A measurement of the top quark's charge

Abstract

The top quark was discovered in 1995 at the Fermilab National Accelerator Laboratory (Fermilab). One way to confirm if the observed top quark is really the top quark posited in the Standard Model (SM) is to measure its electric charge. In the Standard Model the top quark is the isospin partner of the bottom quark and is expected to have a charge of +2/3. However, an alternative "exotic" model has been proposed with a fourth generation exotic quark that has the same characteristics, such as mass, as our observed top but with a charge of -4/3. This thesis presents the first CDF measurement of the top quark's charge via its decay products, a W boson and a bottom quark, using ≈ 1f b −1 of data. The data were collected by the CDF detector from proton anti-proton (pp) collisions at √ s = 1.96 TeV at Fermilab.We classify events depending on the charges of the bottom quark and associated W boson and count the number of events which appear "SM-like" or "exotic-like" with a SM-like event decaying as t → W + b and an exotic event as t → W − b. We find the p-value under the Standard Model hypothesis to be 0.35 which is consistent with the Standard Model. We exclude the exotic quark hypothesis at an 81% confidence level, for which we have chosen a priori that the probability of incorrectly rejecting the SM would be 1%. The calculated Bayes Factor (BF) is 2×Ln(BF)=8.54 which is interpreted as the data strongly favors the Standard Model over the exotic quark hypothesis.