Light Dirac Neutrinos Research Articles

Singlet-doublet fermion Dark Matter with Dirac neutrino mass, (g − 2)μ and ∆Neff

We study the possibility of generating light Dirac neutrino mass via scotogenic mechanism where singlet-doublet fermion Dark Matter (DM) plays non-trivial role in generating one-loop neutrino mass, anomalous magnetic moment of muon: (g − 2)μ as well as additional relativistic degrees of freedom ∆Neff within reach of cosmic microwave background (CMB) experiments. We show that the Dirac nature of neutrinos can bring interesting correlations within the parameter space satisfying the (g − 2)μ, DM relic density and the effective relativistic degrees of freedom ∆Neff. While we stick to thermal singlet-doublet DM with promising detection prospects, both thermal and non-thermal origin of ∆Neff have been explored. In addition to detection prospects of the model at DM, (g − 2)μ and other particle physics experiments, it remains verifiable at future CMB experiments like CMB-S4 and SPT-3G.

Light Dirac neutrino portal dark matter with gauged U(1)B−L symmetry

We propose a gauged U(1)B−L version of the light Dirac neutrino portal dark matter. The U(1)B−L symmetry provides a UV completion by naturally accommodating three right-handed neutrinos from anomaly cancellation requirements which, in combination with the left-handed neutrinos, form the sub-eV Dirac neutrinos after electroweak symmetry breaking. The particle content and the gauge charges are chosen in such a way that light neutrinos remain purely Dirac and dark matter, a gauge singlet Dirac fermion, remain stable. We consider both thermal and nonthermal production possibilities of dark matter and correlate the corresponding parameter space with the one within reach of future cosmic microwave background (CMB) experiments sensitive to enhanced relativistic degrees of freedom ΔNeff. The interplay of dark matter, CMB, structure formation and other terrestrial constraints keep the scenario very predictive leading the U(1)B−L parameter space into tight corners. Published by the American Physical Society 2024

Predictive Dirac neutrino spectrum with strong CP solution in SU(5)L × SU(5)R unification

We develop a grand unified theory of matter and forces based on the gauge symmetry SU(5)L × SU(5)R with parity interchanging the two factor groups. Our main motivation for such a construction is to realize a minimal GUT embedding of left-right symmetric models that provide a parity solution to the strong CP problem without the axion. We show how the gauge couplings unify with an intermediate gauge symmetry SU(3)cL × SU(2)2L × U(1)L × SU(5)R, and establish its consistency with proton decay constraints. The model correctly reproduces the observed fermion masses and mixings and leads to naturally light Dirac neutrinos with their Yukawa couplings suppressed by a factor MI/MG, the ratio of the intermediate scale to the GUT scale. We call this mechanism type II-Dirac seesaw. Furthermore, the model predicts δCP = ±(130.4±1.2)° and \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${m}_{{\ u }_{1}}$$\\end{document} = (4.8 – 8.4) meV for the Dirac CP phase and the lightest neutrino mass. We demonstrate how the model solves the strong CP problem via parity symmetry.

Observable ΔNeff\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {\ extrm{N}}_{\ extrm{eff}}$$\\end{document} in Dirac scotogenic model

We study the possibility of probing the radiative Dirac seesaw model with dark sector particles going inside the loop, popularly referred to as the Dirac scotogenic model via measurements of effective relativistic degrees of freedom ΔNeff\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {\ extrm{N}}_{\ extrm{eff}}$$\\end{document} at cosmic microwave background (CMB) experiments. The loop suppression and additional free parameters involved in neutrino mass generation allow large (∼O(1))\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\sim {\\mathcal {O}}(1))$$\\end{document} coupling of light Dirac neutrinos with the dark sector particles. Such large Yukawa coupling not only dictates the relic abundance of heavy fermion singlet dark matter but also can lead to the thermalisation of the right chiral part of Dirac neutrinos, generating additional relativistic degrees of freedom ΔNeff.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {\ extrm{N}}_{\ extrm{eff}}.$$\\end{document} We find that the parameter space consistent with dark matter phenomenology and neutrino mass bounds can also be probed at future cosmic microwave background experiments like CMB-S4 via precision measurements of ΔNeff.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta {\ extrm{N}}_{\ extrm{eff}}.$$\\end{document} The same parameter space, while leading to loop-suppressed direct detection cross-section of dark matter outside future sensitivities, can also have other interesting and complementary observational prospects via charged lepton flavour violation and collider signatures.

