SPHALERON TRANSITIONS IN THE MINIMAL STANDARD MODEL AND THE UPPER BOUND FOR THE HIGGS-BOSON MASS

We calculate the dissipation of the baryon number after the electrowea k phase transition due to thermal fluctuations above the sphaleron bar rier. We consider not only the classical Boltzmann factor but also fer mionic and bosonic one-loop contributions. We find that both bosonic a nd especially fermionic fluctuations can considerably suppress the tra nsition rate. Assuming the Langer-Affleck formalism for this rate, the condition that an initial baryon asymmetry must not be washed out by sphaleron transitions leads, in the minimal standard model (sin theta( W) = 0), to an upper bound for the Higgs boson mass in the range 60 to 75 GeV.