PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...
PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...
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)