Dark dimension and the standard model landscape

We study the landscape of lower-dimensional vacua of the standard model (SM) coupled to gravity in the presence of the so-called “dark dimension” of size R⊥ in the micron range, focusing on the validity of the swampland conjecture forbidding the presence of nonsupersymmetric anti–de Sitter (AdS) vacua in a consistent quantum gravity theory. We first adopt the working assumption that right-handed neutrinos propagate in the bulk, so that neutrino Yukawa couplings become tiny due to a volume suppression, leading to naturally light Dirac neutrinos. We show that the neutrino Kaluza-Klein (KK) towers compensate for the graviton tower to maintain stable de Sitter (dS) vacua found in the past, but neutrino oscillation data set restrictive bounds on R⊥ and therefore the first KK neutrino mode is too heavy to alter the shape of the radion potential or the required maximum mass for the lightest neutrino to carry dS rather than AdS vacua found in the absence of the dark dimension, m1,max≲7.63 meV. We also show that a very light gravitino (with mass in the meV range) could help relax the neutrino mass constraint m1,max≲50 meV. The differences for the predicted total neutrino mass ∑mν among these two scenarios are within reach of next-generation cosmological probes that may measure the total neutrino mass with an uncertainty σ(∑mν)=0.014 eV. We also demonstrate that the KK tower of a very light gravitino can compensate for the graviton tower to sustain stable dS vacua and thus right-handed neutrinos can (in principle) be locked on the brane. For this scenario, Majorana neutrinos could develop dS vacua, which is not possible in the SM coupled to gravity. Finally, we investigate the effects of bulk neutrino masses in suppressing oscillations of the zero modes into the first KK modes to relax the oscillation bound on R⊥. Published by the American Physical Society 2024

A new viable mass region of dark matter and dirac neutrino mass generation in a scotogenic extension of SM

In this paper, we propose a scotogenic extension of the Standard Model (SM) which can provide a scalar dark matter (DM) candidate in the new, theoretically previously unaddressed, intermediate region ([Formula: see text][Formula: see text]GeV) and also generate light Dirac neutrino masses. In this framework, the SM is extended by three gauge singlet fermions, two singlet scalar fields and one additional scalar doublet along with a continuous global symmetry [Formula: see text] and discrete symmetries [Formula: see text] and [Formula: see text]. These additional symmetries prevent the singlet fermions from obtaining Majorana mass terms along with providing the stability to the DM candidate. It is known that in the case of the scalar singlet DM model, the only region which is not yet excluded is a narrow region close to the Higgs resonance [Formula: see text] — others ruled out from different experimental and theoretical bounds. In the case of the inert doublet model, the mass region ([Formula: see text]–80[Formula: see text]GeV) and the high-mass region (heavier than 550[Formula: see text]GeV) are allowed. This motivates us to explore a parameter range in the intermediate-mass region [Formula: see text][Formula: see text]GeV, which we do in a scotogenic extension of SM with a scalar doublet and scalar singlets. The DM in our model is a mixture of singlet and doublet scalars, in the freeze-out scenario. We constrain the allowed parameter space of the model using the Planck bound on present DM relic abundance, neutrino mass and the latest bound on spin-independent DM-nucleon scattering cross-section from the XENON1T experiment. Further, we constrain the DM parameters from the indirect detection bounds arising from the global analysis of the Fermi-LAT observations of dwarf spheroidal satellite (dSphs) galaxies, Higgs invisible decay and Electroweak Precision Test (EWPT) as well. We find that our model findings may provide a viable DM candidate satisfying all the constraints on DM parameters in the new, previously unexplored mass range ([Formula: see text][Formula: see text]GeV). This new window for the DM candidate could be searched in future experiments along with an explanation of the Dirac mass of neutrinos since so far there is no strong evidence in support of the Majorana nature of neutrino mass.

