The Superstring
David Balin, A. Love · 1994
The bosonic string theory of Chapter 7 is not entirely satisfactory as a complete theory, first because it does not possess any fermionic states, and secondly because the string ground state is tachyonic (as discussed in §7.8 ). Both these difficulties can be overcome by constructing a theory in which, associated with each bosonic degree of freedom X μ ( τ , σ ), μ = 0, … , D –1, there is a world sheet spinor fermionic degree of freedom Ψ μ ( τ , σ ) (described by a two-component Majorana spinor). The action for this theory needs to be formulated in such a way as to avoid the occurrence of ghost states of negative norm.