Helicity Amplitudes for Single-Top Production

TL;DR

The paper computes complete tree-level helicity amplitudes for single-top production channels in hadron collisions, including semileptonic top decay, using spinor-helicity techniques. It provides color-decomposed partial amplitudes for W-gluon fusion, flavor excitation, s-channel, and W-associated production, and validates results against MadGraph/CompHEP while examining the narrow width approximation. The authors analyze momentum and angular distributions, polarization effects, and show that a peak near $m_t$ in the invariant mass distribution isolates the top contribution, preserving sensitivity to the top mass and charged-current coupling. These analytic amplitudes enable efficient, cross-checkable implementations and aid precision studies of single-top processes and potential beyond-Standard-Model signals.

Abstract

Single top quark production at hadron colliders allows a direct measurement of the top quark charged current coupling. We present the complete tree-level helicity amplitudes for four processes involving the production and semileptonic decay of a single top quark: W-gluon fusion, flavor excitation, s-channel production and W-associated production. For the first three processes we study the quality of the narrow top width approximation. We also examine momentum and angular distributions of some of the final state particles.

Figures (7)

• Figure 1: A representative Feynman diagram for single-top production contributing to the partial amplitude $A^{(1)}_{Wg}$ (\ref{['eq:9']}). The top quark line is thickened.
• Figure 2: A representative Feynman diagram for single-top production contributing to the partial amplitude $A^{(2)}_{Wg}$ (\ref{['eq:9']}). The top quark line is thickened.
• Figure 3: A representative Feynman diagram corresponding to the partial amplitude $A^{(1)}_{Wb}$ (\ref{['amp_Wb']}). The top quark line is thickened.
• Figure 4: The normalized $m_{\nu\bar{l}b}$ distribution for W-gluon fusion (solid) and the s-channel process (dashed) at the Tevatron.
• Figure 5: The pseudo-rapidity distribution for W-gluon fusion of the $\bar{b}$-quark (dashed), the $b$-quark (solid) and the light quark $q$ (dot-dashed) at the Tevatron.