Naturally Stable Right-Handed Neutrino Dark Matter

Abstract

We point out that a class of non-supersymmetric models based on the gauge group SU (3) C × SU (2) L × SU (2) R × U (1) Y L × U (1) Y R possesses an automatic, exact Z 2 symmetry under which the fermions in the SU (2) R × U (1) Y R sector (called R-sector) are odd and those in the SU (2) L × U (1) Y L sector (called L-sector or the Standard Model sector) are even. This symmetry, which is different from the usual parity symmetry of the left-right symmetric models, persists in the lepton sector even after the gauge symmetry breaks down to SU (3) C × U (1) EM . This keeps the lightest right-handed neutrino naturally stable, thereby allowing it to play the role of dark matter (DM) in the Universe. There are several differences between the usual left-right models and the model presented here: (i) our model can have two versions, one which has no parity symmetry so that the couplings and masses in the L and R sectors are unrelated, and another which has parity symmetry so that couplings are related but with an extra Higgs doublet in each sector so that the fermion mass patterns are different; (ii) the R-sector fermions are chosen much heavier than the L-sector ones in both scenarios; and finally (iii) both light and heavy neutrinos are Majorana fermions with the light neutrino masses arising from a pure type-II seesaw mechanism. We discuss the DM relic density, direct and indirect detection prospects and associated collider signatures of the model. Comparing with current collider and direct detection constraints, we find a lower bound on the DM mass of order of 1 TeV. We also point out a way to relax the DM unitarity bound in our model for much larger DM masses by an entropy dilution mechanism. An additional feature of the model is that the DM can be made very long lived, if desired, by allowing for weak breaking of the above Z 2 symmetry. Our model also predicts the existence of long-lived colored particles which could be searched for at the LHC.

Heavy right-handed neutrino dark matter in left–right models

“…a The parity symmetric version could be realized at the TeV scale by doubling the doublet scalars in both the SM and heavy sectors, such that one set of the Yukawa couplings y f,1 is responsible for the SM fermion masses and the other one y f,2 dominates the heavy fermion masses. 11 Once the Σ fields acquire VEVs, the heavy-light quark mass mixing terms δ U = λ U Σ 1 and δ D = λ D Σ 2 appear in the Lagrangian, and the heavy quark can then decay into the SM fermions, leaving only the lightest RHN stable due to the leptonic Z 2 symmetry. The singlets Σ 1,2 are charged under The decay rate of heavy quarks Q → q + Z can be estimated as…”

“…those based on the Left-Right (LR) gauge group SU (2) L × SU (2) R × U (1) B−L , [7][8][9] the gauge interactions of RHNs induce further decay modes, which have to be suppressed/forbidden by imposing additional discrete symmetries (or making the model more contrived) to keep the lightest RHN cosmologically stable. 10 In this paper we discuss an alternative class of LR models based on the gauge group SU (3) 11 which has an accidental Z 2 symmetry, at the renormalizable level that keeps the lightest RHN stable making it a natural DM candidate. In this model there exists a right-handed copy of the SM electroweak sector at the TeV scale (or higher), charged under SU (2) R × U (1) Y R .…”

Gravitational waves triggered by $B-L$ charged hidden scalar and leptogenesis

“…For a sequential Z boson with the same couplings as in the SM, the current ATLAS and CMS 13 TeV data requires that M Z > 4.05 TeV at the 95% confidence level [49,50]. The production cross section σ(pp → Z → + − ) in the U (1) B−L model can be obtained by rescaling that of a sequential heavy Z boson, as function of the gauge coupling g BL [51]. To this end, the partial decay widths of the Z boson into the SM fermions, the heavy RHNs and the scalar S are respectively…”

Super heavy thermal dark matter

“…Of course, DM may be much heavier than this bound, if it is not a WIMP. Such models include non-thermal dynamics, decoupled dark sectors, inflationary and gravitational production, nonstandard cosmological histories, and large entropy production [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. In none of these cases, however, is the DM abundance solely determined by its interactions with the SM.…”