GSO projection

introduction

The GSO projection not only eliminates the tachyon in the bosonic NS sector but also yields the same number of bosonic and fermionic states at each mass level.
The GSO projection of the RNS model yields a ten-dimensional string theory, called superstring theory, which is supersymmetric in spacetime and does not contain tachyons.
Gliozzi, Scherk and Olive obtained for the first time a string theory without tachyons.
The RNS formalism has spacetime supersymmetry after projecting out half of the states using the GSO projection. However, it is not manifestly supersymmetric. Green and Schwarz, using the light-cone gauge, have given a formulation with manifest spacetime supersymmetry