NS-branes in 5d brane world models

الأصل · EN

We study codimension-1 brane solutions of the 5d brane world models compactified on S₁ / Z₂. In string theoretical setup they suggest that the background branes located at orbifold fixed points should be NS-branes (in the five dimensional sense), rather than D-branes. Indeed, the existence of the background NS-branes is indispensable to obtain flat geometry M₄ × S₁ / Z₂ where M₄ represents the 4d Minkowski spacetime, and without these branes the 5d metric becomes singular everywhere. This result is very reminiscent of the (p+3)d effective string theory 1 where the NS-NS type p-brane is indispensable to obtain a flat geometry R₂ or R₂ /Zₙ on the transverse dimensions. Without this NS-NS type p-brane the 2d transverse space becomes a pin-shaped singular space. The correspondence between these two theories leads us to a conjecture that the whole flat backgrounds of the string theory inherently invovle the NS-branes implicitly in their ansatz, and hence the true background p-branes immanent in our spacetime may be NS-branes, instead of D-branes. We argue that this result can have a significant consequence in the context of the cosmological constant problem.