Abstract
We compute the top quark contribution to the two-loop amplitude for on-shell Z boson pair production in gluon fusion, gg → ZZ. Exact dependence on the top quark mass is retained. For each phase space point the integral reduction is performed numerically and the master integrals are evaluated using the auxiliary mass flow method, allowing fast computation of the amplitude with very high precision.
Highlights
The master integrals are evaluated efficiently using a system of ordinary differential equations and the auxiliary mass flow method [14, 15]
Symmetries between diagrams are established using REDUZE 2 [31] and we find that all two-loop diagrams can be mapped on to 21 integral families
In all non-factorisable diagrams both Z bosons are attached to the same quark loop leading to a single Dirac trace involving zero or two γ5 matrices
Summary
We consider the two-loop amplitude for Z boson production in gluon fusion g(p1) + g(p2) → Z(p3) + Z(p4). This process is mediated by quark loops. We calculate the contribution where the external Z bosons couple directly to top quarks and disregard other quark flavours. The photon mediated amplitude vanishes identically and calculations for the two latter are available in the literature [16,17,18,19,20]. For four-dimensional external states the amplitude A can be decomposed in terms of 18 tensor structures. Our goal is to calculate the form factors AI=1,...,18
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