The Lorentz-invariant boundary action of the confining string and its universal contribution to the inter-quark potential

Understand

We study the boundary contribution to the low energy effective action of the open string describing the confining flux tube in gauge theories.

• The form of the boundary terms is strongly constrained by the requirement of Lorentz symmetry, which is spontaneously broken by the formation of a long confining flux tube in the vacuum.
• Writing the boundary action as an expansion in the derivatives of the Nambu-Goldstone modes describing the transverse fluctuations of the string, we single out and put in a closed form the first few Lorentz invariant boundary terms.
• We also evaluate the leading deviation from the Nambu-Goto string produced by the boundary action on the vacuum expectation value of the Wilson loop and we test this prediction in the 3d Ising gauge model.