Search

Abstract 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 ∆N eff 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 ∆N eff. While we stick to thermal singlet-doublet DM with promising detection prospects, both thermal and non-thermal origin of ∆N eff 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.

Abstract We propose a minimal setup that realises dynamical inflection point inflation, and, using the same field content, generates neutrino masses, a baryon asymmetry of the universe, and dark matter. A dark SU(2) D gauge sector with a dark scalar doublet playing the role of inflaton is considered along with several doublet and singlet fermions sufficient to realise multiple inflection points in the inflaton potential. The singlet fermions couple to SM leptons and generate neutrino masses via the inverse seesaw mechanism. Those fermions also decay asymmetrically and out of equilibrium, generating a baryon asymmetry via leptogenesis. Some of the fermion doublets are dark matter, and they are produced via inflaton decay and freeze-in annihilation of the same fermions that generate the lepton asymmetry. Reheating, leptogenesis, and dark matter are all at the TeV scale.

Abstract 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 $$\Delta {\textrm{N}}_{\textrm{eff}}$$ Δ N eff at cosmic microwave background (CMB) experiments. The loop suppression and additional free parameters involved in neutrino mass generation allow large $$(\sim {\mathcal {O}}(1))$$ ( ∼ O ( 1 ) ) 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 $$\Delta {\textrm{N}}_{\textrm{eff}}.$$ Δ N eff . 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 $$\Delta {\textrm{N}}_{\textrm{eff}}.$$ Δ N eff . 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.

We propose a scenario where a high scale seesaw origin of light neutrino mass and gravitational dark matter (DM) in MeV-TeV ballpark originating from primordial black hole (PBH) evaporation can be simultaneously probed by future observations of stochastic gravitational wave (GW) background with multiple tilts or spectral breaks. A high scale breaking of an Abelian gauge symmetry ensures the dynamical origin of the seesaw scale while also leading to the formation of cosmic strings responsible for generating stochastic GW background. The requirement of a correct DM relic in this ballpark necessitates the inclusion of a diluter as PBH typically leads to DM overproduction. This leads to a second early matter dominated epoch after PBH evaporation due to the long-lived diluter. These two early matter dominated epochs, crucially connected to the DM relic, lead to multiple spectral breaks in the otherwise scale-invariant GW spectrum formed by cosmic strings. We find interesting correlations between DM mass and turning point frequencies of GW spectrum which are within reach of several near future experiments like LISA, BBO, ET, CE, etc.

Abstract We propose a mechanism for cogenesis of baryon and dark matter (DM) in the universe via the Affleck-Dine (AD) route. An AD field which breaks the lepton number symmetry, leads to the generation of lepton asymmetry by virtue of its cosmic evolution, which then gets transferred into lepton and dark sectors. While the lepton asymmetry gets converted into baryon asymmetry via sphalerons, the dark sector asymmetry leads to the final DM abundance with the symmetric part being annihilated away due to resonantly enhanced annihilation, which we choose to be provided by a gauged B − L portal. Stringent constraints from DM direct detection forces DM and B − L gauge boson masses to be light, in the few GeV ballpark. While a large portion of the model parameter space is already ruled out, the remaining parameter space is within sensitivity of laboratory as well as cosmology based experiments. The AD field also plays the role of inflaton with the required dynamics by virtue of its non-minimal coupling to gravity, consistent with observations.

Abstract The Standard Model, extended with three right-handed (RH) neutrinos, is the simplest model that can explain light neutrino masses, the baryon asymmetry of the Universe, and dark matter (DM). Models in which RH neutrinos are light are generally easier to test in experiments. In this work, we show that, even if the RH neutrinos are super-heavy (M i=1,2,3 > 109 GeV)—close to the Grand Unification scale—the model can be tested thanks to its distinct features on the stochastic Gravitational Wave (GW) background. We consider an early Universe filled with ultralight primordial black holes (PBH) that produce a super-heavy RH neutrino DM via Hawking radiation. The other pair of RH neutrinos generates the baryon asymmetry via thermal leptogenesis, much before the PBHs evaporate. GW interferometers can test this novel spectrum of masses thanks to the GWs induced by the PBH density fluctuations. In a more refined version, wherein a U(1) gauge symmetry breaking dynamically generates the seesaw scale, the PBHs also cause observable spectral distortions on the GWs from the U(1)-breaking cosmic strings. Thence, a low-frequency GW feature related to DM genesis and detectable with a pulsar-timing array must correspond to a mid- or high-frequency GW signature related to baryogenesis at interferometer scales.

