Flavor changing single top quark production channels at e^+e^- colliders in the effective Lagrangian description

TL;DR

This paper analyzes the sensitivity of current and future e^+e^- colliders to flavor-changing single top quark production via effective vertices, setting bounds on new physics scales up to tens of TeV.

Contribution

It provides a comprehensive global analysis of collider sensitivities to flavor-changing top quark interactions within the effective Lagrangian framework, highlighting the potential to constrain new physics.

Findings

LEP2 data constrains four-Fermi operators to rom 0.7 to 1.4 TeV

Future colliders can probe scales up to 27 TeV

Limits are stronger than those on flavor-diagonal couplings

Abstract

We perform a global analysis of the sensitivity of LEP2 and e^+e^- colliders with a c.m. energy in the range 500 - 2000 GeV to new flavor-changing single top quark production in the effective Lagrangian approach. The processes considered are sensitive to new flavor-changing effective vertices such as Ztc, htc, four-Fermi tcee contact terms as well as a right-handed Wtb coupling. We show that e^+ e^- colliders are most sensitive to the physics responsible for the contact tcee vertices. For example, it is found that the recent data from the 189 GeV LEP2 run can be used to rule out any new flavor physics that can generate these four-Fermi operators up to energy scales of \Lambda > 0.7 - 1.4 TeV, depending on the type of the four-Fermi interaction. We also show that a corresponding limit of \Lambda > 1.3 - 2.5 and \Lambda > 17 - 27 TeV can be reached at the future 200 GeV LEP2 run and a 1000 GeV e^+e^- collider, respectively. We note that these limits are much stronger than the typical limits which can be placed on flavor diagonal four-Fermi couplings. Similar results hold for \mu^+\mu^- colliders and for tu(bar) associated production. Finally we briefly comment on the necessity of measuring all flavor-changing effective vertices as they can be produced by different types of heavy physics.