In theoretical physics, a **gauge anomaly** is an example of an anomaly: it is an effect of quantum mechanics—usually a one-loop diagram—that invalidates the gauge symmetry of a quantum field theory; i.e. of a gauge theory.

Anomalies in gauge symmetries lead to an inconsistency, since a gauge symmetry is required in order to cancel unphysical degrees of freedom with a negative norm (such as a photon polarized in the time direction). Therefore all gauge anomalies must cancel out. This indeed happens 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 reparametrization is a gauge symmetry in gravitation.

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**Gauge Anomaly**- Calculation of The Anomaly

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