Search for a fourth generation b-prime-quark at LEP-II at s**(1/2) = 196-GeV - 209-GeV

A search for the pair production of fourth generation b'-quarks was performed using data taken by the DELPHI detector at LEP-II. The analysed data were collected at centre-of-mass energies ranging from 196 to 209 GeV, corresponding to an integrated luminosity of 420 pb^{-1}. No evidence for a signal was found. Upper limits on BR(b' -> bZ) and BR(b' -> cW) were obtained for b' masses ranging from 96 to 103 GeV/c^2. These limits, together with the theoretical branching ratios predicted by a sequential four generations model, were used to constrain the value of R_{CKM}=|V_{cb'}/V_{tb'}V_{tb}|, where V_{cb'}, V_{tb'} and V_{tb} are elements of the extended CKM matrix