Exclusion of an Exotic Top Quark with -4/3 Electric Charge Using Soft Lepton Tagging

TL;DR

This paper measures the top quark's electric charge using soft lepton tagging in proton-antiproton collisions, providing the strongest exclusion of an exotic -4/3 charge scenario and confirming the standard model.

Contribution

It introduces a novel method using soft lepton tagging to determine the top quark's electric charge, excluding the exotic scenario at 95% confidence level.

Findings

Consistent with the standard model top charge of +2/3.

Excludes the exotic -4/3 charge at 95% confidence level.

First use of soft leptons for top charge determination.

Abstract

We present a measurement of the electric charge of the top quark using $\ppbar$ collisions corresponding to an integrated luminosity of 2.7~fb$^{-1}$ at the CDF II detector. We reconstruct $\ttbar$ events in the lepton+jets final state and use kinematic information to determine which $b$-jet is associated with the leptonically- or hadronically-decaying $t$-quark. Soft lepton taggers are used to determine the $b$-jet flavor. Along with the charge of the $W$ boson decay lepton, this information permits the reconstruction of the top quark's electric charge. Out of 45 reconstructed events with $2.4\pm0.8$ expected background events, 29 are reconstructed as $\ttbar$ with the standard model $+$2/3 charge, whereas 16 are reconstructed as $\ttbar$ with an exotic $-4/3$ charge. This is consistent with the standard model and excludes the exotic scenario at 95\% confidence level. This is the strongest exclusion of the exotic charge scenario and the first to use soft leptons for this purpose.