Freeze-in dark matter via a light Dirac neutrino portal

We propose a scenario where dark matter (DM) can be generated nonthermally due to the presence of a light Dirac neutrino portal between the standard model (SM) and dark sector particles. The SM is minimally extended by three right-handed neutrinos (${\ensuremath{\nu}}_{R}$), a Dirac fermion DM candidate ($\ensuremath{\psi}$) and a complex scalar ($\ensuremath{\phi}$), transforming nontrivially under an unbroken ${\mathbb{Z}}_{4}$ symmetry while being singlets under the SM gauge group. While DM and ${\ensuremath{\nu}}_{R}$ couplings are considered to be tiny in order to be in the nonthermal or freeze-in regime, $\ensuremath{\phi}$ can be produced either thermally or nonthermally depending upon the strength of its Higgs portal coupling. We consider both these possibilities and find out the resulting DM abundance via freeze-in mechanism to constrain the model parameters in the light of Planck 2018 data. Since the interactions producing DM also produce relativistic ${\ensuremath{\nu}}_{R}$, we check the enhanced contribution to the effective relativistic degrees of freedom $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}$ in view of existing bounds as well as future sensitivities. We also check the stringent constraints on free-streaming length of such freeze-in DM from structure formation requirements. Such constraints can rule out DM mass all the way up to $\mathcal{O}(100\text{ }\text{ }\mathrm{keV})$ keeping the $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}\ensuremath{\le}\mathcal{O}({10}^{\ensuremath{-}3})$ out of reach from near future experiments. Possible extensions of this minimal model can lead to observable $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}$ which can be probed at next generation experiments.

Type II Dirac seesaw with observable ΔNeff in the light of W-mass anomaly

We propose a type II seesaw model for light Dirac neutrinos to provide an explanation for the recently reported anomaly in W boson mass by the CDF collaboration with 7σ statistical significance. In the minimal model, the required enhancement in W boson mass is obtained at tree level due to the vacuum expectation value of a real scalar triplet, which also plays a role in generating light Dirac neutrino mass. Depending upon the couplings and masses of newly introduced particles, we can have thermally or non-thermally generated relativistic degrees of freedom ΔNeff in the form of right handed neutrinos which can be observed at future cosmology experiments. Extending the model to a radiative Dirac seesaw scenario can also accommodate dark matter and lepton anomalous magnetic moment.

Probing high scale Dirac leptogenesis via gravitational waves from domain walls

We propose a novel way of probing high-scale Dirac leptogenesis, a viable alternative to the canonical leptogenesis scenario where the total lepton number is conserved, keeping light standard model neutrinos purely Dirac. The simplest possible seesaw mechanism for generating light Dirac neutrinos involves heavy singlet Dirac fermions and a singlet scalar. In addition to unbroken global lepton number, a discrete ${Z}_{2}$ symmetry is imposed to forbid direct coupling between right and left chiral parts of light Dirac neutrinos. Generating light Dirac neutrino mass requires the singlet scalar to acquire a vacuum expectation value (VEV) that also breaks the ${Z}_{2}$ symmetry, leading to the formation of domain walls in the early Universe. These walls, if made unstable by introducing a soft ${Z}_{2}$-breaking term, generate gravitational waves (GWs) with a spectrum characterized by the wall tension or the singlet VEV, and the soft symmetry breaking scale. The scale of leptogenesis depends on the ${Z}_{2}$-breaking singlet VEV, which is also responsible for the tension of the domain wall, affecting the amplitude of GWs produced from the collapsing walls. We find that most of the near-future GW observatories will be able to probe Dirac leptogenesis scales all the way up to ${10}^{11}\text{ }\text{ }\mathrm{GeV}$.