Abstract We study a scenario where both dark matter and heavy right handed neutrino (RHN) responsible for leptogenesis acquire masses by crossing the relativistic bubble walls formed as a result of a TeV scale supercooled first order phase transition (FOPT). While this leads to a large out-of-equilibrium abundance of right handed neutrino inside the bubble sufficient to produce the required lepton asymmetry, the dark matter being lighter can still remain in equilibrium with its relic being set by subsequent thermal freeze-out. A classical conformal symmetry ensures the origin of mass via FOPT induced by a singlet scalar while also ensuring supercooling leading to enhanced gravitational wave amplitude within the sensitivity of the LISA experiment. A minimal scenario with three RHN, one inert scalar doublet and one singlet scalar as additional fields beyond the standard model is sufficient to realize this possibility which also favours inert RHN dark matter over inert scalar doublet.

The dark sector is postulated to be invariant under an SU(2) gauge symmetry, spontaneously broken by a Higgs quadruplet to a conserved residual Z3×Z2 symmetry. The resulting dark matter phenomenology is studied.

Abstract We propose a gauged $$B-L$$ 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 $$B-L$$ B - L violation by two units. An additional scalar doublet with appropriate $$B-L$$ 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 $$B-L$$ B - L gauge interactions of the decaying fermion, the criteria of successful Dirac leptogenesis can also constrain the gauge sector couplings. The same $$B-L$$ 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 $$B-L$$ B - L charges, which also keep the model anomaly free. The same $$B-L$$ B - L gauge interactions also lead to additional thermalised relativistic degrees of freedom $$\Delta N_{\mathrm{eff}}$$ Δ N 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 $$\Delta N_{\mathrm{eff}}$$ Δ N eff even after incorporating the latest collider bounds, the currently allowed parameter space can be probed by future measurements of $$\Delta N_{\mathrm{eff}}$$ Δ N eff .

Abstract We study the effects of gravitational lensing on neutrino oscillations in the $$\gamma $$ γ -spacetime which describes a static, axially-symmetric and asymptotically flat solution of the Einstein’s field equations in vacuum. Using the quantum-mechanical treatment for relativistic neutrinos, we calculate the phase of neutrino oscillations in this spacetime by considering both radial and non-radial propagation. We show the dependence of the oscillation probability on the absolute neutrino masses, which in the two-flavour case also depends upon the sign of mass squared difference, in sharp contrast with the well-known results of vacuum oscillation in flat spacetime. We also show the effects of the deformation parameter $$\gamma $$ γ on neutrino oscillations and reproduce previously known results for the Schwarzschild metric. We then extend these to a more realistic three flavours neutrino scenario and study the effects of the parameter $$\gamma $$ γ and the lightest neutrino mass while using best fit values of neutrino oscillation parameters.

Dark matter is postulated to transform under the non-Abelian SU(2)D gauge symmetry. Its connection to Dirac neutrino masses is explored.

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.

Motivated by the recently reported anomaly in W boson mass by the CDF collaboration with 7σ statistical significance, we consider a singlet-doublet (SD) Majorana fermion dark matter (DM) model where the required correction to W boson mass arises from radiative corrections induced by SD fermions. While a single generation of SD fermions, odd under an unbroken Z2 symmetry, can not explain the W boson mass anomaly while being consistent with DM phenomenology, two generations of SD fermions can do so with the heavier generation playing the dominant role in W-mass correction and lighter generation playing the role in DM phenomenology. Additionally, such multiple generations of SD fermions can also generate light neutrino masses radiatively if a Z2-odd singlet scalar is included.

Abstract We study the possibility of realising cosmic inflation, dark matter (DM), baryon asymmetry of the universe (BAU) and light neutrino masses in non-supersymmetric minimal gauged $$B-L$$ B - L extension of the standard model with three right handed neutrinos. The singlet scalar field responsible for spontaneous breaking of $$B-L$$ B - L gauge symmetry also plays the role of inflaton by virtue of its non-minimal coupling to gravity. While the lightest right handed neutrino is the DM candidate, being stabilised by an additional $$Z_2$$ Z 2 symmetry, we show by performing a detailed renormalisation group evolution (RGE) improved study of inflationary dynamics that thermal DM is generally overproduced due to insufficient annihilations through gauge and scalar portals. This happens due to strict upper limits obtained on gauge and other dimensionless couplings responsible for DM annihilation while assuming the non-minimal coupling to gravity to be at most of order unity. The non-thermal DM scenario is viable, with or without $$Z_2$$ Z 2 symmetry, although in such a case the $$B-L$$ B - L gauge sector remains decoupled from the inflationary dynamics due to tiny couplings. We also show that the reheat temperature predicted by the model prefers non-thermal leptogenesis with hierarchical right handed neutrinos while being consistent with other requirements.

