Constructing Manifestly Covariant String Field Theory

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The purpose of this project was to construct "manifestly covariant and gauge invariant string field theory", which is probably essential for revealing the dynamics of string. This aim was largely achieved as listed as follows :1. For bosonic string, both for open and closed, we succeeded in constructing gauge-invariant as well as (gauge-fixed) BRS-invariant string field theories.2. We proved at the full order level that the physical on-shell scattering amplitudes given in our theories are independent of the choice of the string-length parameter <alpha> for external strings.3. In particular, the closed string field action is shown to be cast into "pregeometrical form" consisting of purely cubic interaction term alone, which is completely free from the background geometry. The kinetic term is generated spontaneously by the string field condensation.4. As a step toward superstring field theory, we constructed BRS invariant 3-string vertices for Neveu-Schwarz-Ramond superstring theory.5. We have proved that the <alpha>-symmetry of our theory implies in fact that the Lorentz invariance of the light-cone gauge string field theory. Conversely, we have also presented a general procedure for constructing covariant string field theory starting from the light-cone gauge string field theory. The resultant theory gives a possible solution to the "problem of string length <alpha>" present in our original string field theories.