Width watchers test top quark

DZero physicists precisely determined the width of the top quark, something that cannot be measured with a ruler.

In the Standard Model, the top quark is considered a point particle and lacks any spatial size or volume, yet DZero physicists have recently performed the most precise determination of the width of the top quark to date. This statement seems to contradict itself when considering the kind of width you might measure with a ruler, but is perfectly sensible when discussing the decay width of an unstable particle.

Heisenberg’s uncertainty principle limits how accurately certain pairs of particle properties can be known at the same time. One such relationship exists between the energy of a particle state and the length of time that state lasts. An unstable particle that doesn’t stick around long before it decays, such as a top quark, is not always produced at exactly the same mass. The decay width of a particle measures this inherent range of possible masses. A particle’s lifetime and decay width are directly related through the uncertainty principle, such that a particle with a short lifetime will have a large decay width.

DZero physicists combined two previous results to determine the width of the top quark. The first, a previous result of the week, is the rate of the specific mode of single top quark production where a bottom quark and a W boson fuse to create a top quark. The top quark decay width is proportional to this production rate. The second is the chance for a top quark decay to produce a W boson with a bottom quark, in particular, out of the possible down quark, strange quark or bottom quark decay products. This information is used to help transform the single top quark production rate into the world’s best determination of the top quark width. This best value translates into a top quark decay lifetime of less than a millionth of a billionth of a billionth of a second!

If you are interested in hearing more about this and other single top quark related analyses, they will be the focus of tomorrow’s Joint Experimental-Theoretical Seminar.

—Mike Cooke

These physicists made major contributions to this analysis.

Over the course of Run II, in parallel with data taking and analysis efforts, a number of improvements have been developed for the DZero event reconstruction algorithms. This team is reprocessing a significant part of the DZero Run II data set using the best available algorithms, which will lead to improvements in DZero’s physics results.