On the effective action of confining strings

We study the low-energy effective action on confining strings (in the fundamental representation) in SU(N) gauge theories in D space-time dimensions. We write this action in terms of the physical transverse fluctuations of the string. We show that for any D, the four-derivative terms in the effective action must exactly match the ones in the Nambu-Goto action, generalizing a result of Luscher and Weisz for D = 3. We then analyze the six-derivative terms, and we show that some of these terms are constrained. For D = 3 this uniquely determines the effective action for closed strings to this order, while for D > 3 one term is not uniquely determined by our considerations. This implies that for D = 3 the energy levels of a closed string of length L agree with the Nambu-Goto result at least up to order 1/L-5. For any D we find that the partition function of a long string on a torus is unaffected by the free coefficient, so it is always equal to the Nambu-Goto partition function up to six-derivative order. For a closed string of length L, this means that for D > 3 its energy can, in principle, deviate from the Nambu-Goto result at order 1/L-5, but such deviations must always cancel in the computation of the partition function (so that the sum of the deviations of all states at each energy level must vanish). In particular there is no correction at this order to the ground state energy of a winding string. Next, we compute the effective action up to six-derivative order for the special case of confining strings in weakly-curved holographic backgrounds, at one-loop order (leading order in the curvature). Our computation is general, and applies in particular to backgrounds like the Witten background, the Maldacena-Nunez background, and the Klebanov-Strassler background. We show that this effective action obeys all of the constraints we derive, and in fact it precisely agrees with the Nambu-Goto action (the single allowed deviation does not appear).