Low scale Dirac leptogenesis and dark matter with observable Delta N_{mathrm{eff}}

We propose a gauged B-L extension of the standard model (SM) where light neutrinos are of Dirac type by virtue of tiny Yukawa couplings. To achieve leptogenesis, we include additional heavy Majorana fermions without introducing any B-L violation by two units. An additional scalar doublet with appropriate B-L charge can allow such heavy fermion coupling with SM leptons so that out-of-equilibrium decay of the former can lead to generation of lepton asymmetry. Due to the B-L gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings. The same B-L gauge sector parameter space can also be constrained from dark matter requirements if the latter is assumed to consist of SM singlet particles with non-zero B-L charges, which also keep the model anomaly free. The same B-L gauge interactions also lead to additional thermalised relativistic degrees of freedom Delta N_{mathrm{eff}} from light Dirac neutrinos which are tightly constrained by Planck 2018 data. While there exists parameter space from the criteria of successful leptogenesis, dark matter and Delta N_{mathrm{eff}} even after incorporating the latest collider bounds, the currently allowed parameter space can be probed by future measurements of Delta N_{mathrm{eff}}.

Effective Dirac Neutrino Mass Operator in the Standard Model With a Local Abelian Extension

We present 48 types of solutions to the anomaly cancellation conditions of local Abelian extensions of the Standard Model (SM) with right-handed SM-singlet chiral fermions. At least two of them acquire effective light Dirac neutrino masses, while the others get heavy masses from the spontaneous symmetry breaking of the local Abelian symmetry, forming a dark sector with multi-component and multi-generational fermionic dark matter. The corresponding effective Dirac neutrino mass operator can be realized at tree-level or radiatively by introducing extra scalars, and in some cases after imposing extra scotogenic conditions. The Dirac Zee model with Dirac fermionic dark matter is presented as an example of model where the neutrino and dark matter phenomenology are basically independent of each other.

Ultra-high-energy neutrino scattering in an anomalous U(1) effective field theory

A unique characteristic of exponentially growing scattering amplitude arises in an anomalous Abelian effective field theory when an extremely light Dirac neutrino mass is introduced to break the symmetry. We show that the low energy effective Lagrangian can be made explicitly gauge invariant with the help of a nonlinear representation of the Goldstone or Stueckelberg field. We study the peculiar feature of exponential growth in the ultra-high-energy neutrino-nucleon inelastic scattering. It is found that the inelastic scattering cross section is highly sensitive to the ratio of gauge coupling to the gauge boson mass, gX/mX. When the IceCube measurement of ultra-high-energy neutrinos, which is consistent with the standard model prediction up to Eν∼6 PeV, is taken into account, the inferred constraint on gX/mX is more severe than that obtained from the events of mono-lepton+missing transverse energy at the LHC. A muon collider with a collision energy of 10 TeV can be a good environment other than hadron colliders to probe the novel effect.

Naturally light Dirac neutrinos from SU(6)

A known mechanism for obtaining naturally light Dirac neutrinos is implemented in the context of SU(6) → SU(5) × U(1)N.

Naturally light Dirac neutrinos from SO(10)×U(1)ψ

A new solution is presented where the right-handed neutrino νR in SO(10) pairs up with νL to form a naturally light Dirac neutrino. It is based on the framework of E6→SO(10)×U(1)ψ, then SO(10)→SU(5)×U(1)χ.

Large star/rose extra dimension with small leaves/petals

In this paper, we propose to compactify a single Large Extra Dimension (LED) on a star/rose graph with a large number of identical leaves/petals. The 5D Planck scale can be chosen to be [Formula: see text] TeV which can provide a path to solve the gauge hierarchy problem. The leaf/petal length scale is of [Formula: see text], where [Formula: see text] GeV is the weak scale, without the large geometrical hierarchy of the traditional LED models to stabilize. The 4D fields of the SM are localized on a 3-brane at the central vertex of the star/rose graph. We predict a tower of feebly coupled weak scale Kaluza–Klein (KK) gravitons below a regime of strongly coupled gravitational phenomena above the TeV scale. Moreover, we reformulate in our setup the LED mechanism to generate light Dirac neutrinos, where the right-handed neutrinos are KK-modes of gauge singlet fermions propagating in the bulk. A large number of KK-gravitons and KK-neutrinos interact only gravitationally and thus constitute a hidden sector.

