Physical implications of a double right-handed gauge symmetry

TL;DR

The paper proposes a novel SM extension based on the flipping principle with a double right-handed Abelian gauge structure that renders left-handed fermions neutral and assigns nonuniversal charges to right-handed fermions. It achieves the SM fermion mass hierarchy by generating third-generation masses at tree level while first and second generation masses arise radiatively, and it explains the active neutrino mass hierarchy via a combination of tree-level and two-loop seesaw mechanisms. A residual parity stabilizes a complex scalar singlet DM candidate, which can reproduce the observed relic abundance and satisfy direct-detection bounds. The framework yields distinctive collider and flavor signatures, including Z'–mediated FCNCs, electroweak precision constraints, and novel scalar production channels, making it testable at current and future facilities like the LHC and ILC.

Abstract

Guided by the flipping principle, we propose a novel extension of the Standard Model based on a double right-handed $U(1)$ gauge symmetry. In this framework, all left-handed fermions are neutral, while right-handed fermions of the third generation carry charges distinct from those of the first two generations. This structure naturally explains the observed Standard Model fermion mass hierarchy: the heavy masses of the third generation are generated at tree level, while the lighter masses of the first and second generations arise radiatively at the one-loop level. For the active neutrino sector, the tiny masses are generated through a combination of tree-level and two-loop seesaw mechanisms. Crucially, this approach successfully reproduces the observed neutrino mass hierarchy, with the atmospheric mass-squared difference generated at tree level and the solar neutrino mass squared difference emerging at the two-loop level. These hierarchical patterns stem from the interplay between gauge invariance and a residual parity symmetry that survives the spontaneous breaking of the extended gauge group. The same residual symmetry stabilizes a viable scalar singlet dark matter candidate, which we show can reproduce the observed relic abundance while remaining consistent with current direct detection bounds. After addressing constraints from electroweak precision tests and flavor-changing neutral currents, we explore the discovery prospects for the new neutral bosons at existing and future colliders, including the LEP, LHC, and a future ILC.

Figures (9)

• Figure 1: Feynman diagrams contributing to the entries of the mass matrices for SM charged fermions (first row) and light active neutrinos (second row). Here $a,b=1,2,3$ and $\alpha,\beta=1,2$.
• Figure 2: The red, brown, pink, and orange curves represent the exclusion bounds derived from the $Z$-boson mass shift ParticleDataGroup:2024cfk, the $\rho$ parameter ParticleDataGroup:2024cfk, the $Z$--$Z'$ mixing parameter $\varepsilon$ALEPH:2005abErler:2009jh, and the total decay width of $Z_1$ParticleDataGroup:2024cfk, respectively. The shaded regions are excluded by current experimental data. The dashed blue line indicates the gauge coupling unification condition $g_1=g_2=\sqrt2 g_Y$.
• Figure 3: Lower bounds of $-\mu_{0}^2$ as a function of the new-physics scale $\Lambda_1$ for several values of $g_1$. The gray-shaded region is excluded by the $2\sigma$ flavor constraints, while the white region is allowed.
• Figure 4: Total cross section for $Z^\prime$ production via Drell--Yan mechanism at the LHC as a function of the $Z^\prime$ mass, for $\sqrt{S}=14$ TeV (left) and $\sqrt{S}=28$ TeV (right).
• Figure 5: Total cross section for resonant $pp\rightarrow Z^\prime\rightarrow l^{+}l^{-}$ production via Drell--Yan mechanism at the LHC as a function of the $Z^\prime$ mass, for $\sqrt{S}=14$ TeV (left) and $\sqrt{S}=28$ TeV (right).