We present a self-interacting boosted dark matter (DM) scenario as a possible explanation of the recently reported excess of electron recoil events by the XENON1T experiment. The Standard Model (SM) has been extended with two vector-like fermion singlets charged under a dark U(1)D gauge symmetry to describe the dark sector. While the presence of light vector boson mediator leads to sufficient DM self-interactions to address the small scale issues of cold dark matter, the model with sub-GeV scale DM can explain the XENON1T excess via elastic scattering of boosted DM component with electrons at the detector. Strong annihilation of DM into the light mediator leads to a suppressed thermal relic. A hybrid setup of dark freeze-out and non-thermal contribution from the late decay of a scalar can lead to correct relic abundance. We fit our model with XENON1T data and also find the final parameter space consistent with self-interaction of DM, DM-electron scattering rate, as well as astrophysical and cosmological observations. A tiny parameter space consistent with all these constraints and requirements can be further scrutinized in near-future experiments.

Abstract We study a generic leptophilic U(1) X extension of the standard model with a light gauge boson. The U(1) X charge assignments for the leptons are guided by lepton universality violating (LUV) observables in semileptonic b → sℓℓ decays, muon anomalous magnetic moment and the origin of leptonic masses and mixing. Anomaly cancellation conditions require the addition of new chiral fermions in the model, one of which acts as a dark matter (DM) candidate when it is stabilised by an additional Z 2 $$ {\mathcal{Z}}_2 $$ symmetry. From our analysis, we show two different possible models with similar particle content that lead to quite contrasting neutrino mass origin and other phenomenology. The proposed models also have the potential to address the anomalous results in b → cℓν ℓ decays like R(D), R(D ∗), electron anomalous magnetic moment and the very recent KOTO anomaly in the kaon sector. We also discuss different possible collider signatures of our models which can be tested in future.

Abstract We propose a new gauged $$B-L$$ 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 $$B-L$$ B-L charge for right handed neutrinos than the usual $$-1$$ -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 $$Z_2 \times Z'_2$$ Z2×Z2′ 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 $$\Delta N_{\mathrm{eff}}$$ ΔNeff which gets enhanced in this model due to the thermalisation of the light right handed neutrinos by virtue of their sizeable $$B-L$$ B-L gauge interactions.

Abstract We propose a novel scenario of generating lepton asymmetry via annihilation and coannihilation of dark sector particles including t-channel processes. In order to realistically implement this idea, we consider the scotogenic model having three right handed neutrinos and a new scalar doublet, all of which are odd under an in-built $$Z_2$$ Z2 symmetry. The lightest $$Z_2$$ Z2 odd particle, if electromagnetically neutral, can be a dark matter candidate while annihilation and coannihilation between different $$Z_2$$ Z2 odd particles into standard model leptons serve as the source of lepton asymmetry. The light neutrino masses arise at one loop level with $$Z_2$$ Z2 odd fields going inside the loop. We show that experimental data related to light neutrinos, dark matter relic abundance and baryon asymmetry can be simultaneously satisfied in the model for two different cases: one with fermion dark matter and the other with scalar dark matter. In both scenarios, t-channel annihilation as well as coannihilation of $$Z_2$$ Z2 odd particles play a non-trivial role in producing the non-zero CP asymmetry. Both the scenarios remain allowed from DM direct detection while keeping the scale of leptogenesis as low as TeV or less, lower than the one for vanilla leptogenesis scenario in scotogenic model along with the additional advantage of explaining the baryon-dark matter coincidence to some extent. Due to such low scale, the model is testable through rare decay experiments looking for charged lepton flavour violation.

Motivated by the growing evidence for lepton flavour universality violation after the first results from Fermilab's muon (g−2) measurement, we revisit one of the most widely studied anomaly free extensions of the standard model namely, gauged Lμ−Lτ model, known to be providing a natural explanation for muon (g−2). We also incorporate the presence of dark matter (DM) in this model in order to explain the recently reported electron recoil excess by the XENON1T collaboration. We show that the same neutral gauge boson responsible for generating the required muon (g−2) can also mediate interactions between electron and dark fermions boosted by dark matter annihilation. The required DM annihilation rate into dark fermion require a hybrid setup of thermal and non-thermal mechanisms to generate DM relic density. The tightly constrained parameter space from all requirements remains sensitive to ongoing and near future experiments, keeping the scenario very predictive.

