GUT angle minimises $Z^0$ decay
  The GUT value of Weinberg's angle is also the value that minimises the total square matrix elements of $Z^0$ decay, independently of any GUT consideration, and thus the one that maximises the neutrino branching ratio against total width. We review the proof of this result and some related facts. 
Alejandro Rivero (EUPT, Universidad de Zaragoza)