Editing Gauge anomaly (section)

{{short description|Breakdown of gauge symmetry at the quantum level}} 

In [[theoretical physics]], a '''gauge anomaly''' is an example of an [[anomaly (physics)|anomaly]]: it is a feature of [[quantum mechanics]]—usually a [[one-loop diagram]]—that invalidates the [[gauge symmetry]] of a [[quantum field theory]]; i.e. of a [[gauge theory]].<ref> Treiman, Sam, and Roman Jackiw, (2014). ''Current algebra and anomalies''. Princeton University Press.</ref>

All gauge anomalies must cancel out. Anomalies in gauge symmetries<ref>{{cite book
 |last1=Cheng |first1=T.P.|last2=Li |first2=L.F. |date=1984 |title=Gauge Theory of Elementary Particle Physics|publisher=Oxford Science Publications }}</ref> lead to an inconsistency, since a gauge symmetry is required   in order to cancel degrees of freedom with a negative norm which are unphysical (such as a [[photon]] polarized in the time direction). Indeed, cancellation occurs in the [[Standard Model]].

The term '''gauge anomaly''' is usually used for vector gauge anomalies. Another type of gauge anomaly is the [[gravitational anomaly]], because coordinate reparametrization (called a [[diffeomorphism]]) is the gauge symmetry of [[gravitation]].