Contents

Contents

Idea

The tachyon mode in the open bosonic string spectrum is an indication that as a perturbation theory it describes the perturbation about an unstable vacuum. Ashoke Sen realized (Sen 99) that since the open string technically propagates with its endpoints on the space filling D25-brane, that instability must be the instability of the D25, which wants to decay to a “true bosonic string vacuum”, usually called the tachyon vacuum.

This conjecture became famous as Sen’s conjecture. It was subsequently checked to be true by numerical means in open bosonic string field theory. Moreover, using string field theory it became possible to study that non-perturbative state in which the D25-brane is indeed gone. There were several arguments that indeed in the vicinity of this state the perturbative open string has disappeared and turned into the closed bosonic string.

A breakthrough happened in 2005, when Martin Schnabl found an analytic expression for this “tachyon vacuum” (Schnabl 05).

On the other hand, in superstring theory tachyons modes appear for strings stretching between a D-brane and an anti-D-brane. The analog of Sen’s conjecture here states that the consensation of the tachyon witnesses the annihilation of the brane/anti-brane pair (Sen 98).

References

An analytic solution was given in

For more references see at

For the decay of D-brane/anti-D-brane pairs in superstring theory: