In mathematics, the special unitary group of degree, denoted, is the Lie group of unitary matrices with determinant 1.
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
Cn
\operatorname{SU}(n)\subset\operatorname{U}(n)\subset\operatorname{GL}(n,C).
The groups find wide application in the Standard Model of particle physics, especially in the electroweak interaction and in quantum chromodynamics.[1]
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 (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 in fact identical to one of the symmetry groups of spinors, Spin(3), that enables a spinor presentation of rotations.
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.[2] Algebraically, it is a simple Lie group (meaning its Lie algebra is simple; see below).[3]
Z/nZ
Its outer automorphism group for is
Z/2Z,
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
ak{su}(n)
The Lie algebra
ak{su}(n)
\operatorname{SU}(n)
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
i
TaTb=\tfrac{1}{2n}\deltaabIn+
n2-1 | |
\tfrac{1}{2}\sum | |
c=1 |
\left(ifabc+dabc\right)Tc
As a consequence, the commutator is:
~\left[Ta,Tb\right]~=~i
n2-1 | |
\sum | |
c=1 |
fabcTc ,
\left\{Ta,Tb\right\}~=~\tfrac{1}{n}\deltaabIn+
n2-1 | |
\sum | |
c=1 |
{dabcTc}~.
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
n2-1 | |
\sum | |
c,e=1 |
dacedbce=
n2-4 | |
n |
\deltaab~.
[Ta,[Tb,Tc]]+[Tb,[Tc,Ta]]+[Tc,[Ta,Tb]]=0.
By convention, in the physics literature the generators
Ta
1/2
SU(2)
1 | |
2 |
\sigma1,
1 | |
2 |
\sigma2,
1 | |
2 |
\sigma3
\sigmaa
SU(3)
Ta=
1 | |
2 |
λa
λa
Tr(TaTb)=
1 | |
2 |
\deltaab.
In the -dimensional adjoint representation, the generators are represented by matrices, whose elements are defined by the structure constants themselves:
\left(Ta\right)jk=-ifajk.
See also: Versor, Pauli matrices and Representation theory of SU(2).
Using matrix multiplication for the binary operation, forms a group,[7]
\operatorname{SU}(2)=\left\{\begin{pmatrix}\alpha&-\overline{\beta}\ \beta&\overline{\alpha}\end{pmatrix}: \alpha,\beta\inC,|\alpha|2+|\beta|2=1\right\}~,
If we consider
\alpha,\beta
C2
\alpha=a+bi
\beta=c+di
|\alpha|2+|\beta|2=1
a2+b2+c2+d2=1
This is the equation of the 3-sphere S3. This can also be seen using an embedding: the map
\begin{align} \varphi\colonC2\to{}&\operatorname{M}(2,C)\\[5pt] \varphi(\alpha,\beta)={}&\begin{pmatrix}\alpha&-\overline{\beta}\ \beta&\overline{\alpha}\end{pmatrix}, \end{align}
\operatorname{M}(2,C)
C2
R4
\operatorname{M}(2,C)
R8
\operatorname{M}(2,C)
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.
Quaternions of norm 1 are called versors since they generate the rotation group SO(3):The matrix:
\begin{pmatrix}a+bi&c+di\ -c+di&a-bi \end{pmatrix} (a,b,c,d\inR)
a\hat{1}+b\hat{i}+c\hat{j}+d\hat{k}
This map is in fact a group isomorphism. 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 is isomorphic to the group of versors.[8]
See main article: Quaternions and spatial rotation. 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 ; 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 .
ak{su}(2)=\left\{\begin{pmatrix}i a&-\overline{z}\ z&-i a\end{pmatrix}: a\inR,z\inC\right\}~.
The Lie algebra is then generated by the following matrices,
u1=\begin{pmatrix} 0&i\\ i&0 \end{pmatrix}, u2=\begin{pmatrix} 0&-1\\ 1&0 \end{pmatrix}, u3=\begin{pmatrix} i&0\\ 0&-i \end{pmatrix}~,
This can also be written as
ak{su}(2)=\operatorname{span}\left\{i\sigma1,i\sigma2,i\sigma3\right\}
These satisfy the quaternion relationships
u2 u3=-u3 u2=u1~,
u3 u1=-u1 u3=u2~,
u1u2=-u2 u1=u3~.
