Anomalous Ward identities for on-shell amplitudes at the conformal fixed point

TL;DR

This paper derives anomalous conformal Ward identities for on-shell amplitudes in a conformally invariant quantum field theory, revealing how conformal symmetry constrains scattering processes and simplifies anomaly calculations.

Contribution

It introduces a novel derivation of anomalous conformal Ward identities for on-shell amplitudes at the conformal fixed point, highlighting a universal collinear function and lower-loop contributions.

Findings

Anomalous Ward identities relate amplitudes to lower-loop data.

The second anomaly term involves a universal collinear function.

Results are applicable to dimensionally-regularised amplitudes away from the fixed point.

Abstract

Conformal symmetry underlies many massless quantum field theories, but little is known about the consequences of this powerful symmetry for on-shell scattering amplitudes. Working in a dimensionally-regularised $\phi^3$ model at the conformal fixed point, we show that the on-shell renormalised amplitudes satisfy anomalous conformal Ward identities. Each external on-shell state contributes two terms to the anomaly. The first term is proportional to the elementary field anomalous dimension, and thus involves only lower-loop information. We show that the second term can be given as the convolution of a universal collinear function and lower-order amplitudes. The computation of the conformal anomaly is therefore simpler than that of the amplitude at the same perturbative order, which gives our anomalous conformal Ward identities a strong predictive power in perturbation theory. Finally, we show that our result is also of practical importance for dimensionally-regularised amplitudes away from the conformal fixed point.