Creating Matter at the Electroweak Phase Transition

Although feasible explanations for the observed preponderance of matter over antimatter have long existed, it is only since the idea of creating it at the electroweak phase transition (EWPT) that we have the hope of testing such theories in the laboratory. Since the EWPT occurred when the universe was at a temperature near 100 GeV, any new physics it might need for producing the baryon asymmetry should be within the reach of the next generation of particle accelerators. In fact one of the attractive features of electroweak baryogenesis is that a minimal number of new physics ingredients is needed to produce the baryon asymmetry. Already within the standard model one has baryon number violation via electroweak sphaleron interactions at high temperature. These interactions involve 9 quarks and 3 leptons, thus violating lefthanded B + L by 3 units, and their rate is large up until the EWPT. Furthermore one has departure from thermal equilibrium because the EWPT is first order, proceeding by the nucleation of bubbles of the true vacuum phase (where the Higgs field gets a VEV) from the symmetric phase. Thus two of Sakharov’s conditions for baryogenesis are fulfilled. The third, CP violation, is also present in the standard model, but most consider the form it takes there to be too feeble for the purposes of baryogenesis, being suppressed by factors of all the quark masses and mixing angles, and thermal damping effects. This shortcoming can be remedied by adding a second Higgs doublet, for example. A conceptually attractive way of putting these ingredients together was proposed by Cohen, Kaplan and Nelson in 1991 [1], and elaborated more recently by Joyce, Prokopec and Turok [2]. The idea is that fermions from the symmetric phase have some probability of bouncing off the expanding walls of the bubbles of true vacuum phase. Due to CP violation in the walls, the reflection probability is different for right-handed and left-handed fermions, and so an excess of chirality initially builds up in front of the moving wall. Since sphalerons “see” only left-handed fermions, they act so as to redistribute the chirality asymmetry among all