\left[u3,u1\right]=2 u2, \left[u1,u2\right]=2 u3, \left[u2,u3\right]=2 u1~.
The above generators are related to the Pauli matrices by
u1=i \sigma1~,u2=-i \sigma2
u3=+i \sigma3~.
The Lie algebra serves to work out the representations of .
See also: Clebsch–Gordan coefficients for SU(3).
The group is an 8-dimensional simple Lie group consisting of all unitary matrices with determinant 1.
The group is a simply-connected, compact Lie group.[10] Its topological structure can be understood by noting that acts transitively on the unit sphere
S5
C3\congR6
The -bundles over are classified by
3\right) | |
\pi | |
4d\left(S |
=Z2
5 | |
S | |
N, |
5 | |
S | |
S |
5 | |
S | |
N |
\cap
5 | |
S | |
S |
\simeqS4
Then, all such transition functions are classified by homotopy classes of maps
\left[S4,SU(2)\right]\cong\left[S4,S3\right]=
3\right) | |
\pi | |
4d\left(S |
\congZ/2
\pi4(SU(3))=\{0\}
Z/2
The representation theory of is well-understood.[12] Descriptions of these representations, from the point of view of its complexified Lie algebra
ak{sl}(3;C)
The generators,, of the Lie algebra
ak{su}(3)
Ta=
λa | |
2 |
~,
\begin{align} λ1={}&\begin{pmatrix}0&1&0\ 1&0&0\ 0&0&0\end{pmatrix},&λ2={}&\begin{pmatrix}0&-i&0\ i&0&0\ 0&0&0\end{pmatrix},&λ3={}&\begin{pmatrix}1&0&0\ 0&-1&0\ 0&0&0\end{pmatrix},\\[6pt] λ4={}&\begin{pmatrix}0&0&1\ 0&0&0\ 1&0&0\end{pmatrix},&λ5={}&\begin{pmatrix}0&0&-i\ 0&0&0\ i&0&0\end{pmatrix},\\[6pt] λ6={}&\begin{pmatrix}0&0&0\ 0&0&1\ 0&1&0\end{pmatrix},&λ7={}&\begin{pmatrix}0&0&0\ 0&0&-i\ 0&i&0\end{pmatrix},&λ8=
1 | |
\sqrt{3 |
These span all traceless Hermitian matrices of the Lie algebra, as required. Note that are antisymmetric.
They obey the relations
\begin{align} \left[Ta,Tb\right]&=i
8 | |
\sum | |
c=1 |
fabcTc,\\ \left\{Ta,Tb\right\}&=
1 | |
3 |
\deltaabI3+
8 | |
\sum | |
c=1 |
dabcTc, \end{align}
\begin{align} \left[λa,λb\right]&=2i
8 | |
\sum | |
c=1 |
fabcλc,\\ \{λa,λb\}&=
4 | |
3 |
\deltaabI3+
8{d | |
2\sum | |
abc |
λc}. \end{align}
The are the structure constants of the Lie algebra, given by
\begin{align} f123&=1,\\ f147=-f156=f246=f257=f345=-f367&=
1 | |
2 |
,\\ f458=f678&=
\sqrt{3 | |
The symmetric coefficients take the values
\begin{align} d118=d228=d338=-d888&=
1 | |
\sqrt{3 |
They vanish if the number of indices from the set is odd.
A generic group element generated by a traceless 3×3 Hermitian matrix, normalized as, can be expressed as a second order matrix polynomial in :[13]
\begin{align} \exp(i\thetaH)={} &\left[-
1 | |
3 |
I\sin\left(\varphi+
2\pi | |
3 |
\right)\sin\left(\varphi-
2\pi | |
3 |
\right)-
1 | |
2\sqrt{3 |
\varphi\equiv
1 | \left[\arccos\left( | |
3 |
3\sqrt{3 | |
As noted above, the Lie algebra
ak{su}(n)
The complexification of the Lie algebra
ak{su}(n)
ak{sl}(n;C)
Cn
A choice of simple roots is
\begin{align} (&1,-1,0,...,0,0),\\ (&0,1,-1,...,0,0),\\ &\vdots\\ (&0,0,0,...,1,-1). \end{align}
So, is of rank and its Dynkin diagram is given by, a chain of nodes: ....[17] Its Cartan matrix is
\begin{pmatrix} 2&-1&0&...&0\\ -1&2&-1&...&0\\ 0&-1&2&...&0\\ \vdots&\vdots&\vdots&\ddots&\vdots\\ 0&0&0&...&2 \end{pmatrix}.
