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PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...

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4.3. CONSTRAINTS FROM WALL CROSSING 49<br />

The symplectic transformations given in (4.3.26), if present, give us the additional symmetries<br />

required to have ̂Φ transform as a modular form under a non-trivial subgroup of<br />

Sp(2, Z). We can use this to determine the subgroup of Sp(2, Z) under which we expect ̂Φ<br />

to transform as a modular form and also the weight k of the modular form. However since<br />

we do not know a priori which part of the symmetry groups of φ e and φ m will lift to the<br />

symmetries of ̂Φ, this is not a fool proof method. Nevertheless these can serve as guidelines<br />

for making an educated guess.<br />

The behaviour of ̂Φ near the other zeroes given in (4.3.2) could provide us with additional<br />

information. If the zero of ̂Φ at (4.3.2) is related to the one at ˇv = 0 by an S-duality transformation<br />

then( this information ) is not new. Since S-duality transformation acts by multiplying ( )<br />

a0 b<br />

the matrix<br />

0<br />

a0 b<br />

associated with a wall from the left [44], this means that if<br />

0<br />

c 0 d 0 c 0 d 0<br />

itself is an S-duality transformation then we ( do not)<br />

get a new information. ( To this ) we must<br />

a0 b<br />

also add the information that multiplying<br />

0<br />

λ 0<br />

from the right by<br />

c 0 d 0 0 λ −1 for any λ<br />

( )<br />

0 1<br />

or by<br />

does not change the wall [44]. However in many cases even after imposing<br />

−1 0<br />

these equivalence relations one finds inequivalent walls. 9 In such cases the associated zero of<br />

̂Φ cannot be related ( to the)<br />

zero at ˇv = 0 by an S-duality transformation, and we get new information.<br />

10 a0 b<br />

Let<br />

0<br />

be the matrix associated with such a decay. If the corresponding<br />

c 0 d 0<br />

partition functions ( ) φ m (τ; b( 0 , d 0 ) and)<br />

φ e (τ; a 0 , c 0 ) have modular groups containing matrices of<br />

α1 β<br />

the form<br />

1 p1 q<br />

and<br />

1<br />

respectively, then they may be regarded as symplectic<br />

γ 1 δ 1 r 1 s 1<br />

transformations generated by the matrices<br />

and<br />

⎛<br />

⎞<br />

d 0 b 0 0 0<br />

⎜ c 0 a 0 0 0<br />

⎟<br />

⎝ 0 0 a 0 −c 0<br />

⎠<br />

0 0 −b 0 d 0<br />

⎛<br />

⎞<br />

d 0 b 0 0 0<br />

⎜ c 0 a 0 0 0<br />

⎟<br />

⎝ 0 0 a 0 −c 0<br />

⎠<br />

0 0 −b 0 d 0<br />

⎞ ⎛<br />

⎞<br />

α 1 0 β 1 0 d 0 b 0 0 0<br />

⎜ 0 1 0 0<br />

⎟ ⎜ c 0 a 0 0 0<br />

⎟<br />

⎝ γ 1 0 δ 1 0 ⎠ ⎝ 0 0 a 0 −c 0<br />

⎠ (4.3.29)<br />

0 0 0 1 0 0 −b 0 d 0<br />

−1 ⎛<br />

⎞ ⎛<br />

⎞<br />

1 0 0 0 d 0 b 0 0 0<br />

⎜ 0 p 1 0 q 1<br />

⎟ ⎜ c 0 a 0 0 0<br />

⎟<br />

⎝ 0 0 1 0 ⎠ ⎝ 0 0 a 0 −c 0<br />

⎠ (4.3.30)<br />

0 r 1 0 s 1 0 0 −b 0 d 0<br />

−1 ⎛<br />

( )<br />

1 1<br />

9 For example in Z 6 CHL model with S-duality group Γ 1 (6) the wall corresponding to the matrix<br />

( )<br />

2 3<br />

1 0<br />

is not equivalent to the wall corresponding to . We shall discuss this example in some detail in §4.7.<br />

0 1<br />

10 Typically the number of such additional zeroes is a finite number, providing us with a finite set of additional<br />

information.

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