At current energies, the world is left-handed, i.e. the Standard Model contains an SU(2) group. Left-right symmetry at some larger scale implies the need for an SU(2) group. Thus the particle content is expanded by right-handed and and right-handed neutrinos. The Higgs fields have to be in a triplet representation, leading to doubly-charged Higgs particles, one set for each of the two SU(2) groups. Also the number of neutral and singly-charged Higgs states is increased relative to the Standard Model, but a search for the lowest-lying states of this kind is no different from e.g. the freedom already accorded by the MSSM Higgs scenarios.
PYTHIA implements the scenario of [Hui97]. The expanded particle
content with default masses is:
The has been implemented as a simple copy of the ordinary , with the exception that it couple to right-handed neutrinos instead of the ordinary left-handed ones. Thus the standard CKM matrix is used in the quark sector, and the same vector and axial coupling strengths, leaving only the mass as free parameter. The implementation (without interference with or the ordinary ) allows decays both to left- and right-handed neutrinos, as well as other fermions, according to one specific model ansatz [Fer00]. Obviously both the and the descriptions are likely to be simplifications, but provide a starting point.
The right-handed neutrinos can be allowed to decay further [Riz81,Fer00]. Assuming them to have a mass below that of , they decay to three-body states via a virtual , and , where both choices are allowed owing to the Majorana character of the neutrinos. If there is a significant mass splitting, also sequential decays are allowed. Currently the decays are isotropic in phase space. If the neutrino masses are close to or above the ones, this description has to be substituted by a sequential decay via a real (not implemented, but actually simpler to do than the one here).