30 years, some 700 integrals, and 1 dessert, or: Electroweak two-loop corrections to the Zbb vertex

Abstract

The one-loop corrections to the weak mixing angle $\sin^2θ_{eff}^b$ derived from the $Z{\bar b}b$ vertex, are known since 1985. It took another 30 years to calculate the complete electroweak two-loop corrections to $\sin^2θ_{eff}^b$. The main obstacle was the calculation of the O(700) bosonic two-loop vertex integrals with up to three mass scales, at $s=M_Z^2$. We did not perform the usual integral reduction and master evaluation, but chose a completely numerical approach, using two different calculational chains. One method relies on publicly available sector decomposition implementations. Further, we derived Mellin-Barnes (MB) representations, exploring the publicly available MB suite. We had to supplement the MB suite by two new packages: AMBRE~3, a Mathematica program, for the efficient treatment of non-planar integrals and MBnumerics for advanced numerics in the Minkowskian space-time. Our preliminary result for LL2016, the "dessert", for the electroweak bosonic two-loop contributions to $\sin^2θ_{eff}^b$ is: $Δ\sin^2θ_{eff}^{b(α^2,\rm bos)} = \sin^2θ_W ~ Δκ_b^{(α^2,bos)}$, with $Δκ_b^{(α^2,bos)} = -1.0276 x 10^{-4}$. This contribution is about a quarter of the corresponding fermionic corrections and of about the same magnitude as several of the known higher-order QCD corrections. The $\sin^2θ_{eff}^b$ is now predicited in the Standard Model with a relative error of $10^{-4}$ [1].

Contribution to the proceedings of the conference "Loops and Legs in Quantum Field Theory - LL 2016", 24 - 29 April 2016, Leipzig, Germany. Minor corrections in the text, few equations added. Version 2 compared to v.1: typos in eq. 1.24 corrected