Motivated by the recently reported excess of electron recoil events by the XENON1T experiment, we propose low scale seesaw scenarios for light neutrino masses within U(1)X gauge extension of the standard model that also predicts stable as well as long lived dark sector particles. The new fields necessary for seesaw realisation as well as dark matter are charged under the U(1)X gauge symmetry in an anomaly free way. A singlet scalar field which effectively gives rise to lepton number violation and hence Majorana light neutrino masses either at tree or radiative level, also splits the dark matter field into two quasi-degenerate states. While sub-eV neutrino mass and non-zero dark matter mass splitting are related in this way, the phenomenology of sub-GeV scale inelastic dark matter can be very rich if the mass splitting is of keV scale. We show that for suitable parameter space, both the components with keV splitting can contribute to total dark matter density of the present universe, while opening up the possibility of the heavier dark matter candidate to undergo down-scattering with electrons. We check the parameter space of the model for both fermion and scalar inelastic dark matter candidates which can give rise to the XENON1T excess while being consistent with other phenomenological bounds. We also discuss the general scenario where mass splitting Δm between the two dark matter components can be larger, effectively giving rise to a single component dark matter scenario.

Abstract We propose a B − L gauged extension of the Standard Model where light neutrino masses arise from type III seesaw mechanism. Unlike the minimal B − L model with three right handed neutrinos having unit lepton number each, the model with three fermion triplets is however not anomaly free. We show that the leftover triangle anomalies can be cancelled by two neutral Dirac fermions having fractional B − L charges, both of which are naturally stable by virtue of a remnant ℤ2 × ℤ 2 ′ $$ {\mathbb{Z}}_2^{\prime } $$ symmetry, naturally leading to a two component dark matter scenario without any ad-hoc symmetries. We constrain the model from all relevant phenomenological constraints including dark matter properties. Light neutrino mass and collider prospects are also discussed briefly. Due to additional neutral gauge bosons, the fermion triplets in type III seesaw can have enhanced production cross section in collider experiment.

Abstract We propose an Abelian gauged version of the singlet-doublet fermionic dark matter (DM) model where the DM, combination of a vector like fermion doublet and a fermion singlet, is naturally stabilised by the gauge symmetry without requiring any ad-hoc discrete symmetries. In order to have an enlarged parameter space for the DM, accsessible at collider experiments like the Large Hadron Collider (LHC), we consider the additional gauge symmetry to be based on the quantum B − 3L τ . The restriction to third generation of leptons is chosen in order to have weaker bounds from the LHC on the corresponding gauge boson. The triangle anomalies arising in this model can be cancelled by the inclusion of a right handed neutrino which also takes part in generating light neutrino masses through type I seesaw mechanism. The model thus offers a potential thermal DM candidate, interesting collider signatures and correct neutrino mass along with a stable electroweak vacuum and perturbative couplings all the way up to the Planck scale. We constrain our model parameters from these requirements as well as existing relevant constraints related to DM and colliders.

Abstract We consider the possibility of texture zeros in lepton mass matrices of the minimal left-right symmetric model (LRSM) where light neutrino mass arises from a combination of type I and type II seesaw mechanisms. Based on the allowed texture zeros in light neutrino mass matrix from neutrino and cosmology data, we make a list of all possible allowed and disallowed texture zeros in Dirac and heavy neutrino mass matrices which appear in type I and type II seesaw terms of LRSM. For the numerical analysis we consider those cases with maximum possible texture zeros in light neutrino mass matrix M ν , Dirac neutrino mass matrix M D , heavy neutrino mass matrix M RR while keeping the determinant of M RR non-vanishing, in order to use the standard type I seesaw formula. The possibility of maximum zeros reduces the free parameters of the model making it more predictive. We then compute the new physics contributions to rare decay processes like neutrinoless double beta decay, charged lepton flavour violation. We find that even for a conservative lower limit on left-right symmetry scale corresponding to heavy charged gauge boson mass 4.5 TeV, in agreement with collider bounds, for right-handed neutrino masses above 1 GeV, the new physics contributions to these rare decay processes can saturate the corresponding experimental bound.

