The Neveu–Schwarz sector uses antiperiodic worldsheet Majorana fermions around the closed spatial coordinate of the spinning string, or the corresponding doubled interval for an open string. Their Neveu–Schwarz fermionic oscillator expansion therefore has half-integer labels, with no fermionic zero mode.
The lowest fermionic creation mode has label . In the critical open superstring, applying it to the oscillator vacuum gives the massless vector before the remaining physical-state identifications. The unprojected vacuum is tachyonic; the GSO projection selects the permitted fermion-parity sector and removes that vacuum in the usual supersymmetric construction. This sector supplies spacetime bosons in the open-string spectrum, while the Ramond sector supplies spacetime fermions.
New to topics? Read the docs here!