Variants of the Dark Left-Right Gauge Model: Neutrinos and Scotinos

In the recently proposed dark left-right gauge model (DLRM) of particle interactions, the usual left-handed lepton doublet (ν, e)L transforming under SU(2)L is accompanied by the unusual right-handed fermion doublet (n, e)R transforming under SU(2)R, where nR is not the Dirac mass partner of νL. In this scenario, whereas νL is certainly a neutrino, nR should be considered a scotino, i.e. a dark-matter fermion. Variants of this basic idea are discussed, including its minimal scotogenic realization. Introduction : The gauge group SU(3)C × SU(2)L × U(1)Y of the Standard Model (SM) of particle interactions treats left-handed and right-handed fermions differently, with the electric charge given by Q = T3L + Y . To restore left-right symmetry, it is often proposed that the extension SU(3)C × SU(2)L × SU(2)R × U(1)B−L be considered, where Q = T3L + T3R + (B − L)/2. In that case, the fermion content of the SM gains one extra particle, i.e. νR in the right-handed lepton doublet (ν, l)R. Connecting this with the usual left-handed lepton doublet (ν, l)L through a Higgs bidoublet, νR pairs with νL to obtain a Dirac mass, just as lR does with lL. Assuming SU(2)R × U(1)B−L is broken to U(1)Y through a Higgs triplet transforming as (1, 1, 3, 1), νR gets a large Majorana mass, thereby inducing a small seesaw mass for νL. The above is a well-known scenario for what the addition of νR would do for understanding the existence of tiny neutrino masses. For a more general discussion of the SU(2)R breaking scale, see Ref. [1]. Suppose the mass connection between νR and νL is severed without affecting lR and lL, then νL and νR can be different particles, with their own interactions. Whereas νL is clearly still the well-known neutrino, νR may become something else entirely. As shown in Ref. [2], it may in fact be a scotino, i.e. a dark-matter fermion, and to avoid confusion, it is renamed nR. This is accomplished in a nonsupersymmetric SU(3)C ×SU(2)L×SU(2)R×U(1) model with the imposition of a global U(1) symmetry S, such that the breaking of SU(2)R×S will leave the generalized lepton number L = S − T3R unbroken. It is called the dark left-right model (DLRM), to distinguish it from the alternative left-right model (ALRM) proposed 22 years ago [3, 4] which has the same crucial property that nR is not the mass partner of νL. Fermion content : The fermion structure of the DLRM under SU(3)C×SU(2)L×SU(2)R× U(1)× S is given by [2]