Search for the t and b' quarks in hadronic decays of the Z0 boson

We present a search for the third generation up type quark t and a possible fourth down type quark b' in hadronic Z 0 decays observed in DELPHI at the LEP collider. For any scenario with a decay through the charged current or into a charged Higgs with a mass at least 6 GeV/ c 2 below the t and 3 GeV c 2 below the b' mass, we set a lower limit for the t quark mass at 44.0 GeV/ c 2 and for the b' mass at 44.5 GeV/ c 2 . For specific scenarios the mass limits are slightly higher, e.g. for charged current decays the limits are 44.5 and 45.0 GeV/ c 2 respectively, where all limits are given at a 95% confidence level.