Top Pair Resonances Research Articles

Constraining new resonant physics with top spin polarisation information

We provide a comprehensive analysis of the power of including top quark-polarisation information to kinematically challenging top pair resonance searches, for which ATLAS and CMS start losing sensitivity. Following the general modelling and analysis strategies pursued by the experiments, we analyse the semi-leptonic and the di-lepton channels and show that including polarisation information can lead to large improvements in the limit setting procedures with large data sets. This will allow us to set stronger limits for parameter choices where sensitivity from the invariant mass of the top pair is not sufficient. This highlights the importance of spin observables as part of a more comprehensive set of observables to gain sensitivity to BSM resonance searches.

Up asymmetries from exhilarated composite flavor structures

Abstract We present a class of warped extra dimension (composite Higgs) models which conjointly accommodates the $ t\overline{t} $ forward-backward asymmetry observed at the Tevatron and the direct CP asymmetry in singly Cabibbo suppressed D decays first reported by the LHCb collaboration. We argue that both asymmetries, if arising dominantly from new physics beyond the Standard Model, hint for a flavor paradigm within partial compositeness models in which the right-handed quarks of the first two generations are not elementary fields but rather composite objects. We show that this class of models is consistent with current data on flavor and CP violating physics, electroweak precision observables, dijet and top pair resonance searches at hadron colliders. These models have several predictions which will be tested in forthcoming experiments. The CP asymmetry in D decays is induced through an effective operator of the form $ {{\left( {\overline{u}c} \right)}_{V+A }}{{\left( {\overline{s}s} \right)}_{V+A }} $ at the charm scale, which implies a larger CP asymmetry in the D 0 → K + K − rate relative the D 0 → π + π − channel. This prediction is distinctive from other Standard Model or dipole-based new physics interpretation of the LHCb result. CP violation in $ D-\overline{D} $ mixing as well as an an excess of dijet production of the LHC are also predicted to be observed in a near future. A large top asymmetry originates from the exchange of an axial resonance which dominantly produces left-handed top pairs. As a result a negative contribution to the lepton-based forward-backward asymmetry in $ t\overline{t} $ production, as well as $ \mathcal{O} $ (10%) forward-backward asymmetry in $ b\overline{b} $ production above $ {m_{{b\overline{b}}}} \simeq 600 $ GeV at the Tevatron is expected.

Boosted Top Quarks, Top Pair Resonances, and Top Partner Searches at the LHC

The increased center-of-mass energy of proton-proton collisions at the LHC allows for higher and higher mass scales to be probed in the search for new physics. These searches for new physics can often involve the reconstruction of top quark decays. In this high-mass regime, event topologies can change and top quark decay products become highly boosted, requiring the use of special identification algorithms. This report summarizes the latest techniques used in the reconstruction of boosted top quarks, and highlights several new physics searches from ATLAS and CMS where they are utilized. Such analyses include the search for top quark pair resonances as is expected from Z’ decays, as well as top quark partner searches, such as fourth generation vector-like quarks, excited top quarks, and top quark partners with non-standard charges

A search for ${\rm{t\bar t}}$ resonances in the dilepton channel in 1.04 fb−1of pp collisions at√s= 7 TeV with the ATLAS detector

The first ATLAS result on a search for a high mass top pair resonance at the LHC, in the subset of events where both W bosons from the top decays decay to either a final state electron or muon, is presented. The analysis is performed on 1.04/fb of pp collisions at sqrt(s) = 7 TeV. Numerous models predict the production of new massive particles that decay preferentially to a top-anti-top pair, including Randall-Sundrum models where the observation of a Kaluza-Klein excitation of the gluon may be the first indication of the existence of an extra spatial dimension. In the analysis presented, a simple observable, sensitive to resonance mass, is formed by summing the missing transverse energy, and the transverse momenta of the selected jets and the two candidate leptons. A deviation from the Standard Model prediction for this observable is searched for using Bayesian statistical methods that compare the yields, and shapes, of the Standard Model background and signal predictions for KK-gluons with masses between 500 and 1600 GeV. No excess over the Standard Model is observed and 95% C.L. upper limits are set on the production cross-section times branching ratio to top quarks for KK-gluon-like resonances. The results of the analysis exclude Randall-Sundrum KK-gluons with masses less than 840 GeV.

Direct search limits on the littlest Higgs model

Recent direct searches for new massive particles place constraints on the free parameters of the littlest Higgs model. Depending on the choice of model free parameters, the direct-search limit on the global symmetry-breaking scale $f$ can range from as low as a few hundred GeV to in excess of 4.5 TeV. The most stringent constraints are from exclusion of the ${A}_{H}$ using high-mass dilepton resonance searches. The ${Z}_{H}$ provides the best constraint in parameter regions where the ${A}_{H}$ decouples from leptons. Current top pair resonance data approach but do not yet reach a useful limit in the anomaly-cancelling case, but do provide a constraint for a limited range of parameters in other cases. A neutral gauge boson is shown to be undetectable in dilepton resonances for a significant range of parameter space due to decoupling from standard model leptons, providing a counterexample to broad claims that a new neutral gauge boson (sometimes generically referred to as a ${Z}^{\ensuremath{'}}$) is ruled out to a high-mass scale.