QCDCORRECTIONS TO THE GOLDEN DECAY CHANNEL OF THE HIGGS BOSON

Abstract

Precision studies within the Higgs sector of the Standard Model are crucial to deepen our understanding of the fundamental particle interac tions and uncovering new physics phenomena beyond the Standard Model, thereby guiding us towards a more comprehensive understanding of the uni verse. In this context, future colliders aim to provide highly precise exper imental measurements of the properties and couplings of the Higgs boson. To fully leverage the potential of these precision machines, it is crucial to minimize theoretical errors in the Higgs sector observables, such as produc tion and decay rates, to at least match the level of experimental uncertainties. Reducing theoretical uncertainties involves including higher-order terms in perturbative calculations of the observables. Central to these calculations are Feynman integrals, which form the backbone of the theoretical frame work. In this thesis, we focus on providing precise predictions for the observables associated with one of the most important decay channels of the Higgs boson, the “Golden decay channel”. Our work improves theoretical predictions for the partial decay width of this channel, specifically the H → Z(∗)Z(∗) → e+e−µ+µ− channel, by incorporating NNLO mixed QCD-electroweak correc tions in the perturbation theory. The entire calculation of the decay ampli tude at O(ααs) is performed systematically following the conventional work f low of multi-loop calculations. This approach involves the application of the Feynman rules, the tensor decomposition of contributing amplitudes, and the utilization of integration-by-parts identities to express the amplitudes in terms of master integrals. We describe the full analytical evaluation of these 2-loop master inte grals, specifically those appearing in the O(ααs) corrections to the HZZ vertex with both Z-bosons being off-shell, employing the method of differ ential equations. The calculation retains full dependence on the masses of all the particles, including those in the loop. Despite encountering non rationalizable square roots, we transform the system of differential equations into the canonical dlog-form with a minimal set of independent one-forms. The analytic results for all the master integrals are obtained in terms of 1 Chen’s iterated integrals with logarithmic kernels order-by-order in the di mensional regularization parameter ϵ, along with analytic boundary con stants. After the analytic evaluation of relevant 2-loop master integrals, this the sis also presents a phenomenological study of the H → e+e−µ+µ− decay at O(ααs). The entire calculation of the decay amplitude is implemented in the public code Hto4l to perform the phase space integration over final state lep tons and to obtain improved predictions for the partial decay width with an accuracy of O(ααs). We study the impact of these mixed QCD-electroweak corrections on the invariant mass distribution of the final-state leptons and angular variables, specifically focusing on the angle between the decay planes of the intermediate Z-bosons relative to leading-order predictions and NLO electroweak corrections. Our study shows that there are certain kinematic and angular bins in which the mixed QCD-electroweak corrections dominate the NLO EW corrections, highlighting the importance of these corrections in the context of data analysis aimed at probing new physics in the Higgs sector. Finally, the thesis concludes with a summary of the main results and outlines possible future directions