In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant .
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 in the special case.
The group operation is matrix multiplication. The special unitary group is a normal subgroup of the unitary group , consisting of all unitary matrices. As a compact classical group, is the group that preserves the standard inner product on . It is itself a subgroup of the general linear group,
The groups find wide application in the Standard Model of particle physics, especially in the electroweak interaction and in quantum chromodynamics.
The simplest case, , is the trivial group, having only a single element. The group is isomorphic to the group of quaternions of norm 1, and is thus diffeomorphic to the 3-sphere. Since unit quaternions can be used to represent rotations in 3-dimensional space (uniquely up to sign), there is a surjective homomorphism from to the rotation group whose kernel is . Since the quaternions can be identified as the even subalgebra of the Clifford algebra , is identical to one of the symmetry groups of spinors, Spin(3), that enables a spinor presentation of rotations.
Properties The special unitary group is a strictly real Lie group (vs. a more general complex Lie group). Its dimension as a real manifold is . Topologically, it is compact and simply connected., Proposition 13.11 Algebraically, it is a simple Lie group (meaning its Lie algebra is simple; see below).
The center of is isomorphic to the cyclic group , and is composed of the diagonal matrices for an th root of unity and the identity matrix.
Its outer automorphism group for is , while the outer automorphism group of is the trivial group.
A maximal torus of rank is given by the set of diagonal matrices with determinant . The Weyl group of is the symmetric group , which is represented by signed permutation matrices (the signs being necessary to ensure that the determinant is ).
The Lie algebra of , denoted by , can be identified with the set of traceless anti‑Hermitian complex matrices, with the regular commutator as a Lie bracket. Particle physicists often use a different, equivalent representation: The set of traceless Hermitian complex matrices with Lie bracket given by times the commutator.
Lie algebra
The Lie algebra of consists of skew-Hermitian matrices with trace zero. Proposition 3.24 This (real) Lie algebra has dimension .
The complexification of the Lie algebra is , the space of all complex matrices with trace zero. Section 3.6 A Cartan subalgebra then consists of the diagonal matrices with trace zero, Section 7.7.1 which we identify with vectors in whose entries sum to zero. The roots then consist of all the permutations of .
A choice of simple roots is
So, is of rank and its Dynkin diagram is given by , a chain of nodes: .... Section 8.10.1 Its Cartan matrix is
Its Weyl group or Coxeter group is the symmetric group , the symmetry group of the -simplex.
Fundamental representation In the physics literature, it is common to identify the Lie algebra with the space of trace-zero Hermitian (rather than the skew-Hermitian) matrices. That is to say, the physicists' Lie algebra differs by a factor of from the mathematicians'. With this convention, one can then choose generators that are traceless Hermitian complex matrices, where:
where the are the structure constants and are antisymmetric in all indices, while the -coefficients are symmetric in all indices.
As a consequence, the commutator is:
and the corresponding anticommutator is:
The factor of in the commutation relation arises from the physics convention and is not present when using the mathematicians' convention.
The conventional normalization condition is
The generators satisfy the Jacobi identity:
By convention, in the physics literature the generators are defined as the traceless Hermitian complex matrices with a factor: for , the generators are chosen as , , where are the Pauli matrices, while for the case of one defines where are the Gell-Mann matrices. With these definitions, the generators satisfy the following normalization condition:
Adjoint representation In the -dimensional adjoint representation, the generators are represented by matrices, whose elements are defined by the structure constants themselves:
SU(2)
Using matrix multiplication for the binary operation, forms a group, Exercise 1.5
where the overline denotes complex conjugation.
Diffeomorphism with the 3-sphere S3 If we consider as a pair in where and with real numbers , then the equation becomes
This is the equation of the 3-sphere . This can also be seen using an embedding: the map
where denotes the set of 2-by-2 complex matrices, is an injective real linear map (by considering isomorphic to and isomorphic to ). Hence, the restriction of to the 3-sphere is an embedding of the 3-sphere onto a compact submanifold of , namely .
Therefore, as a manifold, is diffeomorphic to , which shows that is simply connected and that can be endowed with the structure of a compact, connected Lie group.
Isomorphism with group of versors and relation to spatial rotations
The ring of quaternions can be associated with a subring of 2-by-2 complex matrices by the ring isomorphism mapping
to
Additionally, the determinant of the matrix is the squared norm of the corresponding quaternion. Clearly any matrix in is of this form and, since it has determinant , the corresponding quaternion has norm . Thus, restricting the above map gives a group isomorphism between and the group of quaternions with norm .
