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String Theory Demystified

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CHAPTER 7 RNS Superstrings 129<br />

EXAMPLE 7.1<br />

Show that the Dirac matrices on the worldsheet obey an anticommutation relation<br />

known as the Dirac algebra<br />

by explicit computation.<br />

SOLUTION<br />

This result is easy to verify. Since<br />

α β αβ<br />

{ ρ , ρ } =−2 η<br />

(7.4)<br />

η αβ = − ⎛ 1 0⎞<br />

⎝<br />

⎜ 0 1⎠<br />

⎟<br />

The Dirac algebra will be satisfi ed by the ρ α if the following relations hold:<br />

1 0<br />

and likewise for { ρ , ρ } . Now,<br />

0 0 ⎛ 0<br />

ρρ<br />

=<br />

⎝<br />

⎜ i<br />

0 0 0 0 0 0 0 0 00<br />

{ ρ, ρ} = ρρ + ρρ = 2ρρ =− 2η= 2I<br />

1 1 1 1 1 1 1 1 11<br />

{ ρ , ρ } = ρ ρ + ρρ = 2ρρ =− 2η=−2I 0 1 0 1 1 0<br />

{ ρ , ρ } = ρ ρ + ρ ρ =−2ηη<br />

01<br />

−i⎞<br />

⎛ 0<br />

0 ⎠<br />

⎟<br />

⎝<br />

⎜ i<br />

= 0<br />

− ⎞ 0 0 0<br />

0 ⎠<br />

⎟ =<br />

i ⎛ ⋅ + ( −i)⋅i<br />

⋅( −i)<br />

+ ( −i)⋅0⎞⎛1<br />

0⎞<br />

⎜<br />

⎝ ⋅ + ⋅ ⋅( − )+ ⋅<br />

⎟ =<br />

i 0 0 i i i 0 0 ⎠ ⎝<br />

⎜ 0 1 ⎠<br />

⎟ = I<br />

0 0<br />

Hence the fi rst relation { ρ , ρ } = 2I<br />

is satisfi ed. We verify that the second<br />

1 1<br />

relation { ρ , ρ } =−2I is also satisfi ed:<br />

ρρ = ⎛<br />

⎝<br />

⎜<br />

i<br />

1 1 0<br />

Finally, noting that<br />

⎛<br />

ρρ =<br />

⎝<br />

⎜ i<br />

0 1 0<br />

1 0 ⎛ 0<br />

ρρ =<br />

⎝<br />

⎜ i<br />

⎞ 0<br />

0⎠<br />

⎟ ⎛ i<br />

⎝<br />

⎜<br />

i<br />

0 1 1 0<br />

we see that { ρ , ρ } = { ρ , ρ } = 0.<br />

⎞ 0 0 0 0<br />

0⎠<br />

⎟<br />

0<br />

=<br />

i ⎛ ⋅ + i⋅i ⋅ i+ i⋅<br />

⎞ ⎛ − ⎞<br />

⎝<br />

⎜<br />

i ⋅ + ⋅ ⋅ + ⋅ ⎠<br />

⎟ =<br />

⎝<br />

⎜<br />

− ⎠<br />

⎟ =−<br />

1 0<br />

I<br />

0 i i i 0 0 0 1<br />

− ⎞ ⎛ 0<br />

0 ⎠<br />

⎟<br />

⎝<br />

⎜<br />

⎞ 0 0<br />

0⎠<br />

⎟<br />

0<br />

=<br />

i<br />

i<br />

i ⎛ ⋅ + ( −i)⋅i<br />

⎜<br />

⎝ ⋅ + ⋅<br />

⋅ i+ ( −i)⋅⎞<br />

⋅ + ⋅<br />

⎟<br />

⎠<br />

=<br />

i 0 0 i<br />

0 ⎛ 1<br />

i i 0 0 ⎝<br />

⎜ 0<br />

0⎞<br />

−1⎠<br />

⎟<br />

i ⎞ ⎛ 0<br />

0⎠<br />

⎟<br />

⎝<br />

⎜ i<br />

−i⎞<br />

0 0 i i<br />

0 ⎠<br />

⎟ i 0<br />

0 i i 0<br />

=<br />

⎛ ⋅ + ⋅<br />

⎜<br />

⎝ ⋅ +0⋅ ⋅( − )+ ⋅ ⎞ −1<br />

⋅( − )+ 0⋅0 ⎟ =<br />

⎠ 0<br />

0<br />

1<br />

⎛<br />

i i i ⎝<br />

⎜<br />

⎞<br />

⎠<br />

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