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String Theory Demystified

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

Now, let θ → θ+c<br />

and we obtain<br />

∫ ∫<br />

dθ[ a+ b( θ+ c)] = dθ( a+ bθ+ bc)<br />

∫ ∫<br />

= ( a+ bc) dθ+ b dθθ<br />

In order for this integral to be translation invariant, it cannot depend on c, and so we<br />

conclude that<br />

∫ dθ = 0<br />

when θ is a Grassman variable. By convention, the Grassman integral is normalized<br />

to one in the following way:<br />

So that altogether we have the rule<br />

∫<br />

∫ dθθ = 1<br />

dθ( a+ bθ) = b<br />

For double integration over two Grassman coordinates, there is only one rule to<br />

remember<br />

∫<br />

2<br />

d θθθ =−2i<br />

With these rules in hand, you are on your way to becoming a supersymmetry<br />

expert.<br />

A Manifestly Supersymmetric Action<br />

The action written in Chap. 7 [Eq. (7.2)] includes fermionic fi elds but supersymmetry<br />

is not manifest. This situation can be remedied by writing down an action in terms<br />

of superfi elds. The action we use is given by<br />

i<br />

S = d d DY DY<br />

′ ∫<br />

2 2 µ<br />

σ θ<br />

8πα<br />

µ

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