Abstract
We investigate the semiclassical approach to the lensing of photons in a spherically symmetric gravitational background, starting from Born level and include in our analysis the radiative corrections obtained from the electroweak theory for the graviton/photon/photon vertex. In this approach, the cross section is related to the angular variation of the impact parameter ($b$), which is then solved for $b$ as a function of the angle of deflection, and measured in horizon units ($b_h\equiv b/(2 G M)$). Exact numerical solutions for the angular deflection are presented. The numerical analysis shows that perturbation theory in a weak background agrees with the classical Einstein formula for the deflection already at distances of the order of $20$ horizon units ($\sim 20\, b_h$) and it is optimal in the description both of very strong and weak lensings. We show that the electroweak corrections to the cross section are sizeable, becoming very significant for high energy gamma rays. Our analysis covers in energy most of the photon spectrum, from the cosmic microwave background up to very high energy gamma rays, and scatterings with any value of the photon impact parameter. We also study the helicity-flip photon amplitude, which is of $O(\alpha^2)$ in the weak coupling $\alpha$, and its massless fermion limit, which involves the exchange of a conformal anomaly pole. The corresponding cross section is proportional to the Born level result and brings to a simple renormalization of Einsten's formula.
Highlights
Occur when a photon trajectory nears a large concentration of matter
We investigate the semiclassical approach to the lensing of photons in a spherically symmetric gravitational background, starting from Born level and include in our analysis the radiative corrections obtained from the electroweak theory for the graviton/photon/photon vertex
We have presented the explicit expression of the radiative corrections to the cross section describing the scattering of a high energy photon over a spherically symmetric gravitational background, in the SM
Summary
The background metric, over which we expand, is the retarded solution of the linearized Einstein’s equations and coincides with the Schwarzschild metric, once we take its expression in the limit of a weak external field. We will investigate both the polarized and the unpolarized cross sections, and present results for these over a very wide range of energy. As for any gravitational cross section, it grows quadratically with the mass of the source, becoming relevant for scatterings off massive/ supermassive black holes This component of the unpolarized cross section appears at O(α2) and carries information on the anomaly form factor of the graviton/photon/photon vertex
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