Introduction

By 1973 two ghost-free dual models had been constructed, the Dual Resonance Model and the Ramond–Neveu–Schwarz model. The structure underlying the DRM was that of a relativistic string described by the Nambu–Goto action, but it was not clear yet which kind of string was underlying the RNS model. These models were not suitable for describing strong interactions because they both had massless particles with spins one and two in their spectra (together with a tachyon). An additional problem was raised by the deep inelastic experiments: probing the structure of protons at short distances, they showed the existence of pointlike particles that could be interpreted as the quarks. As already mentioned, string theory scattering amplitudes were too soft at large momentum transfer to explain these experiments. Therefore many researchers went back to field theory; in particular, quantum chromodynamics (QCD), the non-Abelian gauge theory for quarks and gluons formulated in 1972, was intensively studied in the following years obtaining convincing experimental support.

Although these events implied that dual theories could not correctly describe the hadronic processes, they did not completely put an end to their study. In fact, some of the earlyworkers in this field were so attracted by the consistent and deep structure of these models that they continued to study them, sometimes at the price of putting their careers at risk.