Glimpse of a naked quark

The birth of the short-lived top quark affords DZero physicists the opportunity to study fundamental aspects of naked quarks.

The birth of a top quark heralds an opportunity unique among the quarks—the chance to study a naked quark. Being so massive that it would fall just a few spots below gold on the periodic table, the top quark’s lifetime is very short. It never has time to join with other quarks to form hadrons, composite particles of quarks held together by gluons. Instead, it decays via the weak force, nearly always into a bottom quark and a W boson, imparting information about its properties into the particles it leaves behind.

One interesting characteristic of the top quark is its spin, the intrinsic angular momentum it carries. When a pair of top quarks is created and their spins are measured with respect to a particular axis, they are either aligned or opposed. The Standard Model makes specific predictions for the frequency of observing aligned versus opposed spins that depend on the mode of production. The spins behave differently based on whether two quarks annihilated or two gluons fused to create the top quark pair. Measuring this spin correlation strength between the top quarks is a sensitive test of the Standard Model, and it opens a window to explore physics beyond the Standard Model.

To extract information about the spin correlation strength from the top quark pair candidate events, the analysis team first calculates the probability that the energy, momentum and location of every particle from the decay are consistent with the Standard Model. They compare this to the probability that the event is consistent with a universe where the spins are completely uncorrelated. By combining their latest efforts with a previous analysis at DZero that uses an independent data sample, they are able to provide an impressive glimpse at the behavior of naked quarks. This is the first evidence that top quark pairs have correlated spins, as predicted by the standard model.

—Mike Cooke

These physicists made major contributions to this analysis.
Meenakshi Narain from Brown University played a leading role in efforts to verify the quality of DZero’s data by running a battery of reconstruction certification tests on each multi-hour block of our data set.