When QCD strings can break ∗

Most of the observed properties of the string breaking in lattice gauge models in presence of charged dynamical matter can be simply understood in terms of a topological property of the loop expansion in the underlying string description of confinement. Similar considerations apply also to the breaking of the adjoint string. The naive string picture of quark confinement is very simple: in the confining phase of non abelian gauge theories a pair of static sources in the fundamental representation is connected by a color flux tube that for large interquark separations behaves as a free vibrating string. The energy of the system is proportional to the string length and gives rise to a confining potential. When quarks or other kinds of matter in the fundamental representation are added to the system, the string can break because it can end on a dynamically created particle. As a consequence one expects that the static potential flattens. A similar screening is expected also in pure Yang-Mills theory for the static potential between a pair of adjoint sources, mediated by a flux tube in the adjoint representation called adjoint string. In this case the role of charged matter is played by the gluons, thus the breaking of the adjoint string seems unavoidable. However these string breaking effects have proved elusive in most analyses of large Wilson loops, both in QCD with dynamical fermions [1] and in pure Yang-Mills theory with adjoint sources in 3+1 D [3] and in 2+1 D [2] , the only exception seems to be the unquenched 2+1 D SU(2) with two flavors [4]. On the contrary, clear signals of string breaking have been observed in studies where the basis of observables used to extract the potential has been enlarged: fundamental string breaking was found in SU(2) Higgs model in 2+1 D [5] and 3+1 D [6] and more recently adjoint string breaking was observed in 2+1 SU(2) pure gauge theory [7]. In a recent paper [8] we pointed out that the operators with a good overlap with the broken string state have as a common feature the presence of two disjoint source lines. This is also evident in the observation of string breaking at finite temperature QCD with dynamical fermions [9]. This lack of macroscopic effects of string breaking in the Wilson loop and its manifestation in operators with disjoint sources have both a simple , natural explanation in a …