Anomaly Cancellation for GUTS: Beginner's Questions

[robousy]

Beginners question :

In GUTS what is the relevance of anomaly cancellation.

Is it in some sense an axiom - like we say we must have local invariance - or is it in just some mathematical requirement?

Thanks in advance.

 

[Haelfix]

We don't care too much about global symmetries being anomalous, for instance the axial anomaly of QCD.

Otoh if we have anomalies that break local gauge invariance, we have a serious problem, the theory is necessarily inconsistent (both theoretically and by experiment, gauge invariance is key). So people devised consistency equations to ensure that anomalous terms were matched with other (often adhoc) terms that precisely cancel.

Now as far as GUTs go, some lie groups furnish anomaly free irreps automatically (eg SO(10)), its usually the GUTs with extra U(1) factors lying around that you ahve to be careful with.

 

[robousy]

So its not really an axiom - more a requirement.