Introduction

Part V deals with the extensions of the Dual Resonance Model (DRM), i.e. the bosonic string, to include additional symmetries and degrees of freedom. These generalizations were originally motivated by the need to overcome the drawbacks of the DRM and obtain a more realistic model of hadrons. Such attempts were only partially successful, though, with hindsight, we can say that they added some essential elements for the construction of modern string theory.

One of the first modifications of the Koba–Nielsen amplitude aimed at incorporating the internal flavour symmetry of hadrons, and was proposed by Chan and Paton in 1969. As discussed in Section 27.2, these authors showed that an internal flavour symmetry can be introduced simply by multiplying the amplitudes by appropriate group theoretical factors. Such factors can be viewed as resulting from the presence of a quark–antiquark pair attached to the open string end-points, and carrying flavour quantum numbers.

However, the incorporation of flavour symmetry was not the only open issue. As discussed in the previous Parts, the main problems of the DRM were: (i) the presence of a tachyon; (ii) the absence of fermions, preventing the description of baryons; (iii) the presence of a critical dimension with an unrealistic value, d = 26. Attempts to solve these problems started very early, in fact immediately after the appearance of the Veneziano formula, and went on more or less in parallel with the understanding of the DRM and its reinterpretation as a quantum string (see Parts III and IV).