Top Quark - Decay

Decay

The only known way that a top quark can decay is through the weak interaction producing a W-boson and a down-type quark (down, strange, or bottom). Because of its enormous mass, the top quark is extremely short-lived with a predicted lifetime of only 5×10−25 s. As a result top quarks do not have time to form hadrons before they decay, as other quarks do. This provides physicists with the unique opportunity to study the behavior of a "bare" quark.

In particular, it is possible to directly determine the branching ratio Γ(W+b) / Γ(W+q (q = b,s,d)). The best current determination of this ratio is 0.99+0.09
−0.08. Since this ratio is equal to | V_tb |2 according to the Standard Model, this gives another way of determining the CKM element | V_tb |, or in combination with the determination of | V_tb | from single top production provides tests for the assumption that the CKM matrix is unitary.

The Standard Model also allows more exotic decays, but only at one loop level, meaning that they are extremely suppressed. In particular, it is possible for a top quark to decay into another up-type quark (an up or a charm) by emitting a photon or a Z-boson. Searches for these exotic decay modes have provided no evidence for their existence in accordance with expectations from the Standard Model. The branching ratios for these decays have been determined to be less than 5.9 in 1,000 for photonic decay and less than 3.7 in 100 for Z-boson decay at 95% confidence.