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

Abstract

In this note we demonstrate the use of top polarization in the study of $t\bar{t}$ 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 $\cos {\varphi }_{\ell }$) 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 $Z^{\prime }$ 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: $\sqrt{s}=14$ TeV and $\sqrt{s}=7$ 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 $M_{Z^{\prime }}\simeq 1200$ GeV, being as high as $10$ for a smaller mass of the $Z^{\prime }$ 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 $3\sigma$ sensitivity over a large part of the range $500\lesssim M_{Z^{\prime }}\lesssim 1500$ GeV.

28 page, LaTeX, requires JHEP style file, 12 figures. Typos corrected and references added

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Cited by in corpus (12)

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• Top Polarization and Stop Mixing from Boosted Jet Substructure
• Data driving the top quark forward--backward asymmetry with a lepton-based handle
• Forward Tevatron Tops and Backward LHC Tops with Associates
• Renormalization group-induced phenomena of top pairs from four-quark effective operators
• Precise probing and discrimination of third-generation scalar leptoquarks
• Quantum coherence, top transverse polarisation and the Tevatron asymmetry $A_{FB}^{\ell }$
• On two-body and three-body spin correlations in leptonic $t\bar{t}Z$ production and anomalous couplings at the LHC
• Asymmetry Observables for Measuring Spin Correlations in Top-Quark Pair Production
• Indian Contributions to LHC Theory
• Tagging a Boosted Top quark with a $\tau$ final state
• Top-quark Polarization and Asymmetries at the LHC in the Effective Description of Squark Interactions