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412

10 Conclusion of Volume 2

A.3 The Charge Conjugation Matrix

Since Γa and their transposed ΓaT satisfy the same Clifford algebras it follows that there must be a similarity transformation connecting these two representations of the same algebra on the same carrier space. Such statement relies on Schur’s lemma and it is proved in the following way. We introduce the notation:

 

 

1

 

 

 

Γa1...an Γ[a1 Γa2 . . . Γan] = n

!

(A.3.1)

 

 

()δP ΓaP (a1) . . . ΓaP (an)

 

D

 

P

 

of

 

 

 

where

P denotes the sum over the n! permutations of the indices and δP the parity

permutation P , i.e. the number of elementary transpositions of which it is composed. The set of all matrices 1, Γa , Γa1a2 , . . . , Γa1...aD constitutes a finite group of 2[D/2]-dimensional matrices. Furthermore the groups generated in this way by Γa , Γa or ΓaT are isomorphic. Hence by Schur’s lemma two irreducible representations of the same group, with the same dimension and defined over the same vector space, must be equivalent, that is there must be a similarity transformation that connects the two. The matrix realizing such a similarity is called the charge conjugation matrix. Instructed by this discussion we define the charge conjugation matrix by means of the following equations:

CΓa C1 = −ΓaT

(A.3.2)

C+Γa C+1 = ΓaT

By definition C± connects the representation generated by Γa to that generated by ±ΓaT . In even dimensions both Cand C+ exist, while in odd dimensions only one of the two is possible. Indeed in odd dimensions ΓD1 is proportional to Γ0Γ1 . . . ΓD2 so that the Cand C+ of D 1 dimensions yield the same result on ΓD1. This decides which C exists in a given odd dimension.

Another important property of the charge conjugation matrix follows from iterating (A.3.2). Using Schur’s lemma one concludes that C± = αC±T so that iterating

Table A.1 Structure of charge conjugation matrices in various space-time dimensions

Charge conjugation matrices

DC+ = C+ (real)

4C+T = −C+; C+2 = −1

5C+T = −C+; C+2 = −1

6C+T = −C+; C+2 = −1

7

8C+T = C+; C+2 = 1

9C+T = C+; C+2 = 1

10C+T = C+; C+2 = 1

11

C= C(real)

CT = −C; C2 = −1

CT = C; C2 = 1 CT = C; C2 = 1 CT = C; C2 = 1

CT = −C; C2 = −1 CT = −C; C2 = −1

A Spinors and Gamma Matrix Algebra

413

again we obtain α2 = 1. In other words C+ and Care either symmetric or antisymmetric. We do not dwell on the derivation which can be obtained by explicit iterative construction of the gamma matrices in all dimensions and we simply collect below the results for the properties of C± in the various relevant dimensions (see Table A.1).

A.4 Majorana, Weyl and Majorana-Weyl Spinors

The Dirac conjugate of a spinor ψ is defined by the following operation:

ψ

ψΓ0

(A.4.1)

and the charge conjugate of ψ is defined as:

ψc = C ψT

(A.4.2)

where C is the charge conjugation matrix. When we have such an option we can either choose C+ or C. By definition a Majorana spinor λ satisfies the following condition:

λ = λc = 0T λ

(A.4.3)

Equation (A.4.3) is not always self-consistent. By iterating it a second time we get the consistency condition:

C Γ0T C = Γ0

(A.4.4)

There are two possible solutions to this constraint. Either Cis antisymmetric or C+ is symmetric. Hence, in view of the results displayed above, Majorana spinors exist only in

D = 4, 8, 9, 10, 11

(A.4.5)

In D = 4, 10, 11 they are defined using the Ccharge conjugation matrix while in D = 8, 9 they are defined using C+.

Weyl spinors, on the contrary, exist in every even dimension; by definition they are the eigenstates of the ΓD+1 matrix, corresponding to the +1 or 1 eigenvalue. Conventionally the former eigenstates are named left-handed, while the latter are named right-handed spinors:

ΓD+1ψ+ L

, = ±ψ+ L ,

(A.4.6)

R

R

 

In some special dimensions we can define Majorana-Weyl spinors which are both eigenstates of ΓD+1 and satisfy the Majorana condition (A.4.3). In order for this to be possible we must have:

C Γ0T ΓD+1ψ = ΓD+1ψ

(A.4.7)

414

10 Conclusion of Volume 2

which implies:

 

C Γ0T ΓD+1Γ0T C 1 = ΓD+1

(A.4.8)

With some manipulations the above condition becomes:

 

C ΓD+1C 1 = −ΓDT+1

(A.4.9)

which can be checked case by case, using the definition of ΓD+1 as product of all the other gamma matrices. In the range 4 D 11 the only dimension where (A.4.9) is satisfied is D = 10 which is the critical dimensions for superstrings. This is not a pure coincidence.

Summarizing we have:

Spinors in 4 D 11

D

Dirac

Majorana

Weyl

Majora-Weyl

 

 

 

 

 

4

Yes

Yes

Yes

No

5

Yes

No

No

No

6

Yes

No

Yes

No

7

Yes

No

No

No

8

Yes

Yes

Yes

No

9

Yes

Yes

No

No

10

Yes

Yes

Yes

Yes

11

Yes

Yes

No

No

 

 

 

 

 

A.5 A Particularly Useful Basis for D = 4 γ -Matrices

In this section we construct a D = 4 gamma matrix basis which is convenient for various purposes. Let us first specify the basis and then discuss its convenient properties.

In terms of the standard matrices (A.2.8) we realize the so(1, 3) Clifford algebra:

{γa , γb} = 2ηab;

ηab = diag(+, , , )

(A.5.1)

by setting:

 

 

γ0 = σ1 σ3;

γ1 = iσ2 σ3

 

γ2 = i1 σ2;

γ3 = iσ3 σ3

(A.5.2)

γ5 = 1 σ1;

C = iσ2 1

 

where γ5 is the chirality matrix and C the generators of the Lorentz algebra

is the charge conjugation matrix. In this basis so(1, 3), namely γab are particularly simple

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