Towards constraining dark matter at the LHC: higher order QCD predictions for t$$ \overline{t} $$ + Z (Z → νℓ$$ \overline{v} $$ℓ)

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 → tt + Z(→ ν ν). This calculation is based on matrix elements for e + ν e µ −ν µ bb ν τντ 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 K-factors are obtained for p miss T , ∆φ , ∆y , cos θ , H T , H 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. Since from the experimental point of view both tt and tt + Z(Z → ν ν) comprise the same final states, we additionally study the impact of the enlarged Open Access, c The Authors. Article funded by SCOAP 3 .

The simplest of them all: $$ t\overline{t}{W}^{\pm } $$ at NLO accuracy in QCD

“…Helac-NLO (ttX, where X = j, γ, Z, W ± [56][57][58][59]), ttW ± is perhaps the simplest one in terms of computational complexity. We draw this conclusion after comparing various criteria, such as the number of Feynman diagrams and subtraction terms involved in the calculation, or the number of partonic subprocesses and color structures of the amplitudes.…”

$$ t\overline{t}b\overline{b} $$ at the LHC: on the size of corrections and b-jet definitions

“…This is the first computation of a 2 → 6 process (the decay products of the W 's are not counted, because they do not couple to colour charged states) carried out within this framework. Even though Helac-Nlo has already been employed for the calculations of NLO QCD corrections to tt + X, X = j, γ, Z(→ νν), W ± (→ ν) with full top quark off-shell effects included [27][28][29][30][31], these processes were at most 2 → 5 processes from the QCD point of view. For the gg → e + ν e µ −ν µ bb bb partonic reaction there are 3904 LO diagrams.…”

Off-shell vs on-shell modelling of top quarks in photon associated production

“…Because Helac-Nlo is designed to efficiently obtain one-loop helicity amplitudes and calculate total cross sections at NLO in QCD for multi-particle processes in the SM the top-down approach is more natural and conceptually easier to implement. Furthermore, it allows us to exploit highly optimised recursion algorithms that have already been employed to provide NLO QCD results for tt, ttj, ttγ and ttZ processes with the complete top quark off-shell effects included [30,[54][55][56][57][58].…”