What is the electric charge of the top quark?

Shown is the product of the charge of the W boson and the associated b quark jet. An excess of data in the exotic-model-like right-hand side would indicate that the top quark is not what we think it is.

Since the discovery of the top quark, several of its properties have been measured to confirm the Standard Model predictions. As the top quark decays, one can study its decay products, which are a W boson and a b quark. Determining that the top quark decays into a W+ boson, whose charge is +1, and a b quark, whose charge is -1/3, would ensure indirectly that the electric charge of the top quark is indeed +2/3, as expected in the Standard Model. However, if events were found to contain decays into a W and b quark final state, the charge of the decaying particle would be -4/3. That would be incompatible with the Standard Model top quark. Such an exotic-decay combination has already been constrained experimentally; however, this new measurement provides a more sensitive result.

CDF physicists have analyzed top-antitop events in the final state that has jets from the decay of a W boson and an electron or muon from the decay of a second W boson. They also look at the decay’s two b-jets, associating each with its W boson by examining the event’s kinematics.

To reconstruct the electric charge, scientists determine the charge of the W bosons using the electron or muon charge and the charge of the b-jets using a jet-charge procedure. This procedure determines the b-jet charge by weighing the charges of the tracks inside the jet. These tracks are predominantly negative if the b-jet is initiated by a b quark and predominantly positive if the b-jet is initiated by an anti-b quark. The probability that the sign of b or b-bar quark charge will be determined correctly is about 60 percent.

Using a data set of 5.6 inverse femtobarns, CDF physicists classified 774 W + b-jet pairs. Of these, 416 pairs are Standard Model-like (left side of the above figure) and 358 pairs are exotic-model-like (right side of the above figure). This means we can exclude the exotic-quark hypothesis with the -4/3 electric charge at the 99 percent confidence limit.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis.