Global and optimal probes for the top-quark effective field theory at future lepton colliders

Abstract

We study the sensitivity to physics beyond the standard model of precise top-quark pair production measurements at future lepton colliders. A global effectivefield-theory approach is employed, including all dimension-six operators of the Warsaw basis which involve a top-quark and give rise to tree-level amplitudes that interfere with standardmodel e + e − → tt → bW +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 CLIClike 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 bW +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.

The electro-weak couplings of the top and bottom quarks — Global fit and future prospects

“…The LEP and SLC measurements of bottom-quark pair production provide a powerful, complementary handle on the operator coefficients C 1 ϕQ and C 3 ϕQ . Combining measurements of bottom-quark production at LEP/SLC with measurements in top-quark production yield solid constraints on both operator coefficients in a global fit [6]. We consider the measurements of R b and A bb F BLR at the Z pole [61].…”

“…Beam polarization, foreseen in ILC and CLIC, allows to disentangle the photon and Z-boson vertices [17]. In a multi-parameter EFT fit, the initial-state polarization is helpful to simultaneously constrain the coefficients of O tB and O tW [6]. For the sake of brevity we focus on the ILC scenario.…”

“…Ref. [6] demonstrates that at least two center-ofmass energies are needed if one wants to constrain all two-fermion and four-fermion operators coefficients simultaneously. The experimental uncertainties are studied in full simulation in Ref.…”

“…In Ref. [6], a tenparameter fit of the two-fermion and four-fermion operator coefficients that affect the EW couplings of the top quark is shown to provide stringent bounds when at least two well-separated energy stages are available.…”

O new physics, where art thou? A global search in the top sector

“…Its biggest success has been the application to Higgs and electroweak boson measurements at the LHC [8][9][10][11][12][13]. Most recently, the same approach [14][15][16] has been used to systematically analyze top quark measurements at the LHC [17][18][19][20][21] and at future colliders [22,23] and their link to bottom sector [24]. These efforts pave the way towards a global SMEFT analysis at the LHC.…”