Constraints on the Top-Quark Charge from Top-Pair Events

The top quark is the electroweak isospin partner of the bottom quark and is expected to have an electric charge of $+2/3\, \Pe$. We present a measurement that discriminates between two top quark charge hypotheses using the muon+jets final state of top-pair events detected with the CMS experiment. The measurement is performed with proton-proton collisions at $\sqrt{s} = 7$ \TeV, corresponding to an integrated luminosity of $4.6 \fbinv$. We exploit charge correlations between high-\pt muons from W boson decays and soft muons from B-hadron decays in \cPqb \ jets to constrain the top-quark charge. A normalised asymmetry of the events categorised with a charge of either $+2/3\, \Pe$ or $-4/3\, \Pe$ is calculated. The scenario with an exotic top quark charge of $-4/3\, \Pe$ would correspond to an asymmetry between the two categories of $A=-1$. This can be excluded with high significance and the measured asymmetry of $A_{\rm meas}=0.97\pm 0.12\text{(stat.)}\pm 0.31\text{(sys.)}$ agrees nicely with the standard-model expectation of $A=+1$.