Dirac dark matter, dark radiation and type-II seesaw in alternative $U(1)_X$ standard model

Abstract

We propose an extra U (1) X model with an alternative charge assignment for right-handed (RH) neutrinos. The type-II seesaw mechanism by a triplet Higgs field is promising for neutrino mass generation because of the alternative charge assignment. The small vacuum expectation value (VEV) of an additional Higgs doublet naturally leads to a very small VEV of the triplet Higgs field, and as a result, the smallness of neutrino mass can be understood. With the minimal Higgs field for U (1) X with the charge 1, RH neutrinos are candidates for Dirac dark matter (DM) and dark radiation (DR). We have derived and imposed the LHC bound, the DR constraint and the bound from DM direct searches in the wide range of parameter space. Among various U (1) X choices, the DM direct search bound is found to be weakest for U (1) R where the constraints from thermal DM and non-negligible DR can be compatible. Such a number of the effective neutrino species would be interesting from the viewpoint of the so-called Hubble tension.

Affleck-Dine cogenesis of baryon and dark matter

“…Predictions for ∆N eff . An interesting aspect of such light Dirac neutrino scenarios is the enhancement of the effective relativistic degrees of freedom N eff which can be probed at CMB experiments, as can be found in recent works [69][70][71][72][73][74][75][76][77][78][79][80]. The current 2σ limit (95% CL) on N eff from the Planck 2018 data is N eff = 2.99 +0.34 −0.33 [1], consistent with the SM prediction N SM eff = 3.045.…”

“…where n ν R denotes the equilibrium number density and σv is the thermal averaged cross section of ν R [79], we get an upper bound on the gauge coupling g BL in M Z T D limit as Table 2. Three sets of benchmark parameters highlighted in figure 3.…”

Concealing Dirac neutrinos from cosmic microwave background

“…(including the baryon acoustic oscillation (BAO) data) which perfectly agrees with the Standard Model (SM) prediction N SM eff = 3.045 [2,4,5]. The next generation CMB experiments particularly CMB-S4 [6] will be sensitive to a precision of ∆N eff = N eff − N SM eff = 0.06 at 95% C.L., which is expected to test all such beyond Standard Model (BSM) scenarios with light degrees of freedom (DOF) that were in equilibrium with the SM at some point of the evolution of our universe or produced non-thermally from the decay or annihilation of other heavy species [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. In many of such BSM scenarios, the primary motivation is to explain the tiny nonzero neutrino masses (see for example [9,10]) as suggested by neutrino oscillation experiments [22][23][24][25][26].…”