Searching for string resonances in e+e- and γγ collisions

If the fundamental mass scale of superstring theory is as low as a few TeVs, the massive modes of vibrating strings, Regge excitations, will be copiously produced at the Large Hadron Collider (LHC). We discuss the complementary signals of low-mass superstrings at the proposed electron-positron facility (CLIC), in e+e- and γγ collisions. We examine all relevant four-particle amplitudes evaluated at the center-of-mass energies near the mass of lightest Regge excitations and extract the corresponding pole terms. The Regge poles of all four-point amplitudes, in particular, the spin content of the resonances, are completely model-independent, universal properties of the entire landscape of string compactifications. We show that γγ→e+e- scattering proceeds only through a spin-2 Regge state. We estimate that for this particular channel, string scales as high as 4 TeV can be discovered at the 11σ level with the first fb-1 of data collected at a center-of-mass energy ≅5 TeV. We also show that for e+e- annihilation into fermion-antifermion pairs, string theory predicts the precise value, equal to 1/3, of the relative weight of spin 2 and spin 1 contributions. This yields a dimuon angular distribution with a pronounced forward-backward asymmetry, which will help distinguishing between low-mass strings and other beyond the standard model scenarios.