The Tevatron’s top saga

We’ve come a long ways since the first Tevatron collisions in 1985. Not only did Fermilab scientists discover the top quark in 1995, but we can now measure its mass to a precision better than 1 percent.

In 1985, the first pair of protons and antiprotons collided in the Fermilab Tevatron. One of the main purposes for which the Tevatron was built was to search for the top quark and, after 10 years, DZero and CDF jointly announced the particle’s discovery in February of 1995. Discovering the top quark wasn’t easy. In order to claim observation of the top quark, the collaborations had to show that it was observed in several different decay paths.

More than 16 years later and with a data set that is about 100 times larger; our ability to measure properties of the top quark is much improved. Specifically, by combining all of the various measurements in 1995, we estimated the mass of the top quark with a precision of about 10-15 percent. Now, collaborators can achieve about 1 percent precision with individual measurements and can make an even more accurate measurement by combining them. (And this is only exploiting half the data we’ve taken. Analyses as delicate as the one described here take time and a measurement using all the data will be reported in due course.)

Top quarks are produced in pairs and essentially always decay into a bottom quark and a W boson. The W boson can decay in many possible ways, for example into a pair of quarks or either an electron or muon and its corresponding neutrino. Today’s result describes a measurement of the mass of the top quark using events in which both W bosons decay into electrons or muons. Because the generic word for electron and muon is lepton, these are called dilepton decays.

These kind of dilepton decays occur only 4 percent of the time, but have a very striking signature of two bottom quarks, two leptons and missing energy caused by neutrinos escaping the detector undetected. This measurement is the most precise dilepton measurement to date and determined by itself the mass of the top quark better than 2 percent. When combined with other measurements, the mass of the top quark is now known to a precision of better than 1 percent. By any measure, this is a scientific success that was only a dream a quarter century ago, when those first collisions occurred.

Don Lincoln

These physicists performed this analysis.
These physicists are the main system administrators for the DZero Linux cluster, which is a crucial component to the DZero experiment’s ability to analyze their data.