5.2. NON–K ¨AHLER BACKGROUNDS IN TYPE I THEORY 71
closer to the case considered in [70] in the sense that the B–field is completely “used up”, which it has to be since BN S is not part of the type I spectrum.
Therefore, we find an explicit realization of a non–K¨ahler manifold for string theory back-grounds in type I. This should still be a complex manifold [118, 119]4, though, and we need to determine the precise value of the fluxes to ensure that the supergravity equations of motion are solved. So, let us finish the analysis of the type I background before transition by evaluating the RR fields and the dilaton. The type of background we consider, a toroidal orbifold with non–trivial complex structure, has already been studied in [118]. The complex structure will be fixed by the fluxes and one particular simple choice is the assumption that the RR and NS field strengths are constant (althoughBN S andC2 are not). This assumption was shown to be con-sistent with a metric of type (5.2) where Reτi = 0, a choice which is possible for our orientifold setup. Under this condition, the following constraint is imposed on the fields [118, 119]
BN S[xµ Cνy]RR = 0, (5.18)
where [,] indicates antisymmetrization of all enclosed indices. This has important consequences for the RR field we find after T–duality. As derived in [118], only those RR fields with one leg along the T–duality direction survive and we find for the RR three-form fieldstrength and the string coupling
F3I = Fxz1IIBdy∧dz∧dθ2+Fxz2IIBdy∧dz∧dθ1−Fyz2IIBdx∧dz∧dθ1
−Fyz1IIBdx∧dz∧dθ2+FrxzIIBdr∧dy∧dz+Frx1IIBdr∧dy∧dθ2 (5.19) +Frx2IIBdr∧dy∧dθ1−FryzIIBdr∧dx∧dz−Fry1IIBdr∧dx∧dθ2
−Fry2IIBdr∧dx∧dθ1
gI = eφI = √
α , (5.20)
whereFijkIIB are the components of the RR field strength we started with in IIB, see (5.10). Note that the string coupling is still a constant in our local limit, but it could in principle depend on (r, θ1, θ2) through α, if we leave the local limit. In any case, the dilaton does not vanish anymore. It is interesting that there is no B–field dependent fibration in the RR three form.
This agrees with observations made in [118, 119] and is due to the constraint (5.18). Note that this was not the case for the IIA mirror, see equation (3.26) for example, where the fibration structure is encrypted in the hatted coordinates.
In type IIB, the RR fieldstrength F3IIB and the NS fieldstrength HN SIIB are related due to the linearized equation of motion [5, 68]
F3IIB = ∗HN SIIB. (5.21)
In other words we can fix
FryzIIBdr∧dy∧dz = ∂rc4(r, z, θ1, θ2)dr∧dy∧dz (5.22)
= ∗
(∂θ2bxθ1 −∂θ1bxθ2)dx∧dθ1∧dθ2
and similarly for the other components, where we use the convention rxyzθ1θ2 = +1. This is completely analogous to the discussion following equation (3.54), we will therefore not repeat
4Non–complex manifolds in heterotic theory have been considered in [120], for example. Our orientifold construction is similar to models considered in [118], therefore we would expect it to yield complex manifolds as well.
5.2. NON–K ¨AHLER BACKGROUNDS IN TYPE I THEORY 73 it for all components. Let us simply state that this orientifold setup is much less restrictive than the one considered in chapter 3 and we can have more components ofF3 and HN S turned on. This is due to the fact that we have more B–field components that are consistent with the orientifold action and that the coefficients are now also allowed to depend onz. Even requiring bxθi andbyθj to be functions of r only will not result in any of theci to be forced to a constant.
We would like to address the question if the background we derived here can indeed show any geometric transition, i.e. can we shrink the two–cycle the D5 branes are wrapped on and blow up a dual three-cycle with fluxes on it? To answer this question we can consider the type IIB metric away from the orientifold point. T–dualizing this gives another IIB background which turns out to be surprisingly similar to the type I we just derived. Since we know that IIB away from the orientifold point shows geometric transition (this is the original Vafa model), we can infer that the type I background does, too, since it is dual to this IIB background.
Starting with the full IIB metric before orientifolding (5.2) and the same ansatz forBN S as in (5.8) we would have found
dˆs2IIB = dr2+α dz2+|dz2|2−2αAB(dx−bxθidθi)(dy−byθjdθj)
+α(1 +B2) (dx−bxθidθi)2+α(1 +A2) (dy−byθjdθj)2, (5.23) but now with non–vanishingBN S
BN S = −αA(dx−bxθidθi)∧dz−αB(dy−byθjdθj)∧dz , (5.24) which was to be expected because these are precisely the dx dzand dy dz cross–terms from the starting metric, so they turn into B–field components via Buscher’s rules (B.11). We see that the only difference in the metric is a warp factor for dz2. In our local limit this is simply a constant and we can rescale
z −→ z0 = √
α z , (5.25)
then the type I and type IIB metrics after T–duality agree completely. Since (5.23) is dual to the IIB background that shows transition, we can infer that also the type I background (5.17) has a two–cycle that can be shrunk and be exchanged for a blown–up three cycle.
The dual background with blown up three–cycle is found by T–dualizing the orientifold ansatz after transition (5.15). The steps are the same as for the background before transition and pretty straightforward. There is no extra boost of the complex structures required. This brings us to the type I metric after transition
d˜s2I = dr2+ (α0CDe2φ)−1dz2+ζ α0D1|dz2|2
+C(dx−bxidθi)2+D(dy−byjdθj)2. (5.26) Again, the B–field is completely used up under T–duality. The RR fluxes will take the same form as in (5.19), although the precise coefficients may differ. The string coupling is evaluated to be
gI = eφI = √
CD , (5.27)
which is not the same value we found before transition (5.20), but in the local limit both are constant.
Again we want to compare this to the IIB background away from the orientifold point that we would have obtained after two T–dualities. This is done by T–dualizing (5.11), which still
has the dx dz and dy dz cross–terms. After a little algebra one finds almost exactly the same metric apart from a warp factor fordz2, which turns out to be surprisingly simple
dsˆ˜2IIB = dr2+e−2φdz2+ζ α0(D+α2B2e2φ)|dz2|2
+C(dx−bxidθi)2+D(dy−byjdθj)2. (5.28) After a rescaling
z −→ z0 = (α0CD)−1/2z (5.29)
this agrees with the type I metric. The only difference is, as before transition, that this IIB background has a non–vanishing BN S field and we do not have to restrict the ans¨atze for the fluxes to be invariant under orientifold operation. But note that we have established a connection between the semi–flat IIB background after transition (5.11) with the type I background after transition (5.26), instead of considering the full IIB background (with restored coshziand sinhzi terms).
We can therefore conclude that the type I backgrounds constructed in (5.17) and (5.26) are transition dual. Let us repeat the argument: Each of the type I metrics is essentially identical to a IIB background which is T–dual to one of the IIB backgrounds discussed in section 3.4. The backgrounds in section 3.4 are transition duals because we found them by following the duality chain. If now one type I background is T–dual to the IIB background before and the other to the IIB background after transition, this implies that they are also transition duals. They are connected via an even longer duality chain than the one we followed in chapter 3.
Both type I backgrounds are non–K¨ahler, because they are T–dual to IIB backgrounds with NS field. It would be interesting to confirm that they are really complex as anticipated in [73, 118], but we cannot show this conclusively if we only know the local metric. Again, we would require knowledge of the global metric of the F–theory fourfold to be able to extend this analysis to global backgrounds.