On measurement of top polarization as a probe of $ t\bar{t} $ production mechanisms at the LHC

Abstract

Abstract:In this note we demonstrate the use of top polarization in the study of tt resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of cos φ ℓ ) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive z axis is not uniquely defined. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous tbW couplings, to linear order. We study, for purposes of demonstration, the case of a Z ′ as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: √ s = 14 TeV and √ s = 7 TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for M Z ′ ≃ 1200 GeV, being as high as 10% for a smaller mass of the Z ′ of 700 GeV and for the largest allowed coupling in the model, the values being higher for the 7 TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about 4% (7%) for 14 (7) TeV LHC, whereas for the Tevatron, values as high as 12% are attained. For the 14 TeV LHC with an integrated luminosity of 10 fb −1 , these numbers translate into a 3σ sensitivity over a large part of the range 500 M Z ′ 1500 GeV.

Probing top charged-Higgs production using top polarization at the Large Hadron Collider

“…We show that the azimuthal distribution of the lepton is sensitive to top polarization and can be used to probe the coupling parameter tan β in the type II THDM. This approach has been recently used to probe new physics in the case of top pair production in a model with an extra heavy vector resonance (Z ′ ) with chiral couplings [14]. The effects of top polarization in tW and tH − production have been studied previously in [15], where the effects of 1-loop electroweak SUSY corrections have been considered; however, they do not consider top decay.…”

“…As in [14,25,28,29], we can quantify this difference by defining a normalized azimuthal asymmetry for the lepton as…”

Top physics at the LHC

“…Other angular distributions can function as quite selective probes of new physics [88,89]. For instance, if a Z ′ would polarize tops at production, the azimuthal asymmetry…”

Renormalization group-induced phenomena of top pairs from four-quark effective operators

“…Although the polarization is currently measured only at the LHC with a low accuracy [60,61], the Tevatron has been measuring spin correlations based on related techniques. It is also known that many new physics can be efficiently measured and distinguished through the polarization measurements [31,32,35,[62][63][64]. At tree-level, the top polarization is induced by O terns: O…”