Neutrino and astroparticle physics : Working group report

It was shown in [] and [] that topological defects such as domain walls and cosmic strings are generic to the left-right (L-R) symmetric model with two triplet and one bidoublet Higgs fields. In particular, the discrete L-R symmetry of the triplet Higgs potential implies that at the scale vR, in a given region in the Universe, either the SU(2)L or the SU(2)R could break. Immediately after this phase transition therefore we would find domains with either of the above groups broken. Such domains will be separated by walls, dubbed L-R walls in []. Phenomenology demands that the SU(2)R bosons must eventually acquire the larger masses, and that the domain structure disappears in order to not dominate the energy density in the Universe. This will be achieved if GUT scale physics induces small corrections to the triplet Higgs potential, making it energetically favourable for the SU(2)L bosons to remain the lighter species. This will be assumed in the following. Recently with the increasing difficulties in explaining the baryon asymmetry either in the Standard Model or its minimal supersymmetric extension, the idea of leptogensis which then produces the baryon asymmetry has gained ground. In the L-R model, the disappearance of the domains with broken SU(2)L provides a preferred direction for the motion of the domain walls. This can fulfill the out-of-thermal-equilibrium requirement needed for leptogenesis. Consider the interaction of neutrinos from the L-R wall which is moving into the energetically disfavored phase. The left-handed neutrinos, νL, are massive in this domain, whereas they are massless in the phase behind the wall. This can be seen from the Majorana mass term hM∆Lν LνL, and the fact that 〈∆L〉 has a kink-like profile, being zero behind the wall and O(vR) in front of it.