Longitudinal top polarisation measurement and anomalous Wtb coupling

Abstract

Kinematical distributions of decay products of the top quark carry information on the polarisation of the top as well as on any possible new physics in the decay of the top quark. We construct observables in the form of asymmetries in the kinematical distributions to probe their effects. Charged-lepton angular distributions in the decay are insensitive to anomalous couplings to leading order. Hence these can be a robust probe of top polarisation. However, these are difficult to measure in the case of highly boosted top quarks as compared to energy distributions of decay products. These are then sensitive, in general, to both top polarisation and top anomalous couplings. We compare various asymmetries for their sensitivities to the longitudinal polarisation of the top quark as well as to possible new physics in the W tb vertex, paying special attention to the case of highly boosted top quarks. We perform a χ 2 analysis to determine the regions in the plane of longitudinal polarisation of the top quark and the couplings of the W tb vertex constrained by different combinations of the asymmetries. Moreover, we find that the use of observables sensitive to the longitudinal top polarisation can add to the sensitivity to which the W tb vertex can be probed.

High- and low-energy constraints on top-Higgs couplings

“…However, this indirect observable alone allows for a free direction, which is only ruled out by direct probes (the helicity fractions). This further motivates collider observables which are directly sensitive to CP violation such as the ones proposed in [41][42][43].…”

Anomalous top-Higgs couplings and top polarisation in single top and Higgs associated production at the LHC

“…It is therefore expected that non-standard top-Higgs couplings will affect the polarisation states of top quark and change the angular distributions of the top quark decay products [36]. Such a polarisation asymmetry has been widely used to probe the anomalous top quark interactions at the LHC [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69]. The precise measurement of thj channel opens a new window to probe the top quark Yukawa couplings and new physics at the LHC.…”

Jets with electrons from boosted top quarks

“…Indeed, the polarisation measurement of the top jet alone can distinguish top quarks due to QCD interactions (which are vector-like and therefore give rise to unpolarised top particles) from polarised top quarks due to chiral electroweak interactions. Even the energy distribution of the lepton in the jet carries nontrivial information about the anomalous top decay vertices [37][38][39], and consequently, provides a means to detect physics beyond SM. The need for a dedicated method that identifies these semi-leptonic top quark jets and, more importantly, efficiently gives the energy and the orientation of the lepton inside the jet can not be overstated.…”