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PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...

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4.5. EXAMPLES 61<br />

is an integer. Since (ˇρ s , ˇσ s , ˇv s ) are conjugate to (P 2 s /2, Q 2 s/2, Q s · P s ), the partition function<br />

(5.0.4) will be periodic under<br />

(ˇρ s , ˇσ s , ˇv s ) → (ˇρ s + 2, ˇσ s , ˇv s ), (ˇρ s , ˇσ s + 2, ˇv s ), (ˇρ s , ˇσ s , ˇv s + 2), (ˇρ s + 1, ˇσ s + 1, ˇv s + 1) . (4.5.3.6)<br />

The group generated by these transformations can be collectively represented by symplectic<br />

matrices 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, ã 1 + ã 2 , ã 2 + ã 3 , ã 1 + ã 3 ∈ 2 Z , (4.5.3.7)<br />

0 0 0 1<br />

acting on the variables (ˇρ s , ˇσ s , ˇv s ). For future reference we note that the change of variables<br />

from (ˇρ, ˇσ, ˇv) to (ˇρ s , ˇσ s , ˇv s ) can be regarded as a symplectic transformation of the form<br />

⎛<br />

⎞<br />

2 0 0 0<br />

⎜ 0 2 0 0<br />

⎟<br />

⎝ 0 0 1/2 0 ⎠ . (4.5.3.8)<br />

0 0 0 1/2<br />

We now need to determine the subgroup of the S-duality group that leaves the set B<br />

invariant. If we did not have the restriction that Q( 2 /2 and ) P 2 /2 are even, then this subgroup<br />

a b<br />

would consist of SL(2, Z) matrices of the form subject to the restriction a + b ∈<br />

c d<br />

2 Z + 1 and c + d ∈ 2 Z + 1 [48], – these conditions guarantee that the new charge vectors<br />

(Q ′′ , P ′′ ) are each primitive and hence have the same set of discrete T-duality invariants (r 1 =<br />

1, r 2 = 1, r 3 = 2, u 1 = 1). We shall now argue that the same subgroup also leaves the set B<br />

invariant. For this we need to note that if we begin with a (Q, P ) for which Q 2 /2, P 2 /2 and<br />

Q · P are all even then their S-duality transforms given in (4.2.2) will automatically have the<br />

same properties. Thus requiring the transformed pair (Q ′′ , P ′′ ) to have even Q ′′2 /2 and P ′′2 /2,<br />

as is required for (Q ′′ , P ′′ ) to belong to the set B, does not put any additional restriction on<br />

the S-duality transformations. Since both Q and P are scaled by the same amount to get the<br />

rescaled charges Q s and P s , the S-duality group action on (Q s , P s ) is identical to that on (Q, P )<br />

and hence its action on (ˇρ s , ˇσ s , ˇv s ) is identical to that on (ˇρ, ˇσ, ˇv). Using (4.2.14) we see that<br />

the representations of these symmetries as symplectic matrices are given by<br />

⎛<br />

⎞<br />

d b 0 0<br />

c a 0 0 ⎟<br />

⎜<br />

⎝<br />

0 0 a −c<br />

0 0 −b d<br />

⎟<br />

⎠ , a, b, c, d ∈ Z, ad − bc = 1, a + c ∈ 2 Z + 1, b + d ∈ 2 Z + 1 ,<br />

acting on the variables (ˇρ, ˇσ, ˇv) and also on (ˇρ s , ˇσ s , ˇv s ).<br />

(4.5.3.9)

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