Higgs decay into two photons, dispersion relations and trace anomaly

We examine the contribution of W boson loops to the amplitude of the process H → γγ within the dispersion relation approach, taking up an issue raised in this context recently. We show that the non-vanishing limit of the relevant form factor for mw → 0 is due to a finite subtraction induced by the value of the corresponding trace anomaly. The argument can be turned around and one thus arrives at a dispersive (“infrared”) derivation of the trace anomaly.