The Ramond sector uses periodic worldsheet Majorana fermions, giving integer oscillator labels and fermionic zero modes. The zero modes obey a Clifford algebra, so their action on the ground state generates a spacetime spinor representation rather than an ordinary scalar vacuum.
This explains how a worldsheet field with a periodic boundary condition leads to spacetime fermions in the open-string spectrum. The GSO projection selects an allowed chirality in the critical construction. A closed string has separate left-moving and right-moving choices; combining Ramond and Neveu–Schwarz sectors generates the familiar type-II field sectors. Worldsheet fermionic character and spacetime spin are distinct notions connected by the zero-mode and physical-state constraints.
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