Breakdown of the KLN Theorem for Charged Particles in Condensed Matter
  The Kinoshita-Lee-Nauenberg (KLN) theorem describes the fact that inclusive electromagnetic and weak production processes in the vacuum do not contain singularities in the ultra-relativistic limit of zero mass. When these production processes occur in condensed matter, the KLN theorem fails. One consequence of this failure is that precision lifetime determinations of stopped muons will depend on the nature of the surrounding material. 
Y. N. Srivastava and A. Widom