Its Weyl group or Coxeter group is the symmetric group, the symmetry group of the -simplex.
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
\operatorname{GL}(n,R)
M\in\operatorname{SU}(p,q,R)
\begin{align} M*AM&=A\\ \detM&=1. \end{align}
Often one will see the notation without reference to a ring or field; in this case, the ring or field being referred to is
C
\operatorname{F}=C
A=\begin{bmatrix} 0&0&i\\ 0&In-2&0\\ -i&0&0 \end{bmatrix}.
However, there may be better choices for for certain dimensions which exhibit more behaviour under restriction to subrings of
C
\operatorname{SU}(2,1;Z[i])
\operatorname{SL}(2,9;Z)
A further example is
\operatorname{SU}(1,1;C)
\operatorname{SL}(2,R)
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,
\operatorname{SU}(n)\supset\operatorname{SU}(p) x \operatorname{SU}(n-p) x \operatorname{U}(1),
For completeness, there are also the orthogonal and symplectic subgroups,
\begin{align} \operatorname{SU}(n)&\supset\operatorname{SO}(n),\\ \operatorname{SU}(2n)&\supset\operatorname{Sp}(n). \end{align}
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,
\begin{align} \operatorname{SO}(2n)&\supset\operatorname{SU}(n)\\ \operatorname{Sp}(n)&\supset\operatorname{SU}(n)\\ \operatorname{Spin}(4)&=\operatorname{SU}(2) x \operatorname{SU}(2)\\ \operatorname{E}6&\supset\operatorname{SU}(6)\\ \operatorname{E}7&\supset\operatorname{SU}(8)\\ \operatorname{G}2&\supset\operatorname{SU}(3) \end{align}
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.
SU(1,1)=\left\{\begin{pmatrix}u&v\ v*&u*\end{pmatrix}\inM(2,C):uu*-vv*=1\right\},
~u*~
This group is isomorphic to and [19] where the numbers separated by a comma refer to the signature of the quadratic form preserved by the group. The expression
~uu*-vv*~
An early appearance of this group was as the "unit sphere" of coquaternions, introduced by James Cockle in 1852. Let
j=\begin{bmatrix}0&1\ 1&0\end{bmatrix}, k=\begin{bmatrix}1& ~0\ 0&-1\end{bmatrix}, i=\begin{bmatrix} ~0&1\ -1&0\end{bmatrix}~.
Then
~jk=\begin{bmatrix}0&-1\ 1& ~0\end{bmatrix}=-i~,~
~ijk=I2\equiv\begin{bmatrix}1&0\ 0&1\end{bmatrix}~,~
~ki=j~,
ij=k ,
i
~j2=k2=+I2~
The coquaternion
~q=w+xi+yj+zk~
~q=w-xi-yj-zk~
~qq*=w2+x2-y2-z2.
\left\{xi+yj+zk:x2-y2-z2=1\right\}
The hyperboloid is stable under, illustrating the isomorphism with . The variability of the pole of a wave, as noted in studies of polarization, might view elliptical polarization as an exhibit of the elliptical shape of a wave with The Poincaré sphere model used since 1892 has been compared to a 2-sheet hyperboloid model,[20] and the practice of interferometry has been introduced.
When an element of is interpreted as a Möbius transformation, it leaves the unit disk stable, so this group represents the motions of the Poincaré disk model of hyperbolic plane geometry. Indeed, for a point in the complex projective line, the action of is given by
l[ z, 1 r]\begin{pmatrix}u&v\ v*&u*\end{pmatrix}=[ uz+v*,vz+u* ]=\left[
uz+v* | |
vz+u* |
,1 \right]
( uz+v*, vz+u* )\thicksim\left(
uz+v* | |
vz+u* |
, 1 \right)~.
Writing
suv+\overline{suv}=2\Redl(suvr) ,
l|uz+v*r|2=S+zz* and l|vz+u*r|2=S+1~,
S=vv*\left(zz*+1\right)+2\Redl(uvzr).
zz*<1\impliesl|uz+v*r|<l|vz+u*r|