Abstract Motivated by the recent resurrection of the evidence for an eV scale sterile neutrino from the MiniBooNE experiment, we revisit one of the most minimal seesaw model known as the minimal extended seesaw that gives rise to a $$3+1$$ 3+1 light neutrino mass matrix. We consider the presence of $$A_{4}$$ A4 flavour symmetry which plays a non-trivial role in generating the structure of the neutrino mass matrix. Considering a diagonal charged lepton mass matrix and generic vacuum alignments of $$A_{4}$$ A4 triplet flavons, we classify the resulting mass matrices based on their textures. Keeping aside the disallowed texture zeros based on earlier studies of $$3+1$$ 3+1 neutrino textures, we categorise the remaining ones based on texture zeros, $$\mu $$ μ –$$\tau $$ τ symmetry in the $$3\times 3$$ 3×3 block and hybrid textures. After pointing out the origin of such $$3+1$$ 3+1 neutrino textures to $$A_{4}$$ A4 vacuum alignments, we use the latest $$3+1$$ 3+1 neutrino oscillation data and numerically analyse the texture zeros and $$\mu $$ μ –$$\tau $$ τ symmetric cases. We find that a few of them are allowed from each category predicting interesting correlations between neutrino parameters. We also find that all of these allowed cases prefer normal hierarchical pattern of light neutrino masses over inverted hierarchy.

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.

Motivated by the recently reported excess in electron recoil events by the XENON1T collaboration, we propose an inelastic fermion dark matter (DM) scenario within the framework of a gauged Lμ−Lτ extension of the standard model which can also accommodate tiny neutrino masses as well as anomalous muon magnetic moment (g−2)μ. A Dirac fermion DM, naturally stabilised due to its chosen gauge charge, is split into two pseudo-Dirac mass eigenstates due to Majorana mass term induced by singlet scalar which also takes part in generating right handed neutrino masses responsible for type I seesaw origin of light neutrino masses. The inelastic down scattering of heavier DM component can give rise to the XENON1T excess for keV scale mass splitting with lighter DM component. We fit our model with XENON1T data and also find the final parameter space by using bounds from (g−2)μ, DM relic, lifetime of heavier DM, inelastic DM-electron scattering rate, neutrino trident production rate as well as other flavour physics, astrophysical and cosmological observations. A tiny parameter space consistent with all these bounds and requirements will face further scrutiny in near future experiments operating at different frontiers.

We study the possibility of right-handed neutrino dark matter (DM) in gauged U(1)B−L×Z2 extension of the standard model augmented by an additional scalar doublet, being odd under the Z2 symmetry, to give rise the scotogenic scenario of radiative neutrino masses. Due to lepton portal interactions, the right-handed neutrino DM can have additional co-annihilation channels apart from the usual annihilations through ZB−L which give rise to much more allowed mass of DM from relic abundance criteria, even away from the resonance region like MDM≈MZB−L/2. This enlarged parameter space is found to be consistent with neutrino mass constraints while being sensitive to direct detection experiments of DM as well as rare decay experiments looking for charged lepton flavour violating decays like μ→eγ. Due to the possibility of the Z2 odd scalar doublet being the next to lightest stable particle that can be sufficiently produced in colliders by virtue of its gauge interactions, one can have interesting signatures like displaced vertex or disappearing charged tracks provided that the mass splitting δM between DM and the next to lightest stable particle (NLSP) is small. In particular, if δM

We study the possibility of keV neutrino dark matter in the minimal U(1)B−L gauge extension of the standard model where three right handed neutrinos are automatically included due to the requirement of anomaly cancellations. Without considering extra additional particles or symmetries, we consider the lightest right handed neutrino to be in the keV mass range which is kinematically long lived. Due to gauge interactions, such a keV neutrino can be thermally produced in the early Universe followed by decoupling while being relativistic. The final relic abundance of such keV neutrino typically overclose the Universe requiring additional mechanism to bring it under observed limits. We propose a non-standard cosmological history where a scalar field ϕ, that redshifts faster than radiation dominates the Universe prior to the radiation dominated era. We show that such a non-standard phase can keep the abundance of thermally generated keV neutrino dark matter within observed relic abundance. We constrain the non-standard phase, U(1)B−L parameters from these requirements and also briefly comment upon the observational aspects of such keV neutrino dark matter.