Connecting light dirac neutrinos to a multi-component dark matter scenario in gauged B-L model

We propose a new gauged B-L extension of the standard model where light neutrinos are of Dirac type, naturally acquiring sub-eV mass after electroweak symmetry breaking, without any additional global symmetries. This is realised by choosing a different B-L charge for right handed neutrinos than the usual -1 so that the Dirac Yukawa coupling involves an additional neutrinophilic scalar doublet instead of the usual Higgs doublet. The model can be made anomaly free by considering four additional chiral fermions which give rise to two massive Dirac fermions by appropriate choice of singlet scalars. The choice of scalars not only helps in achieving the desired particle mass spectra via spontaneous symmetry breaking, but also leaves a remnant Z_2 times Z'_2 symmetry to stabilise the two dark matter candidates. Apart from this interesting link between Dirac nature of light neutrinos and multi-component dark matter sector, we also find that the dark matter parameter space is constrained mostly by the cosmological upper limit on effective relativistic degrees of freedom Delta N_{mathrm{eff}} which gets enhanced in this model due to the thermalisation of the light right handed neutrinos by virtue of their sizeable B-L gauge interactions.

Mirror Dirac leptogenesis

We consider a mirror world scenario, in which light Dirac neutrinos are generated from a seesaw mechanism and leptogenesis occurs at high scale without violating lepton number. After leptogenesis, the conservation laws of the theory imply the visible baryon-minus-lepton asymmetry to be equal to the mirror baryon-minus-lepton asymmetry. We extend previous work by presenting a detailed study of this Dirac leptogenesis mechanism by constructing the full set of Boltzmann Equations (BEs) for both cases of unflavored and flavored regimes. We show that Z2 breaking and lepton/mirror lepton flavor effects can be exploited to enhance the final baryon-minus-lepton asymmetry in our world by several orders of magnitude.

Dirac neutrino mass generation from a Majorana messenger

The radiative type-I seesaw has been already implemented to explain the lightness of Majorana neutrinos with both Majorana and Dirac heavy fermions, and the lightness of Dirac neutrinos with Dirac heavy fermions. In this work we present a minimal implementation of the radiative type-I seesaw with light Dirac neutrinos and heavy Majorana fermions. An inert doublet and a complex singlet scalar complete the dark sector which is protected by an Abelian fermiophobic gauge symmetry that also forbids tree level mass contributions for the full set of light neutrinos. A fermion vector-like extension of the model is also proposed where the light right-handed neutrinos can thermalize in the primordial plasma and the extra gauge boson can be directly produced at colliders. In particular, the current upper bound on $\Delta N_{\text{eff}}$ reported by PLANCK points to large ratios $M_{Z'}/g'\gtrsim 40\ \text{TeV}$ which can be competitive with collider constraint for $g'$ sufficiently large in the ballpark of the Standard Model values, while future cosmic microwave background experiments may probe all the no minimal models presented here.

Comment on the article by D. Borah and B. Karmakar “Linear seesaw for Dirac neutrinos with A4 flavour symmetry”, Phys. Lett. B 789 (2019) 59–70, arXiv:1806.10685

D. Borah and B. Karmakar [1] have proposed an A4 flavoured linear seesaw model to realise light Dirac neutrinos. In this comment article, we show that some neutrino Yukawa interactions were missed in the model Ref. [1], thus implying that a different formula would be needed to determine the effective neutrino mass matrix, with significantly different results. Our result shows that, unlike stated in Ref. [1], that the inverted neutrino mass spectrum is not ruled out.

Linear seesaw for Dirac neutrinos with A4 flavour symmetry

We propose a linear seesaw model to realise light Dirac neutrinos within the framework of A4 discrete flavour symmetry. The additional fields and their transformations under the flavour symmetries are chosen in such a way that naturally predicts the hierarchies of different elements of the seesaw mass matrix and also keeps the unwanted terms away. For generic choices of flavon alignments, the model predicts normal hierarchical light neutrino masses with the atmospheric mixing angle in the lower octant. Apart from predicting interesting correlations between different neutrino parameters as well as between neutrino and model parameters, the model also predicts the leptonic Dirac CP phase to lie in a specific range −π/2≲δ≲−π/5 and π/5≲δ≲π/2 that includes the currently preferred maximal value. The predictions for the absolute neutrino masses in one specific version of the model can also saturate the cosmological upper bound on sum of absolute neutrino masses.