Top quark couplings at the muon collider

Abstract

We present an overview of the sensitivity of a muon collider to the flavor-diagonal electroweak couplings of the top quark, using dimension-six operators of the Warsaw basis. Of the nine fermion-boson operators involving the top quark, the bounds on three are not competitive with existing limits, and we focus on the remaining six. We estimate individual $95\%$ CL limits on the corresponding Wilson coefficients at $\sqrt{s} = 3$, $6$, $10$, and $14$~TeV from $2σ$ statistical cross-section uncertainties, and assess the effect of an assumed systematic uncertainty. We consider six top-quark production modes: $t\bar t$, $t \bar t h$, $t \bar t ν\bar ν$, $t \bar t μ^+ μ^-$, single-top $t\bar b μ^- \bar ν_μ$, and single-top-plus-Higgs, the last three for several rapidity coverages of the final-state muons. The $s$-channel $t\bar t$ and $t\bar t h$ processes give the strongest sensitivity to the electroweak dipole couplings, with allowed intervals narrower than the current LHC ones by factors of about $3$ to $6$ at $\sqrt{s} = 3$ and $6$ TeV, and by an order of magnitude or more at $10$ and $14$ TeV. The left- and right-handed $ttZ$ couplings, as well as the Yukawa $tth$ couplings (for $\sqrt{s} \geq 6$ TeV), are best probed in $t \bar t ν\bar ν$ production, while the right-handed $tbW$ coupling is best probed in single-top production. For the $ttZ$ and $tbW$ couplings, the limits improve on the current LHC bounds by factors from about $1.5$ to more than an order of magnitude. Our results agree with previous analyses of the Yukawa and the dipole couplings, and provide a roadmap for more in-depth studies.