Gell-Mann matrices explained

The Gell-Mann matrices, developed by Murray Gell-Mann, are a set of eight linearly independent 3×3 traceless Hermitian matrices used in the study of the strong interaction in particle physics.They span the Lie algebra of the SU(3) group in the defining representation.

Matrices

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}

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}

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

} \begin 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & -2 \end .

Properties

See main article: Generalizations of Pauli matrices. These matrices are traceless, Hermitian, and obey the extra trace orthonormality relation, so they can generate unitary matrix group elements of SU(3) through exponentiation.^[1] These properties were chosen by Gell-Mann because they then naturally generalize the Pauli matrices for SU(2) to SU(3), which formed the basis for Gell-Mann's quark model.^[2] Gell-Mann's generalization further extends to general SU(n). For their connection to the standard basis of Lie algebras, see the Weyl–Cartan basis.

Trace orthonormality

In mathematics, orthonormality typically implies a norm which has a value of unity (1). Gell-Mann matrices, however, are normalized to a value of 2. Thus, the trace of the pairwise product results in the ortho-normalization condition

\operatorname{tr}(λ_iλ_j)=2\delta_ij,

where

\delta_ij

is the Kronecker delta.

This is so the embedded Pauli matrices corresponding to the three embedded subalgebras of SU(2) are conventionally normalized. In this three-dimensional matrix representation, the Cartan subalgebra is the set of linear combinations (with real coefficients) of the two matrices

λ₃

and

λ₈

, which commute with each other.

There are three significant SU(2) subalgebras:

\{λ_1,λ_2,λ_3\}

\{λ_4,λ_5,x\},

and

\{λ_6,λ_7,y\},

where the and are linear combinations of

λ₃

and

λ₈

. The SU(2) Casimirs of these subalgebras mutually commute.

However, any unitary similarity transformation of these subalgebras will yield SU(2) subalgebras. There is an uncountable number of such transformations.

Commutation relations

The 8 generators of SU(3) satisfy the commutation and anti-commutation relations^[3]

\begin{align} \left[λ_a,λ_b\right]&=2i\sum_cf^abcλ_c,\\ \{λ_a,λ_b\}&=

	4
	3

\delta_abI+2\sum_cd^abcλ_{c,
\end{align}}

with the structure constants

\begin{align} f^abc&=-

	1
	4

i\operatorname{tr}(λ_a[λ_b,λ_c]),\\ d^abc&=

	1
	4

\operatorname{tr}(λ_a\{λ_b,λ_c\}). \end{align}

The structure constants

f^abc

are completely antisymmetric in the three indices, generalizing the antisymmetry of the Levi-Civita symbol

\epsilon_jkl

of . For the present order of Gell-Mann matrices they take the values

f¹²³=1 , f¹⁴⁷=f¹⁶⁵=f²⁴⁶=f²⁵⁷=f³⁴⁵=f³⁷⁶=

	1
	2

, f⁴⁵⁸=f⁶⁷⁸=

	\sqrt{3

} \ .

In general, they evaluate to zero, unless they contain an odd count of indices from the set, corresponding to the antisymmetric (imaginary) s.

Using these commutation relations, the product of Gell-Mann matrices can be written as

λ_aλ_b=

	1
	2

([λ_a,λ_b]+\{λ_a,λ_b\})=

	2
	3

\delta_abI+\sum_c\left(d^abc+if^abc\right)λ_c,

where is the identity matrix.

Fierz completeness relations

Since the eight matrices and the identity are a complete trace-orthogonal set spanning all 3×3 matrices, it is straightforward to find two Fierz completeness relations, (Li & Cheng, 4.134), analogous to that satisfied by the Pauli matrices. Namely, using the dot to sum over the eight matrices and using Greek indices for their row/column indices, the following identities hold,

	\alpha
\delta
	\beta

\delta^\gamma_\delta=

	1
	3

	\alpha
\delta
	\delta

	\gamma
\delta		+
	\beta

	1
	2

	\alpha
λ
	\delta

⋅

	\gamma
λ
	\beta

and

	\alpha
λ
	\beta

⋅ λ^\gamma_\delta=

	16
	9

	\alpha
\delta
	\delta

	\gamma
\delta		-
	\beta

	1
	3

	\alpha
λ
	\delta

⋅

	\gamma
λ
	\beta

One may prefer the recast version, resulting from a linear combination of the above,

	\alpha
λ
	\beta

⋅ λ^\gamma_\delta=2

	\alpha
\delta
	\delta

	\gamma
\delta		-
	\beta

	2
	3

	\alpha
\delta
	\beta

	\gamma
\delta
	\delta

Representation theory

See main article: Clebsch–Gordan coefficients for SU(3). A particular choice of matrices is called a group representation, because any element of SU(3) can be written in the form

exp(i\theta^jg_j)

using the Einstein notation, where the eight

\theta^j

are real numbers and a sum over the index is implied. Given one representation, an equivalent one may be obtained by an arbitrary unitary similarity transformation, since that leaves the commutator unchanged.

The matrices can be realized as a representation of the infinitesimal generators of the special unitary group called SU(3). The Lie algebra of this group (a real Lie algebra in fact) has dimension eight and therefore it has some set with eight linearly independent generators, which can be written as

g_i

, with i taking values from 1 to 8.^[1]

Casimir operators and invariants

The squared sum of the Gell-Mann matrices gives the quadratic Casimir operator, a group invariant,

	8
\sum
	i=1

λ_iλ_i=

	16
	3

where

is 3×3 identity matrix. There is another, independent, cubic Casimir operator, as well.

Application to quantum chromodynamics

See main article: Color charge and Quantum chromodynamics. These matrices serve to study the internal (color) rotations of the gluon fields associated with the coloured quarks of quantum chromodynamics (cf. colours of the gluon). A gauge colour rotation is a spacetime-dependent SU(3) group element

U=\exp\left(

	i
	2

	k({r},t) λ
\theta
	k\right)

where summation over the eight indices is implied.

References

Gell-Mann . Murray . Symmetries of Baryons and Mesons . Physical Review . American Physical Society (APS) . 125 . 3 . 1962-02-01 . 0031-899X . 10.1103/physrev.125.1067 . 1067–1084. 1962PhRv..125.1067G . free.
Book: T.-P. . Cheng . L.-F. . Li . Gauge Theory of Elementary Particle Physics . . 1983 . 0-19-851961-3 .
Book: Georgi, H. . 1999 . Lie Algebras in Particle Physics . 2nd . . 978-0-7382-0233-4.
Book: Arfken . G. B. . Weber . H. J. . Harris . F. E. . 2000 . Mathematical Methods for Physicists . 7th . . 978-0-12-384654-9.
Book: Kokkedee, J. J. J. . 1969 . The Quark Model . registration . . 69014391.

Notes and References

Stefan Scherer . Matthias R. Schindler . A Chiral Perturbation Theory Primer. hep-ph/0505265. 31 May 2005. 1–2.
Book: David Griffiths. Introduction to Elementary Particles (2nd ed.). John Wiley & Sons. 978-3-527-40601-2. 2008. 283–288,366–369.
Web site: Haber . Howard . Properties of the Gell-Mann matrices . Physics 251 Group Theory and Modern Physics . U.C. Santa Cruz . 1 April 2019.