The figure above shows the bounds on the top-quark width at 68% and 95% confidence levels and Δχ2, a parameter used to set the bounds.

As a member of the third generation of the particle family in the standard model, the top quark has received tremendous attention ever since its discovery at the Tevatron in 1995. Its unique feature in mass -- much heavier than any other elementary particles ever observed -- has won it extraordinary fame in the family. Its lifetime, yet-to-be-measured, is another unique feature; unlike other quarks, the top quark is expected to decay even before it is hadronized. In the unit of seconds, the hadronization time scale is 0.0000000000000000000000033 s or 3.3 x 10-24s, whilst the predicted top quark lifetime is 0.0000000000000000000000005 s or 5 x 10-25s. The top quark does not live long! This feature has been assumed when physicists probe various properties of the top quark. In fact, this particular feature enabled physicists to do a much better job in doing that. However, whether this prediction is correct thus comes into question.

How do physicists measure the lifetime of such short-lived particle? They can do by measuring the width of the particle since the lifetime of a particle (τ) is inversely proportional to its width (Γ) and the width would affect the mass distribution of the particle. The top-quark width, if measured precisely, could not only test the standard model, but also favor or rule out specific new physics models. For example, a top quark could decay into a charged Higgs boson and a bottom quark in a supersymmetric extension of the standard model (in the standard model, a top quark almost exclusively decays into a W boson and a bottom quark), resulting in a larger width or a shorter lifetime. Comparing the shape of the reconstructed mass distribution of top quarks with Monte Carlo simulation with various top width values, physicists at the CDF attempted to measure the top-quark width. From 4.3 fb-1 of collision data recorded by CDF, physicists quote the upper limit of the top-quark width of 7.6 GeV (or its lifetime of 8.7 x 10-26 s) at the 95% confidence level, and the 68% confidence level bound of 0.3 GeV < Γtop < 4.4 GeV or 1.5 x 10-25 s < τtop < 2.2 x 10-24 s. This supports that the top quark decays before hadronization.

The above individuals contributed to this analysis. From left to right:Hyun Su Lee(the University of Chicago), Jian Tang(the University of Chicago), and Young-Kee Kim(Fermilab, the University of Chicago).