Strings and quantum fields

We present a brief background on perturbative string theory, D-branes and the gauge theory on their worldvolume, solitonic NS5-branes and Little String Theory (LST) in their vicinity, and the holographic dualities relating strings propagating in the near horizon of such branes to theories without gravity, such as gauge theory or LST. We proceed by considering four dimensional string theories where the string scale is set at the electro-weak scale: M_s ~ TeV. One possibility consists of theories on 1 + 3 dimensional D-branes, where the gauge couplings are related to the string coupling, g_Y²_M ~ gs, thus requiring large extra dimensions in order to set the correct Newton's constant G ~ M_P^-2. A very different possibility consists of string theory with a small coupling constant g_s ~ M_s/M_P ~ 10^-16, in which case the compactification scales can be set at a TeV. Charged particles are due to D-branes wrapped on small cycles in the compact six dimensional space, or in a dual picture to D-branes stretched between NS5-branes. The gauge couplings are determined by geometric data - the two dimensional surface on which the fivebranes are wrapped - and thus can be set at the desired values independent of gs. Such theories consist of low lying “Standard Model” particles coupled to neutral strings without gravity at a TeV - LST at a TeV.