Search for a fourth generation b’-quark at LEP-II at √(s)= 196–209 GeV

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.