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Register a new userThe impact of top-quark modelling on the exclusion limits in $boldsymbol{tbar{t}}+text{DM}$ searches at the LHC
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New Physics searches at the LHC rely very heavily on the precision and accuracy of Standard Model background predictions. Applying the spin-$0$ $s$-channel mediator model, we assess the importance of properly modelling such backgrounds in $tbar{t}$ associated Dark Matter production and discuss higher-order corrections and off-shell effects for the $tbar{t}$ and $tbar{t}Z$ background processes in the presence of extremely exclusive cuts. Exclusion limits are calculated for state-of-the-art NLO full off-shell $tbar{t}$ and $tbar{t}Z$ predictions and compared to those computed with backgrounds in the NWA and / or at LO. We perform the same comparison for several new-physics sensitive observables and evaluate which of them are affected by the top-quark modelling. Additionally, we make suggestions as to which observables should be used to obtain the most stringent limits assuming integrated luminosities of $300$ fb$^{-1}$ and $3000$ fb$^{-1}$.
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A precise measurement of the top quark mass, a fundamental parameter of the Standard Model, is among the most important goals of top quark studies at the Large Hadron Collider. Apart from the standard methods, numerous new observables and reconstruction techniques are employed to improve the overall precision and to provide different sensitivities to various systematic uncertainties. Recently, the normalised inverse invariant mass distribution of the $tbar{t}$ system and the leading extra jet not coming from the top quark decays has been proposed for the $pp to tbar{t}j$ production process, denoted as ${cal R}(m_t^{pole},rho_s)$. In this paper, a thorough study of different theoretical predictions for this observable, however, with top quark decays included, is carried out. We focus on fixed order NLO QCD calculations for the di-lepton top quark decay channel at the LHC with $sqrt{s}=13$ TeV. First, the impact on the extraction of $m_t$ is investigated and afterwards the associated uncertainties are quantified. In one approach we include all interferences, off-shell effects and non-resonant backgrounds. This is contrasted with a different approach with top quark decays in the narrow width approximation. In the latter case, two cases are employed: NLO QCD corrections to the $ppto tbar{t}j$ production process with leading order decays and the more sophisticated case with QCD corrections and jet radiation present also in top quark decays. The top quark mass sensitivity of ${cal R}(m_t^{pole},rho_s)$ is investigated and compared to other observables: the invariant mass of the top anti-top pair, the minimal invariant mass of the $b$-jet and a charged lepton as well as the total transverse momentum of the $tbar{t}j$ system.
We consider the top quark charge asymmetry in the process $pp to tbar{t}+gamma$ at the 13 TeV LHC. The genuine tree level asymmetry in the $qbar{q}$ channel is large with about -12%. However, the symmetric $gg$ channel, photon radiation off top quark decay products, and higher order corrections wash out the asymmetry and obscure its observability. In this work, we investigate these effects at next-to-leading order QCD and check the robustness of theoretical predictions. We find a sizable perturbative correction and discuss its origins and implications. We also study dedicated cuts for enhancing the asymmetry and show that a measurement is possible with an integrated luminosity of 150 fb$^{-1}$.
Precision studies of the properties of the top quark represent a cornerstone of the LHC physics program. In this contribution we focus on the production of $tbar{t}$ pairs in association with one hard jet and in particular on its connection with precision measurements of the top quark mass at the LHC. We report a summary of a full calculation of the process $pp to e^+ u_emu^-bar{ u}_mu b bar{b}j$ at NLO QCD accuracy, which describes $tbar{t}j$ production with leptonic decays beyond the Narrow Width Approximation (NWA), and discuss the impact of the off-shell effects through comparisons with NWA. Finally we explore the sensitivity of $tbar{t}j$ in the context of top-quark mass extraction with the template method, considering two benchmark observables as case studies.
With the goal of increasing the precision of NLO QCD predictions for the $ppto tbar{t} gamma$ process in the di-lepton top quark decay channel we present theoretical predictions for the ${cal R}= sigma_{tbar{t}gamma}/sigma_{tbar{t}}$ cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of $sqrt{s} = 13$ TeV. Fully realistic NLO computations for $tbar{t}$ and $tbar{t}gamma$ production are employed. They are based on matrix elements for $e^+ u_e mu^- bar{ u}_mu bbar{b}$ and $e^+ u_e mu^- bar{ u}_mu bbar{b}gamma$ processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the $W$ gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain $1%-3%$ percent precision on ${cal R}$. Moreover, for differential cross section ratios theoretical uncertainties in the range of $1%-6%$ have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, ${cal R}$ at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the $t$-$bar{t}$-$gamma$ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.
The top-quark is the heaviest known particle of the Standard Model (SM); its heavy mass plays a crucial role in testing the electroweak symmetry breaking mechanism and for searching for new physics beyond the SM. In this paper, we determine the top-quark pole mass from recent measurements at the LHC at $sqrt{S}=13$ TeV center-of-mass energy to high precision by applying the Principle of Maximum Conformality (PMC) to the $tbar{t}$ pQCD production cross-section at NNLO. The PMC provides a systematic method which rigorously eliminates QCD renormalization scale ambiguities by summing the nonconformal $beta$ contributions into the QCD coupling constant. The PMC predictions satisfy the requirements of renormalization group invariance, including renormalization scheme independence, and the PMC scales accurately reflect the virtuality of the underlying production subprocesses. By using the PMC, an improved prediction for the $tbar{t}$ production cross-section is obtained without scale ambiguities, which in turn provides a precise value for the top-quark pole mass. The resulting determination of the top-quark pole mass $m_t^{rm pole}=172.5pm1.2$ GeV from the LHC measurement at $sqrt{S}=13$ TeV is in agreement with the current world average cited by the Particle Data Group (PDG). The PMC prediction provides an important high-precision test of the consistency of pQCD and the SM at $sqrt{S}=13$ TeV with previous LHC measurements at lower CM energies.
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