# Towards constraining Dark Matter at the LHC: Higher order QCD predictions for $t\bar{t}+Z(Z\to \nu_\ell \bar{\nu}_\ell)$

## TL;DR

This paper calculates next-to-leading order QCD corrections for the process $pp\to t\bar{t}+ Z(\to \nu\bar{\nu})$ at the LHC, improving the theoretical understanding of Standard Model backgrounds in dark matter searches involving top quarks.

## Contribution

It provides the first comprehensive NLO QCD predictions for $t\bar{t}+ Z$ with off-shell and non-resonant effects, aiding dark matter search strategies at the LHC.

## Key findings

- Non-flat differential K-factors for key observables.
- Good theoretical control over Standard Model background.
- Results are robust across different scale choices and PDFs.

## Abstract

Triggered by ongoing dark matter searches in the top quark sector at the Large Hadron Collider we report on the calculation of the next-to-leading order QCD corrections to the Standard Model process $pp\to t\bar{t}+ Z(\to \nu_\ell \bar{\nu}_\ell)$. This calculation is based on matrix elements for $e^+\nu_e \, \mu^- \bar{\nu}_\mu \, b\bar{b} \, \nu_\tau \bar{\nu}_\tau$ production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Non-resonant and off-shell effects due to the finite $W$-boson width are also consistently taken into account. As it is common for such studies, we present results for both integrated and differential cross sections for a few renormalisation and factorisation scale choices and three different parton distribution functions. Already with the fairly inclusive cut selection and independently of the scale choice and the parton distribution function non-flat differential ${\cal K}$-factors are obtained for $p_T^{miss}, \Delta \phi_{\ell\ell}, \Delta y_{\ell\ell}, \cos\theta_{\ell\ell}, H_T, H^\prime_T$ observables that are relevant for new physics searches. Good theoretical control over the Standard Model background is a fundamental prerequisite for a correct interpretation of possible signals of new physics that may arise in this channel. Thus, these observables need to be carefully reexamined in the presence of more exclusive cuts before any realistic strategies for the detection of new physics signal can be further developed.