

The simple complex finite-dimensional Lie superalgebras were classified by Victor Kac.

The basic classical compact Lie superalgebras (that are not Lie algebras) are


SU(m/n) These are the superunitary Lie algebras which have invariants:

:$z.\overline+iw.\overline$

This gives two orthosymplectic (see below) invariants if we take the m z variables and n w variables to be non-commutative and we take the real and imaginary parts. Therefore, we have

:$SU(m/n)=OSp(2m/2n)\cap OSp(2n/2m)$

SU(n/n)/U(1) A special case of the superunitary Lie algebras where we remove one U(1) generator to make the algebra simple.

OSp(''m''/2''n'') These are the orthosymplectic groups. They have invariants given by:

:$x.x+y.z-z.y$

for ''m'' commutative variables (''x'') and ''n'' pairs of anti-commutative variables (''y'',''z''). They are important symmetries in supergravity theories.

D(2/1;$\alpha$) This is a set of superalgebras parameterised by the variable $\alpha$. It has dimension 17 and is a sub-algebra of OSp(9, 8). The even part of the group is O(3)×O(3)×O(3). So the invariants are:

:$A_\mu A_\mu+B_\mu B_\mu+C_\mu C_\mu +\psi^\psi^\varepsilon_\varepsilon_\varepsilon_$

:$A_ + B_ + C_ + A_\mu \Gamma^_\mu \psi\psi + B_\mu \Gamma^_\mu \psi\psi + C_\mu \Gamma^_\mu \psi\psi$

for particular constants $\gamma$.

F(4)
This exceptional Lie superalgebra has dimension 40 and is a sub-algebra of OSp(24, 16). The even part of the group is O(3)xSO(7) so three invariants are:

:$B_ + B_ = 0$
:$A_\mu A_\mu + B_B_ + \psi_^\alpha$
:$A_ + B_ + B_ \sigma_^ \psi^\alpha_k \psi^\beta_k + A_\mu \Gamma_\mu^ \psi^k_\alpha \psi^k_\beta + (\text)$

This group is related to the octonions by considering the 16 component spinors as two component octonion spinors and the gamma matrices acting on the upper indices as unit octonions. We then have $f^\sigma_ \equiv \gamma_\mu$ where ''f'' is the structure constants of octonion multiplication.

G(3)
This exceptional Lie superalgebra has dimension 31 and is a sub-algebra of OSp(17, 14). The even part of the group is O(3)×G2. The invariants are similar to the above (it being a subalgebra of the ''F''(4)?) so the first invariant is:

:$A_\mu A_\mu + C^\mu_\alpha C^\mu_\alpha + \psi_^\nu$

There are also two so-called strange series called p(''n'') and q(''n'').

Classification of infinite-dimensional simple linearly compact Lie superalgebras

The classification consists of the 10 series W(''m'', ''n''), S(''m'', ''n'') ((m, n) ≠ (1, 1)), H(2m, n), K(2''m'' + 1, ''n''), HO(m, m) (''m'' ≥ 2), SHO(''m'', ''m'') (''m'' ≥ 3), KO(''m'', ''m'' + 1), SKO(m, m + 1; β) (''m'' ≥ 2), SHO âˆ¼ (2''m'', 2''m''), SKO âˆ¼ (2''m'' + 1, 2''m'' + 3) and the five exceptional algebras:

::E(1, 6), E(5, 10), E(4, 4), E(3, 6), E(3, 8)

The last two are particularly interesting (according to Kac) because they have the standard model gauge group SU(3)×SU(2)×U(1) as their zero level algebra. Infinite-dimensional (affine) Lie superalgebras are important symmetries in superstring theory. Specifically, the Virasoro algebras with $\mathcal$ supersymmetries are $K(1, \mathcal)$ which only have central extensions up to $\mathcal = 4$.

Category-theoretic definition

In category theory, a Lie superalgebra can be defined as a nonassociative superalgebra whose product satisfies

* cdot,\cdotcirc (+\tau_)=0
* cdot,\cdotcirc ( cdot,\cdototimes \circ(+\sigma+\sigma^2)=0
where σ is the cyclic permutation braiding $( \otimes\tau_) \circ (\tau_\otimes )$. In diagrammatic form:

:

See also

* Gerstenhaber algebra
* Anyonic Lie algebra
* Grassmann algebra
* Representation of a Lie superalgebra
* Superspace
* Supergroup
* Universal enveloping algebra

Notes

References

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Historical

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External links

Irving Kaplansky + Lie Superalgebras

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Supersymmetry
Lie algebras