Measuring the heaviest mass from the lightest fragments

In the insert figure, the measured top quark mass is shown at the bottom of the curve. In the larger image, the blue points show the data from the most recent measurement and the red lines show how well the data fit with a prediction of the signal and background combined. The black line shows the predicted background only.

The top quark is the heaviest currently known elementary particle, with a mass nearly equal to that of a gold atom. Because of its very large mass, it may play a special role in our understanding of the fundamental theory that describes the electromagnetic interaction and the weak interaction in a unified way. This theory could also explain the origin of particle masses.

Because of the importance of the top quark mass, experimenters at the Tevatron have focused much of their efforts to improve the level of precision of its measurement. CDF scientists recently used a new strategy to make a precise measurement of the top quark mass.

Top quarks exist only for tiny fractions of a second before they decay, so scientists cannot directly detect them. However, scientists can infer the top quark from tracing backward from the particle’s decay products: light particles called leptons (electrons and muons) and jets, which are sprays of heavier particles called hadrons (such as pions, protons and neutrons).

CDF has recently measured the mass of the top quark using only the information taken from the leptons in top quark decays. This measurement is possible because the characteristic shape of the momentum (or energy) distribution of the leptons in top quark decays is sensitive to the mass of the top quark. A heavier top quark has a stiffer, or higher momentum spectrum, than a top quark with a lighter mass.

Costas Vellidis was responsible for this analysis.

To make a precise measurement, CDF scientists plotted the momentum distribution of leptons in potential top quark events and then compare that shape to simulations of top quark masses.

Although this method uses only one part of the available information in the top quark decay and has a limited statistical precision, it has very different measurement uncertainties that make it an important cross check to the more traditional approaches.

This measurement agrees well with a measurement reported in the Nov. 18 issue of Fermilab Today.

— Andrew Beretvas