A Way to Reopen the Window for Electroweak Baryogenesis

We reanalyse the sphaleron bound of electroweak baryogenesis when allowingdeviations to the Friedmann equation. These modifications are well motivated inthe context of brane cosmology where they appear without being in conflict withmajor experimental constraints on four-dimensional gravity. While suppressed atthe time of nucleosynthesis, these corrections can dominate at the time of theelectroweak phase transition and in certain cases provide the amount ofexpansion needed to freeze out the baryon asymmetry without requiring astrongly first order phase transition. The sphaleron bound is substantiallyweakened and can even disappear so that the constraints on the higgs and stopmasses do not apply anymore. Such modification of cosmology at early timestherefore reopens the parameter space allowing electroweak baryogenesis whichhad been reduced substantially given the new bound on the higgs mass imposed byLEP. In contrast with previous attempts to turn around the sphaleron boundusing alternative cosmologies, we are still considering that the electroweakphase transition takes place in a radiation dominated universe. The universe isexpanding fast because of the modification of the Friedmann equation itselfwithout the need for a scalar field and therefore evading the problem of thedecay of this scalar field after the completion of the phase transition and therisk that its release of entropy dilutes the baryon asymmetry produced at thetransition.Comment: 19 pages, 3 figures; v2: minor changes, remark added at end of section 5 and in caption of figure 1; v3: references added, version to be publishe