Search for production of four top quarks in final states with same-sign or multiple leptons in proton-proton collisions at s = 13 TeV

Precise Predictions for the Associated Production of a W Boson with a Top-Antitop Quark Pair at the LHC

The production of a top-antitop quark pair in association with a W boson (tt[over ¯]W) is one of the heaviest signatures currently probed at the Large Hadron Collider. Since the first observation reported in 2015, the corresponding rates have been found to be consistently higher than the standard model predictions, which are based on next-to-leading order calculations in the QCD and electroweak interactions. We present the first next-to-next-to-leading order QCD computation of tt[over ¯]W production at hadron colliders. The calculation is exact, except for the finite part of the two-loop virtual corrections, which is estimated using two different approaches that lead to consistent results within their uncertainties. We combine the newly computed next-to-next-to-leading order QCD corrections with the complete next-to-leading order QCD plus electroweak results, thus obtaining the most advanced perturbative prediction available to date for the tt[over ¯]W inclusive cross section. The tension with the latest ATLAS and CMS results remains at the 1σ-2σ level.

Generic Tests of CP Violation in High-p_{T} Multilepton Signals at the LHC and Beyond

We introduce a modification to the standard expression for tree-level CP violation in scattering processes at the LHC, which is important when the initial state is not self-conjugate. Based on that, we propose a generic and model-independent search strategy for probing tree-level CP violation in inclusive multilepton signals. We then use TeV-scale 4-fermion operators of the form tuℓℓ and tcℓℓ with complex Wilson coefficients as an illustrative example and show that it may generate O(10%) CP asymmetries that should be accessible at the LHC with an integrated luminosity of O(1000)  fb^{-1}.

Looking beyond the Standard Model with Third Generation Quarks at the LHC

The Large Hadron Collider (LHC) is at the frontier of collider physics today, probing new physics at unprecedented energy scales. Many theories of physics beyond the Standard Model seek to elucidate the underlying mechanism of electroweak symmetry breaking. Given their large Yukawa couplings to the Higgs boson, third generations quarks of the Standard Model, and especially the top quark, play a key role in such theories. Therefore, new particles predicted by these theories often couple preferentially to top and bottom quarks. The favoured coupling to third generation can also be used to explain recently observed flavour physics anomalies in the LHCb, Babar or Belle experiments. This article will review recent searches for new physics performed by the ATLAS and CMS experiments at the LHC, in final states containing top and bottom quarks. In particular, searches for vector-like quarks, leptoquarks, and heavy scalar and gauge bosons will be discussed.

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at s = 13 Te

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 Te at the CERN LHC, corresponding to an integrated luminosity of 137 fb - 1 collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on the model, the combined result excludes a top squark mass up to 1325 Ge for a massless neutralino, and a neutralino mass up to 700 Ge for a top squark mass of 1150 Ge . Top squarks with masses from 145 to 295 Ge , for neutralino masses from 0 to 100 Ge , with a mass difference between the top squark and the neutralino in a window of 30 Ge around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 Ge .

Sub-TeV H^{+} Boson Production as Probe of Extra Top Yukawa Couplings

We suggest searching for the charged Higgs boson at the Large Hadron Collider (LHC) via cg→bH^{+}→btb[over ¯]. In the general two Higgs doublet model, extra top Yukawa couplings ρ_{tc} and ρ_{tt} can drive the disappearance of antimatter from the Universe, while c[over ¯]bH^{+} and t[over ¯]bH^{+} couple with strength ρ_{tc}V_{tb} and ρ_{tt}V_{tb}, respectively. For ρ_{tc},ρ_{tt}∼0.5, and m_{H^{+}}∼300-500  GeV, evidence could emerge from LHC run 2 data at hand and discovery by adding run 3 data in the near future.