Despite the shutdown of the Fermilab Tevatron collider, two years ago, and the subsequent disassembling of the glorious CDF detector, the CDF Collaboration continues to produce excellent physics results using the large bounty of data they have accumulated in the course of the past 10 years.

Today you can find in the Cornell arxiv a new paper by CDF, which describes a new very interesting measurement of a property of the top quark - the particle discovered at Fermilab in 1995, the heaviest known elementary particle we know. The property measured is the lifetime of top quarks.

Note: this is the fourth, and last, part of a four-part article (see part I, part II, part III) on the five-sigma criterion for discovery claims in particle physics. If you haven't read the first three installments, the text below may or may not make much sense to you...

Note: this is the third part of a four-part article on the Five-Sigma criterion in particle physics. See part 1 and part 2 to make more sense of the discussion below.

In the previous installment of this longish article, I have introduced some of the issues that may affect the correct interpretation of a statistically significant effect.

A pre-emptive warning to the reader: the article below is too long to publish as a single post. I have broken it out in four installments. After reading the text below you should continue with part II, part III, and part IV (which includes a summary).

Do you remember the X(3872) ? This is a hadron containing charm and anticharm quarks, which was observed to decay into a J/Psi meson, a positive, and a negative pion. When it was discovered, by the Belle experiment in 2003, the X caused a lot of interest among spectroscopists, because it is an "exotic" charmonium state: its nature is not totally clear, as it might be interpreted as a "molecule" of two charmed mesons loosely bound together. Or maybe a four-quark system ? Or just conventional charmonium, a bit at odds with the expected set of spin-parity states but otherwise just a honest meson ?