Four Generations, the Electroweak Phase Transition, and Supersymmetry

We calculate the strength of the electroweak phase transition in a supersymmetric model with four chiral generations. The additional chiral fermions (and scalar partners) lower the critical temperature and thus strengthen the first-order phase transition. The scalar partners stabilize the potential, leading to an effective theory that is bounded from below. We identify the ensemble of parameters where phi_c/T_c \gsim 1 simultaneous with obtaining a large enough Higgs mass. Our calculations focus on a subset of the full four generational supersymmetric parameter space: We take the pseudoscalar heavy, tan(beta)=1, and neglect all subleading contributions to the effective potential. We find that the region of parameter space with a strong first-order phase transition requires m_tilde{q}'/m_q' \lsim 1.1 while the constraint on the lightest Higgs mass requires m_tilde{q}'/m_q' /gsim 1 with m_q' \gsim 300 GeV. We are led to an intriguing prediction of quarks and squarks just beyond the current Tevatron direct search limits that are poised to be discovered quickly at the LHC.