Improved measurement of the top quark width

The figure shows the limits on the top quark width at the 68 percent and 95 percent confidence levels and the result of a fit to the data.

The top quark is the heaviest currently known elementary particle. The only way the top quark can decay is through the weak interaction, which is the only interaction that allows quarks to change flavor (tW + b). Its large mass endows it with the largest so-called decay width, and hence, the shortest lifetime of any of the known fermions, which include both the quark and lepton classes of particles. The time it would take for the top quark to form hadrons — particles that contain either two or three quarks — is incredibly short (3.3 × 10-24 seconds), and scientists predict using the Standard Model that the top quark lifetime is even shorter (5.0 x 10-25 seconds). Thus physicists believe that, when they measure the properties of the top quark system, they are looking directly at the top quark itself.

Precise measurements of the top quark decay width allow us to measure its lifetime, exploiting a principle of quantum mechanics called the Heisenberg uncertainty principle. This principle states that certain complementary variables, such as energy and time, are subject to a mathematical inequality that limits the precision with which they can be simultaneously measured. Here we measure the width of the top mass distribution, which is related to the particle’s energy. By measuring the top’s mass distribution, we can also determine its lifetime, since the lifetime is inversely proportional to the mass.

This is CDF’s final measurement of the top quark decay width. Earlier results showed that the decay width is shorter than the time required to form hadrons. The experiment improves on an earlier measurement by using a larger sample, as well as through a better understanding of the systematic errors that go into the top mass distribution.

CDF scientists have measured the top quark width using the full Run II data set by comparing the shape of the reconstructed mass distribution of top quarks with samples whose top quark widths are already known. Scientists extract most-likely top quark widths for the data by deciding which known sample looks the most like the unknown data. Using the Heisenberg uncertainty principle, CDF determined the top quark lifetime at the 68 percent confidence level to be greater than 1.6 x 10-25 seconds and less than 6.0 x 10-25 seconds. This measurement supports the Standard Model lifetime of 5.0 x 10-25 and thus places limits on non-Standard Model decays.

Learn more

edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Hyun Su Lee, Ewha Womans University, Seoul, Korea; Jian Tang, University of Chicago; and Young-Kee Kim, University of Chicago.