Single Top Production in $e^+ e^-$, $e^- e^-$, $γe$ and $γγ$ Collisions

TL;DR

This study assesses single top production across $e^+ e^-$, $e^- e^-$, $\gamma e$, and $\gamma\gamma$ collisions at a future linear collider to probe $|V_{tb}|$ and anomalous $Wtb$ couplings. It computes complete tree-level SM cross sections for all four modes with polarized and unpolarized beams, employs a gauge-invariant subtraction of $t\bar{t}$ backgrounds, and analyzes dimension-6 operators affecting the $Wtb$ vertex, mapping them to $f_{2L}$ and $f_{2R}$. The results identify $\gamma_+ e^-_L$ in $\gamma e$ collisions as the most favorable channel due to large rates and absence of $t\bar{t}$ background, with $e^- e^-$ also promising for precision studies; best bounds on anomalous couplings are obtained from a combination of these channels, notably $-0.02 \le f_{2L} \le 0.06$ and $-0.1 \le f_{2R} \le 0.1$ at $\sqrt{s}=0.5$ TeV, tightening with higher luminosity. The work demonstrates that linear colliders provide precise, complementary probes of top-quark properties beyond what hadron colliders can offer, while highlighting theoretical considerations for consistent EFT analyses and the need for higher-order corrections in future refinements.

Abstract

Single top quark cross section evaluations for the complete sets of tree-level diagrams in the $e^+ e^-$, $e^- e^-$, $γe$ and $γγ$ modes of the next linear collider with unpolarized and polarized beams are performed within the Standard Model and beyond. From comparison of all possibilities we conclude that the process $γ_+ e^-_L \to e^- t \bar b$ is extremely favoured due to large cross section, no $t \bar t$ background, high degrees of beam polarization, and exceptional sensitivities to $V_{tb}$ and anomalous $Wtb$ couplings. Similar reasons favour the process $e^- e^- \to e^- ν_e \bar t b$ for probing top quark properties despite a considerably lower cross section. Less favourable are processes like $e^+ e^-, γγ\to e^- ν_e t \bar b$. Three processes were chosen to probe their sensitivity to anomalous $Wtb$ couplings, with best bounds found for $γ_+ e^-_L \to e^- t \bar b$ and $e^+_R e^-_R \to e^- ν_e t \bar b$.

Figures (13)

• Figure 1: $s$-channel $CC10$ diagrams for the process $e^- e^+ \to e^- \bar{\nu}_e t \bar{b}$, unpolarized beams
• Figure 2: $t$-channel $CC10$ diagrams for the process $e^- e^+ \to e^- \bar{\nu}_e t \bar{b}$, unpolarized beams
• Figure 3: Diagrams for the process $\gamma \gamma \to e^- \nu_e \bar{t} b$
• Figure 4: Diagrams for the process $e^- e^- \to e^- \nu_e \bar{t} b$, unpolarized beams
• Figure 5: Diagrams for the process $\gamma e \to \nu_e \bar{t} b$