The leading chiral electromagnetic correction to the nonleptonic ΔI=3/2 amplitude in kaon decays

In kaon decay, electromagnetic radiative corrections can generate shifts in the apparent ΔI=3/2 amplitude of order αA0/A2∼22α. In order to know the true ΔI=3/2 amplitude for comparison with lattice calculations and phenomenology, one needs to subtract off this electromagnetic effect. We provide a careful estimate of the leading electromagnetic shift in the chiral expansion of the amplitude, which shows that it is smaller than naive expectations, with a fractional shift of δA2(em)/A2=−0.016±0.01.