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PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...

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4.5. EXAMPLES 69<br />

Note that Q and P are both primitive. Since Q · P is quantized in units of 2, we shall define<br />

Q s = Q √<br />

2<br />

, P s = P √<br />

2<br />

, ˇρ s = 2ˇρ, ˇσ s = 2ˇσ, ˇv s = 2ˇv . (4.5.5.3)<br />

Thus we have<br />

Q 2 s = − (2m + 1) , P 2 s = 2K + 1, Q s · P s = −J . (4.5.5.4)<br />

Since Q 2 s/2 is quantized in units of 1/2, we expect the partition function to have ˇσ s period<br />

2. However except for an overall additive factor of 1/2, Q 2 s/2 is actually quantized in integer<br />

units. Thus the partition function has the additional property that it is odd under ˇσ s → ˇσ s +1.<br />

Similarly since Ps<br />

2 is an odd integer, the partition function picks up a minus sign under ˇρ s →<br />

ˇρ s + 1. We shall call these symmetries of ̂Φ. Finally since Q s · P s is quantized in integer units,<br />

the period in the ˇv s direction is also unity. The corresponding symplectic transformations<br />

acting on (ˇρ s , ˇσ s , ˇv s ) are of the form<br />

⎛<br />

⎞<br />

1 0 ã 1 ã 3<br />

⎜ 0 1 ã 3 ã 2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ , ã 1, ã 2 , ã 3 ∈ Z . (4.5.5.5)<br />

0 0 0 1<br />

Under this transformation the partition function picks up a multiplier factor of (−1) ea 1+ea 2<br />

.<br />

Our next task is to determine the subgroup of the S-duality group Γ 0 (2) that leaves the<br />

set B – defined( as the)<br />

T-duality orbit of A – invariant. For this let us apply the S-duality<br />

a b<br />

transformation ∈ Γ<br />

c d 0 (2) on the charge vector (4.5.5.1). This gives<br />

⎛<br />

Q ′ = aQ + bP = ⎜<br />

⎝<br />

b(2K + 1)<br />

(2m + 1)a/2 + bJ<br />

b<br />

−2a<br />

⎞<br />

⎟<br />

⎠ , P ′ = cQ + dP =<br />

⎛<br />

⎜<br />

⎝<br />

d(2K + 1)<br />

(2m + 1)c/2 + dJ<br />

d<br />

−2c<br />

(4.5.5.6)<br />

We need to choose a, b, c, d such that (4.5.5.6) is inside the set B, ı.e. it can be brought to the<br />

form (4.5.5.1) after a T-duality transformation. The T-duality transformations acting within<br />

this four dimensional subspace are generated by matrices of the form [24]:<br />

⎛<br />

⎞ ⎛<br />

n 1 −m 1<br />

⎜ −l 1 k 1 ⎟<br />

⎝<br />

k 1 l 1<br />

⎠ and ⎜<br />

⎝<br />

m 1 n 1 −m 2<br />

k 2 −l 2<br />

k 2 l 2<br />

m 2 n 2<br />

n 2<br />

⎞<br />

⎟<br />

⎠ , (<br />

ki l i<br />

⎞<br />

⎟<br />

⎠ .<br />

)<br />

∈ Γ<br />

m i n 0 (2) .<br />

i<br />

(4.5.5.7)

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