infrared divergence
英[ˈinfrəˈred daɪˈvɜ:dʒɵns]美[ˌɪnfrəˈrɛd daɪˈvədʒəns]
In physics, an infrared divergence or infrared catastrophe is a situation in which an integral, for example a Feynman diagram, diverges because of contributions of objects with very small energy approaching zero, or, equivalently, because of physical phenomena at very long distances.