Top quark mass team wages war on two fronts

To improve the precision of the top quark mass measurement, physicists must contend with both statistical and systematic sources of uncertainty.

Two major factors contribute to the ultimate precision of a measurement of the top quark mass: the amount of data used to make that measurement and the understanding of the uncertainty introduced by the detector. The amount of data used affects the size of the statistical uncertainty of the measurement, while accounting for the bias of the detector effects leads to the systematic uncertainty. Since the final precision of a measurement can’t be smaller than the larger of these two uncertainties, it is possible to have a measurement that is limited by the systematics. A systematically limited measurement won’t improve by simply taking more data. The most recent top quark mass measurement at DZero succeeded in turning a systematically limited analysis channel into a statistically limited one.

The top quark always decays into a W boson and a bottom quark. The W boson can decay into a neutrino and a charged lepton, such as an electron or muon, or into quarks. The major distinction between top quark pair analysis channels is the number of leptonic W boson decays allowed. In the dilepton channel, both W bosons decay leptonically and two neutrinos are produced. However, the incomplete reconstruction of neutrinos in the DZero detector leads to ambiguity when studying these top quark pair events. To account for this ambiguity, DZero physicists considered all possible values of the neutrino parameters to determine the value of the top quark mass that best fits the DZero data set.

The major source of systematic uncertainty for this measurement is the response of our calorimeter to the energy deposit from the spray of particles, or jet, produced by each bottom quark. A previous DZero top quark mass measurement in the single charged lepton channel performed a precise calibration of jet energy by making use of the jets produced by the W boson that decayed to quarks. To reduce their systematic uncertainty, the dilepton analysis team based their analysis on that precision calibration while carefully accounting for the differences that might arise from applying that calibration in their particular channel. The improved systematic uncertainty helps make this result the world’s most precise measurement in the dilepton channel and has now switched this channel from being systematically limited back to being statistically limited. Since this result uses about half of the full DZero Run II data set, it will improve when the additional data is added.

—Mike Cooke

These physicists, along with Jason South, Southern Methodist University, and Huanzhao Liu, Southern Methodist University, made major contributions to this analysis.
This team maintained and monitored the operations of the DZero calorimeter and its contribution to the first stage of the triggering system that determined which proton-antiproton collisions were worth recording for use in analyses, such as the one above.