Search for scalar quarks in e+e− collisions at s up to 209 GeV

Searches for scalar top, scalar bottom and mass-degenerate scalar quarks are performed in the data collected by the ALEPH detector at LEP, at centre-of-mass energies up to 209 GeV, corresponding to an integrated luminosity of 675 pb−1. No evidence for the production of such particles is found in the decay channels t̃→c/uχ, t̃→bℓν̃, b̃→bχ, q̃→qχ or in the stop four-body decay channel t̃→bχff̄ ′ studied for the first time at LEP. The results of these searches yield improved mass lower limits. In particular, an absolute lower limit of 63 GeV/c2 is obtained for the stop mass, at 95% confidence level, irrespective of the stop lifetime and decay branching ratios.