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Naturally small Dirac neutrino mass and $B-L$ dark matter

Ernest Ma, Partha Kumar Paul, Narendra Sahu

Abstract

In the conventional gauged ${B-L}$ extension of the standard model, the $B-L$ charge of the singlet scalar $χ$, responsible for the breaking of $U(1)_{B-L}$ symmetry, is taken to be 2 such that it can anchor type-I seesaw by giving Majorana masses to the right-handed neutrinos, $ν_R$. In this paper, we consider instead the cases $χ\sim 3$ or 4 under $B-L$, so that $ν_R$ may not acquire any Majorana mass and neutrinos are Dirac fermions. We then consider a vector-like fermion $S$ with 2 units of $B-L$ charge, which becomes a good candidate for dark matter, either Dirac for $χ\sim 3$ or Majorana for $χ\sim 4$. In both cases, spontaneous $B-L$ breaking can induce a strong first-order phase transition, producing stochastic gravitational waves (GW) which can be tested at GW experiments. Moreover, the presence of light $ν_R$s gives rise to an additional contribution to the effective number of relativistic degrees of freedom, $Δ{N}_{\rm eff}$, providing complementary constraints from current and upcoming CMB observations.

Naturally small Dirac neutrino mass and $B-L$ dark matter

Abstract

In the conventional gauged extension of the standard model, the charge of the singlet scalar , responsible for the breaking of symmetry, is taken to be 2 such that it can anchor type-I seesaw by giving Majorana masses to the right-handed neutrinos, . In this paper, we consider instead the cases or 4 under , so that may not acquire any Majorana mass and neutrinos are Dirac fermions. We then consider a vector-like fermion with 2 units of charge, which becomes a good candidate for dark matter, either Dirac for or Majorana for . In both cases, spontaneous breaking can induce a strong first-order phase transition, producing stochastic gravitational waves (GW) which can be tested at GW experiments. Moreover, the presence of light s gives rise to an additional contribution to the effective number of relativistic degrees of freedom, , providing complementary constraints from current and upcoming CMB observations.
Paper Structure (1 section, 18 equations, 8 figures, 2 tables)

This paper contains 1 section, 18 equations, 8 figures, 2 tables.

Table of Contents

  1. Feynman diagrams responsible for thermalization of $\nu_R$

Figures (8)

  • Figure 1: Tree-level Dirac neutrino mass.
  • Figure 2: [Left:] Parameter space delineating the thermal and non-thermal regions in the $m_\eta$–$y_R$ plane. The color code denotes $\mu_1$ values. [Right:] $\Delta{N}_{\rm eff}$ as a function of $y_R$ for three values of $\mu_1$, as indicated in the inset. Constraints from current CMB observations and projected sensitivities of future CMB experiments are shown by different colored dashed lines.
  • Figure 3: Correct DM relic parameter space in $\textsl{g}_{\rm BL}-m_{Z_{\rm BL}}$ plane for six choices of DM masses shown with different colored lines. Constraints from CMS CMS:2021ctt, ATLAS ATLAS:2019erb, LHCb LHCb:2019vmc, NA64 Banerjee:2019pds, BaBar BaBar:2014zli, COHERENT Cadeddu:2020nbr are shown with different shaded regions.
  • Figure 4: Correct DM relic parameter space in $\textsl{g}_{\rm BL}-m_{Z_{\rm BL}}$ plane. Constraints from CMS CMS:2021ctt, ATLAS ATLAS:2019erb, LHCb LHCb:2019vmc, NA64 Banerjee:2019pds, BaBar BaBar:2014zli, COHERENT Cadeddu:2020nbr are shown with different shaded regions.
  • Figure 5: Feynman diagram for the freeze-in production of DM from SM Higgs decay.
  • ...and 3 more figures