Renormalon

In physics, a renormalon (a term suggested by 't Hooft^[1]) is a particular source of divergence seen in perturbative approximations to quantum field theories (QFT). When a formally divergent series in a QFT is summed using Borel summation, the associated Borel transform of the series can have singularities as a function of the complex transform parameter.^[2] The renormalon is a possible type of singularity arising in this complex Borel plane, and is a counterpart of an instanton singularity. Associated with such singularities, renormalon contributions are discussed in the context of quantum chromodynamics (QCD)^[2] and usually have the power-like form ${\displaystyle \left(\Lambda /Q\right)^{p}}$ as functions of the momentum ${\displaystyle Q}$ (here ${\displaystyle \Lambda }$ is the momentum cut-off). They are cited against the usual logarithmic effects like ${\displaystyle \ln \left(\Lambda /Q\right)}$.