Factorization of Chiral String Amplitudes

TL;DR

This paper investigates a modified closed-string model with altered boundary conditions, revealing new spin-2 states and analyzing their implications for string amplitudes and spectrum, especially in supersymmetric and heterotic contexts.

Contribution

It introduces a novel boundary condition modification in string theory that leads to the emergence of new spin-2 states and clarifies their role in the spectrum.

Findings

Modified boundary conditions change the spacetime metric sign in one open-string factor.

New spin-2 states appear at specific mass levels, including tachyonic and tardyonic.

Tachyonic states are ghosts but can be avoided in heterotic models.

Abstract

We re-examine a closed-string model defined by altering the boundary conditions for one handedness of two-dimensional propagators in otherwise-standard string theory. We evaluate the amplitudes using Kawai-Lewellen-Tye factorization into open-string amplitudes. The only modification to standard string theory is effectively that the spacetime Minkowski metric changes overall sign in one open-string factor. This cancels all but a finite number of states: As found in earlier approaches, with enough supersymmetry (e.g., type II) the tree amplitudes reproduce those of the massless truncation of ordinary string theory. However, we now find for the other cases that additional fields, formerly thought to be auxiliary, describe new spin-2 states at the two adjacent mass levels (tachyonic and tardyonic). The tachyon is always a ghost, but can be avoided in the heterotic case.