Search for a fourth generation b'-quark at LEP-II

In this thesis, a search for the pair production of sequential fourth generation quarks at LEP ($e^+ e^-\to b'\bar{b'}$) is presented. The data collected by DELPHI at centre-of-mass energies ranging from 196 GeV to 209~GeV, corresponding to an integrated luminosity of about 420~pb$^{-1}$, were analysed. This search was motivated by the electroweak precision data and by previous searches for the pair production of $b'$ quarks at LEP-I and at the TEVATRON, which still allow a $b'$ heavier than $m_{Z}/2$. In the considered mass range ($96$~GeV$/c^2<m_{b'}<103$~GeV$/c^2$) the $b'\to cW$ (charged currents) and $b'\to bZ$ (flavour changing neutral currents) decay modes are expected to be dominant. In the absence of a signal, upper limits on the branching ratios of these decay modes were obtained at 95% confidence level. The obtained limits on $BR_{b'\to cW}$ and $BR_{b'\to bZ}$, together with the theoretical predictions within a sequential model, were used to constrain the values of $|\frac{V_{cb'}}{V_{tb'}V_{tb}}|$, where $V_{cb'}$, $V_{tb'}$ and $V_{tb}$ are elements of the extended $4\times 4$ Cabbibo-Kobayashi-Maskawa matrix. Two extreme cases were considered: the almost degenerate case ($m_{t'}-m_{b'}sim 1$~GeV$/c^2$) and the case where the mass difference is close to the largest possible value ($m_{t'}-m_{b'}sim 50$~GeV$/c^2$).