Experimental constraints on fourth generation quark masses

Understand

The existing bounds from CDF on the masses of the fourth generation quarks, t' and b', are reexamined.

• The bound of 256 GeV on the t' mass assumes that the primary decay of the t' is into q+W, which is not the case for a substantial region of parameter space.
• The bound of 268 GeV on the b' mass assumes that the branching ratio for b' --> b+Z is very large, which is not only not true for much of parameter space, but is {\em never} true for b' masses above 255 GeV.
• In addition, it is assumed that the heavy quarks decay within the silicon vertex detector, and for small mixing angles this will not be the case.