Abstract
We study the sensitivity to physics beyond the standard model of precise top-quark pair production measurements at future lepton colliders. A global effective-field-theory approach is employed, including all ten dimension-six operators of the Warsaw basis which involve a top-quark and give rise to tree-level amplitudes that interfere with standard-model {e}^{+}{e}^{-}to toverline{t}to b{W}^{+}overline{b}{W}^{-} ones in the limit of vanishing b-quark mass. Four-fermion and CP-violating contributions are taken into account. Circular-collider-, ILC- and CLIC-like benchmark run scenarios are examined. We compare the constraining power of various observables to a set of statistically optimal ones which maximally exploit the information contained in the fully differential b{W}^{+}overline{b}{W}^{-} distribution. The enhanced sensitivity gained on the linear contributions of dimension-six operators leads to bounds that are insensitive to quadratic ones. Even with statistically optimal observables, two centre-of-mass energies are required for constraining simultaneously two- and four-fermion operators. The impact of the centre-of-mass energy lever arm is discussed, that of beam polarization as well. A realistic estimate of the precision that can be achieved in ILC- and CLIC-like operating scenarios yields individual limits on the electroweak couplings of the top quark that are one to three orders of magnitude better than constraints set with Tevatron and LHC run I data, and three to two hundred times better than the most optimistic projections made for the high-luminosity phase of the LHC. Clean global constraints can moreover be obtained at lepton colliders, robustly covering the multidimensional effective-field-theory space with minimal model dependence.
Highlights
The particle content of the standard model (SM) is experimentally confirmed through the discovery of a state with properties compatible with that of its Higgs boson
We study the sensitivity to physics beyond the standard model of precise top-quark pair production measurements at future lepton colliders
A proper definition of optimal observables beyond leading order is more involved. To assess their theoretical robustness, we evaluate the optimal observables on a standard-model sample produced for 500 GeV centre-of-mass energy with P (e+, e−) = (+30%, −80%) beam polarization using MG5 aMC@NLO [23] which implements the complex-mass scheme for the next-to-leading order computation including non-resonant contributions
Summary
The particle content of the standard model (SM) is experimentally confirmed through the discovery of a state with properties compatible with that of its Higgs boson. ILC studies [1, 2] relying on a full simulation of the detector response and on estimates for the main systematic uncertainties have shown that cross section and forward-backward asymmetry measurements would yield percent-level determinations of the anomalous couplings of the top quark to the photon and Z boson, with 500 fb−1 of integrated luminosity shared between two beam polarization configurations at a centre-of-mass energy of 500 GeV These are one to three orders of magnitudes better than individual constraints presently available [3, 4]. We include all ten dimension-six operators of the so-called Warsaw basis [20] involving a top quark and interfering with the resonant standard-model e+e− → t t → bW + ̄bW − amplitudes, at leading order and in the limit of vanishing bottom-quark mass This set contains four-fermion operators that are absent in anomalous coupling descriptions as well as the CP-violating components of top-quark electroweak dipole operators. Useful computer codes and numerical results are made available at https://github.com/gdurieux/optimal observables ee2tt2bwbw
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