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String Theory Demystified

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118 <strong>String</strong> <strong>Theory</strong> Demystifi ed<br />

⎛ i i 1 k r i i j ⎞<br />

= + − ( − )<br />

⎝<br />

⎜cKi<br />

cKU i fij ∑ c δr cbr cbk<br />

2<br />

⎠<br />

⎟ ψψ [Apply Eq. (6.3)]<br />

⎛ i i 1<br />

= cKi + cKU− f<br />

⎝<br />

⎜<br />

2<br />

= + − 1<br />

⎛<br />

⎝<br />

⎜<br />

r<br />

∑<br />

k i j<br />

k i r j<br />

i ij ccbk + fij c cbcb r k<br />

2 r<br />

1<br />

⎞<br />

⎠<br />

⎟ ψ<br />

i i<br />

k i j<br />

k i r j j<br />

cKi cKU i fij ccbk<br />

+ fij c∑c ( r −c<br />

br) bk<br />

2 2 r<br />

1<br />

i<br />

(Use the properties of δr )<br />

⎞<br />

δ<br />

⎠<br />

⎟ ψ<br />

[Apply Eq. (6.3) again]<br />

⎛ i 1 k i r j j ⎞<br />

= Q+ c KU i + fij c c ( δr<br />

−c<br />

br) bk<br />

⎝<br />

⎜<br />

∑ 2<br />

⎠<br />

⎟ ψ [Use Eq. (6.5) to write Q]<br />

r<br />

⎛ i 1 k i j 1 k i j r ⎞<br />

= Q+ c KU i + fij cc bk − fij cc∑c bkbr<br />

⎝<br />

⎜<br />

2 2<br />

⎠<br />

⎟ ψ<br />

⎛ i 1 k i j 1 k i j k r ⎞<br />

= Q + cKU i + fij ccbk − fij cc∑ ( δr −bc<br />

k ) br ⎝<br />

⎜<br />

2 2<br />

⎠<br />

⎟ ψ<br />

= Q+ c i ⎛<br />

1 k i j r ⎞<br />

KU i − fij cc c br<br />

⎝<br />

⎜<br />

∑<br />

r ⎠<br />

⎟ ψ<br />

2<br />

= ( Q+ QU) ψ = ( 1+<br />

m)(Q<br />

ψ ) Apply U ψ = m ψ<br />

r<br />

r<br />

( )<br />

Hence the BRST operator raises the ghost number by 1.<br />

BRST-Invariant States<br />

(Kroneker delta again)<br />

[Yet again use Eq. (6.3)]<br />

A state ψ is called BRST invariant if it is annihilated by the BRST operator<br />

[Eq. (6.5)]:<br />

Q ψ = 0 ⇒ ψ is BRST invariant (6.8)<br />

BRST-invariant states are the physical states of the theory. Since Q2 = 0, it follows<br />

that any state ψ = Q χ ≠0<br />

is BRST invariant, since<br />

2<br />

Q ψ = Q χ = 0

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