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 → t overline{t} + Z (Z → νℓ overline{v} ℓ). This calculation is based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}kern0.33em {nu}_{tau }{overline{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 mathcal{K} -factors are obtained for {p}_T^{miss} , ∆\U0001d719ℓℓ, ∆\U0001d4b4ℓℓ, cos θℓℓ, HT, {H}_T^{prime } 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 overline{t} and t overline{t} + Z (Z → νℓ overline{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}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}kern0.33em {nu}_{tau }{overline{nu}}_{tau } and pp → {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b} are compared.
Highlights
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 )
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 pmT iss, ∆φ, ∆y, cos θ, HT, HT observables that are relevant for new physics searches
Our next-to-leading order (NLO) analysis of pp → e+νeμ−νμbb ντ ντ at the LHC is a necessary step towards a correct interpretation of the possible signals of new physics that may arise in this channel
Summary
At the LO level in perturbative expansion the e+νeμ−νμbbντ ντ final states are produced via the scattering of either two gluons or one quark and the corresponding anti-quark. In table 1 the number of one-loop Feynman diagrams, that corresponds to each type of correction for the dominant gg → e+νeμ−νμbbντ ντ partonic subprocess as obtained with Qgraf [33], is given. For the calculation of the real emission contributions, the package Helac-Dipoles is employed It implements the dipole formalism of Catani and Seymour [39, 40] for arbitrary helicity eigenstates and colour configurations of the external partons [25] and the NagySoper subtraction scheme [41], which makes use of random polarisation and colour sampling of the external partons. Having two independent subtraction schemes in Helac-Dipoles allow us to cross check the correctness of the real corrections by comparing the two results. All partonic subprocesses that are taken into account for the real emission contributions are listed, together with the number of the corresponding Feynman diagrams, the number of Catani-Seymour dipoles and Nagy-Soper subtraction terms. Storing events shows clear advantages when different observables are to be studied, or different kinematical cuts are to be applied, since no additional time-consuming running of the code is required
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