Towards constraining dark matter at the LHC: higher order QCD predictions for tt̄ + Z (Z → νℓv̄ℓ)

A bstract 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 → t t̄ t ¯ + Z ( Z → ν ℓ v̄ v ¯ ℓ ). This calculation is based on matrix elements for e⁺νₑμ⁻ν̄μbb̄0.33em ντν̄τ e + ν e μ − ν ¯ μ b b ¯ ν τ ν ¯ τ 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 K -factors are obtained for pTᵐⁱˢˢ p T miss , ∆ 𝜙 ℓℓ , ∆ 𝒴 ℓℓ , cos θ ℓℓ , H T , HT 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 t t̄ t ¯ and t t̄ t ¯ + Z ( Z → ν ℓ v̄ v ¯ ℓ ) comprise the same final states, we additionally study the impact of the enlarged missing transverse momentum on various differential cross section distributions. To this end normalised differential distributions for pp → e⁺νₑμ⁻ν̄μbb̄0.33em ντν̄τ e + ν e μ − ν ¯ μ b b ¯ ν τ ν ¯ τ and pp → e⁺νₑμ⁻ν̄μbb̄ e + ν e μ − ν ¯ μ b b ¯ are compared.