Mixing effects in the finite-temperature effective potential of the MSSM with a light stop

We incorporate the effects of mixing arising from the trilinear terms in th e MSSM potential to the effective three-dimensional theory for the MSSM at high temperature in the limit of large m(A). There are relevant one-loop ef fects that modify the 3D parameters of the effective theory. We calculate t he two-loop effective potential of the 3D theory for the Higgs and the righ t handed stop to analyse the possible phase transitions and to determine th e precise region in the m(h)-m((t) over tilde 2) plane for which the sphale ron constraint for preservation of the baryon asymmetry is satisfied. There is an upper bound on the value of the mixing parameter coming from stop se arches. We also compare with previous results obtained using 4D calculation s of the effective potential for the regime of large m(Q). A two-stage phas e transition persists for a small range of values of m((t) over tilde 2) fo r given values of the mixing parameter and tan beta. This can further const rain the allowed region of parameter space. Electroweak baryogenesis requir es a value of m((t) over tilde 2) less than or similar to 170 and m(h) less than or similar to 105 GeV for m(Q) = 300 GeV. (C) 2000 Elsevier Science B .V. All rights reserved.