The research of SMU physicist Robert Kehoe, a professor in the SMU Department of Physics, has been featured by Fermilab Today. The magazine is the official publication of the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago. Fermi is a high-energy particle physics laboratory credited in 1995 with discovery of the fundamental particle, the top quark.

The article, “Top quark mass team wages war on two fronts,” appears in Fermilab Today‘s Jan. 26 edition as the “Result of the Week.”

Kehoe is part of the DZero collaboration of scientists who seek measurements of the top quark to determine the mass of the Higgs boson, another fundamental particle that has never been observed but which theoretical physicists have theorized generates mass for all particles that comprise matter. Others from SMU who were instrumental in the analysis include doctoral student Yuriy Ilchenko and post doctoral researcher Peter Renkel.

The paper “Measurement of the top quark mass in collisions using events with two leptons” was published by Fermilab. It reports DZero has obtained the world’s most precise measurement in the dilepton channel of the top quark mass.

“The measurement precision is now down to 1.6% in these events, which is astounding given how rare dilepton events are,” Kehoe said. “Perhaps more importantly, we have pursued a new way of calibrating these events that dramatically lowered the systematic uncertainty, and will allow it to decrease with more data — we have half the data yet to analyze.”

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By Mike Cooke
Fermilab Today

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.

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