Polarization of the top quark as a probe of its chromomagnetic and chromoelectric couplings in single-top production at the Large Hadron Collider

TL;DR

This paper investigates how top quark polarization measurements at the LHC can be used to probe anomalous chromomagnetic and chromoelectric couplings, providing new limits on these couplings through decay-lepton angular distributions.

Contribution

It introduces a method using top polarization and decay-lepton angular distributions in single-top Wt production to set limits on anomalous top couplings at the LHC.

Findings

Limits on real part of CMDM $Re ho_2$

Limits on imaginary part of CEDM $Im ho_3$

Limits on real part of Wtb tensor coupling $Ref_{2r}$

Abstract

We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of the anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top-quark couplings. The Wt single-top production mode helps to isolate the anomalous $ttg$ and $Wtb$ couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We determine individual limits on the dominant couplings, viz., the real part of the CMDM $Re\rho_2$, the imaginary part of the CEDM $Im\rho_3$, and the real part of the tensor Wtb coupling $Ref_{2r}$, which may be obtained by utilizing these asymmetries at the LHC. We also obtain simultaneous limits on pairs of these couplings taking two couplings to be non-zero at a time.