The effective theory of long strings

We present the low-energy effective theory on long strings in quantum field theory, including a streamlined review of previous literature on the subject. Such long strings can appear in the form of solitonic strings, as in the 4d Abelian Higgs model, or in the form of confining strings, as in Yang-Mills theories. The bottom line is that upon expanding in powers of 1/L the energy levels of long (closed) strings (where L is the length of the string), all the terms up to (and including) order 1/L ⁵ are universal. We argue that for excited strings in D > 3 space-time dimensions there is a universal deviation at order 1/L ⁵ from the naive formula that is usually used to fit lattice results. For D = 3 this naive formula is valid even at order 1/L ⁵. At order 1/L ⁷ non-universal terms generically appear in all cases. We explain the physical origin of these results, and illuminate them in three different formulations of the effective action of long strings (the relationships among which we partly clarify). In addition, we corroborate these results by an explicit computation of the effective action on long strings in confining theories which have a gravitational dual. These predictions can be tested by precise simulations of 4d Yang-Mills theory on the lattice.