Allowable Low-Energy E6 Subgroups from Leptogenesis

There are only two viable low-energy E6 subgroups: SU(3)C ×SU(2)L ×U(1)Y × U(1)N or SU(3)C × SU(2)L × SU(2)′R × U(1)YL+Y ′ R , which would not erase any preexisting lepton asymmetry of the Universe that may have been created by the decay of heavy singlet (right-handed) neutrinos or any other mechanism. They are also the two most favored E6 subgroups from a recent analysis of present neutral-current data. We study details of the leptogenesis, as well as some salient experimental signatures of the two models. In the energy range of 100 GeV to 1 TeV, physics beyond the standard model (SM) may appear in two ways. One is the possible addition of supersymmetry; the other is the possible extension of the SU(3)C×SU(2)L×U(1)Y gauge group to a larger symmetry group G. Both of these options are realized in the E6 superstring models which predict the existence of new particles, such as an extra gauge boson Z , at O(1) TeV [1]. As required by the solar and atmospheric neutrino data [2], any extension of the SM should include a mechanism for generating small nonzero neutrino masses. It should also be consistent with the present observed baryon asymmetry of the Universe. If it contains B−L violating interactions at energy scales in the range 102−1012 GeV, these together with the B + L violating electroweak sphalerons [3] would erase [4] whatever lepton or baryon asymmetry that may have been created at an earlier epoch of the Universe [5]. In this Letter we show that if G is a subgroup of E6, and if G survives down to O(1) TeV as is expected in these theories, then the constraint of successful leptogenesis [6, 7] from the decay of heavy singlet (right-handed) neutrinos N results uniquely in only two possible candidates. One is G1 = SU(3)C × SU(2)L × U(1)Y × U(1)N [8], and the other is G2 = SU(3)C × SU(2)L × SU(2)′R × U(1)YL+Y ′ R [9], where SU(2) ′ R is not the conventional SU(2)R. Only these groups allow N to have zero quantum numbers with respect to all of their transformations. Any other subgroup of E6 would result in lepton-number violating interactions at O(1) TeV as it is broken down to the SM. Remarkably, G1,2 happen to be also the two most favored E6 subgroups from a recent analysis [10] of present neutral-current data. This is a possible hint that one of these two models may in fact be correct. Whereas there is only one version [9] of the model based on G2, we find 2 (and only 2) phenomenologically viable versions of G1, and work out the details of the leptogenesis in all 3 cases. In addition to specific Z ′ properties at colliders, we also predict the discovery of W R in the G2 model. Among other distinctive experimental signatures are the s-channel diquark