Quaternions of norm are called versors since they can be associated with rotations in 3-dimensional space: Every versor is naturally associated to a spatial rotation in 3 dimensions, and the product of versors is associated to the composition of the associated rotations. Furthermore, every rotation arises from exactly two versors in this fashion. In short: there is a 2:1 surjective homomorphism from to the rotation group ; consequently is isomorphic to the quotient group , the manifold underlying is obtained by identifying antipodal points of the 3-sphere , and is the universal cover of .
Lie algebra The Lie algebra of consists of skew-Hermitian matrices with trace zero. Proposition 3.24 Explicitly, this means
The Lie algebra is then generated by the following matrices,
which have the form of the general element specified above.
This can also be written as using the Pauli matrices.
These satisfy the quaternion relationships , , and . The commutator bracket is therefore specified by
The above generators are related to the Pauli matrices by and . This representation is routinely used in quantum mechanics to represent the spin of fundamental particles such as electrons. They also serve as unit vectors for the description of our 3 spatial dimensions in loop quantum gravity. They also correspond to the Pauli X, Y, and Z gates, which are standard generators for the single qubit gates, corresponding to 3D rotations about the axes of the Bloch sphere.
The Lie algebra serves to work out the representations of .
SU(3)
The group is an 8-dimensional simple Lie group consisting of all unitary matrices with determinant .
Topology The group is a simply-connected, compact Lie group. Proposition 13.11 Its topological structure can be understood by noting that acts transitively on the unit sphere in . The stabilizer of an arbitrary point in the sphere is isomorphic to , which topologically is a 3-sphere. It then follows that is a fiber bundle over the base with fiber . Since the fibers and the base are simply connected, the simple connectedness of then follows by means of a standard topological result (the long exact sequence of homotopy groups for fiber bundles). Section 13.2
The -bundles over are classified by since any such bundle can be constructed by looking at trivial bundles on the two hemispheres and looking at the transition function on their intersection, which is a copy of , so
Then, all such transition functions are classified by homotopy classes of maps
and as rather than , cannot be the trivial bundle , and therefore must be the unique nontrivial (twisted) bundle. This can be shown by looking at the induced long exact sequence on homotopy groups.
Representation theory The representation theory of is well-understood. Chapter 6 Descriptions of these representations, from the point of view of its complexified Lie algebra , may be found in the articles on Lie algebra representations or the Clebsch–Gordan coefficients for .
Lie algebra The generators, , of the Lie algebra of in the defining (particle physics, Hermitian) representation, are
where , the Gell-Mann matrices, are the analog of the Pauli matrices for :
These span all traceless Hermitian matrices of the Lie algebra, as required. Note that , , are antisymmetric.
They obey the relations
or, equivalently,
The are the structure constants of the Lie algebra, given by
while all other not related to these by permutation are zero. In general, they vanish unless they contain an odd number of indices from the set .
The symmetric coefficients take the values
They vanish if the number of indices from the set is odd.
A generic group element generated by a traceless Hermitian matrix , normalized as , can be expressed as a second order matrix polynomial in :;
where
Important subgroups In physics the special unitary group is used to represent fermionic symmetries. In theories of symmetry breaking it is important to be able to find the subgroups of the special unitary group. Subgroups of that are important in GUT physics are, for ,
where denotes the direct product and , known as the circle group, is the multiplicative group of all complex numbers with absolute value 1.
For completeness, there are also the orthogonal and symplectic subgroups,
Since the rank of is and of is 1, a useful check is that the sum of the ranks of the subgroups is less than or equal to the rank of the original group. is a subgroup of various other Lie groups,
See Spin group and Simple Lie group for , , and .
There are also the accidental isomorphisms: , , and .
One may finally mention that is the double covering group of , a relation that plays an important role in the theory of rotations of 2-spinors in non-relativistic quantum mechanics.
Generalized special unitary group For a field , the generalized special unitary group over F, , is the group of all linear transformations of determinant 1 of a vector space of rank over which leave invariant a nondegenerate, Hermitian form of signature . This group is often referred to as the special unitary group of signature over . The field can be replaced by a commutative ring, in which case the vector space is replaced by a free module.
Specifically, fix a Hermitian matrix of signature in , then all
satisfy
Often one will see the notation without reference to a ring or field; in this case, the ring or field being referred to is and this gives one of the classical Lie groups. The standard choice for when is
However, there may be better choices for for certain dimensions which exhibit more behaviour under restriction to subrings of .
Example An important example of this type of group is the Picard modular group which acts (projectively) on complex hyperbolic space of dimension two, in the same way that acts (projectively) on real hyperbolic space of dimension two. In 2005 Gábor Francsics and Peter Lax computed an explicit fundamental domain for the action of this group on .
A further example is , which is isomorphic to .
SU(1, 1) where denotes the complex conjugate of the complex number .