We propose two different seesaw models namely, type I and inverse seesaw 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 matrices in these two types of seesaw mechanisms. For generic choices of flavon alignments, both the models predict 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/3 to π/3. While the type I seesaw model predicts smaller values of absolute neutrino mass, the inverse seesaw predictions for the absolute neutrino masses can saturate the cosmological upper bound on sum of absolute neutrino masses for certain choices of model parameters.

Abstract We consider a simple extension of the minimal left-right symmetric model (LRSM) in order to explain the PeV neutrino events seen at the IceCube experiment from a heavy decaying dark matter. The dark matter sector is composed of two fermions: one at PeV scale and the other at TeV scale such that the heavier one can decay into the lighter one and two neutrinos. The gauge annihilation cross sections of PeV dark matter are not large enough to generate its relic abundance within the observed limit. We include a pair of real scalar triplets Ω L,R which can bring the thermally overproduced PeV dark matter abundance into the observed range through late time decay and consequent entropy release thereby providing a consistent way to obtain the correct relic abundance without violating the unitarity bound on dark matter mass. Another scalar field, a bitriplet under left-right gauge group is added to assist the heavier dark matter decay. The presence of an approximate global U(1) X symmetry can naturally explain the origin of tiny couplings required for long-lived nature of these decaying particles. We also show, how such an extended LRSM can be incorporated within a non-supersymmetric SO(10) model where the gauge coupling unification at a very high scale naturally accommodate a PeV scale intermediate symmetry, required to explain the PeV events at IceCube.

We consider a class of left-right symmetric model with enlarged gauge group SU(3)c×SU(3)L×SU(3)R×U(1)X without having scalar bitriplet. In the absence of scalar bitriplet, there is no Dirac mass term for fermions including usual quarks and leptons. We introduce new isosinglet vector-like fermions so that all the fermions get their masses through a universal seesaw mechanism. We extend our discussion to neutrino mass and its implications in neutrinoless double beta decay (0νββ). We show that for TeV scale SU(3)R gauge bosons, the heavy-light neutrino mixing contributes dominantly to 0νββ that can be observed at ongoing experiments. The new physics contributions arising from purely left-handed currents via exchange of keV scale right-handed neutrinos and the so called mixed helicity λ-diagram can saturate the KamLANDZen bound. We show that the right handed neutrinos in this model can have mass in the sub keV range and can be long lived compared to the age of the Universe. The contributions of these right handed neutrinos to flavour physics observables like μ→eγ and muon g−2 is also discussed. Towards the end we also comment on different possible symmetry breaking patterns of this enlarged gauge symmetry to that of the standard model.

We study an extension of the Inert Higgs Doublet Model (IHDM) by three copies of right handed neutrinos and heavy charged leptons such that both the inert Higgs doublet and the heavy fermions are odd under the Z2 symmetry of the model. The neutrino masses are generated at one loop in the scotogenic fashion. Assuming the neutral scalar of the inert Higgs to be the dark matter candidate, we particularly look into the region of parameter space where dark matter relic abundance is primarily governed by the inert Higgs coupling with the leptons. This corresponds to tiny Higgs portal coupling of dark matter as well as large mass splitting within different components of the inert Higgs doublet suppressing the coannihilations. Such lepton portal couplings can still produce the correct relic abundance even if the Higgs portal couplings are arbitrarily small. Such tiny Higgs portal couplings may be responsible for suppressed dark matter nucleon cross section as well as tiny invisible branching ratio of the standard model Higgs, to be probed at ongoing and future experiments. We also briefly discuss the collider implications of such a scenario.

An anomaly-free U(1) gauge extension of the standard model (SM) is presented. Only one Higgs doublet with a nonzero vacuum expectation is required as in the SM. New fermions and scalars as well as all SM particles transform nontrivially under this U(1), resulting in a model of three active neutrinos and one sterile neutrino, all acquiring radiative masses. Charged-lepton masses are also radiative as well as the mixing between active and sterile neutrinos. At the same time, a residual Z2 symmetry of the U(1) gauge symmetry remains exact, allowing for the existence of dark matter.

We study a very specific type of neutrino mass and mixing structure based on the idea of Strong Scaling Ansatz (SSA) where the ratios of neutrino mass matrix elements belonging to two different columns are equal. There are three such possibilities, all of which are disfavored by the latest neutrino oscillation data. We focus on the specific scenario which predicts vanishing reactor mixing angle θ13 and inverted hierarchy with vanishing lightest neutrino mass. Motivated by several recent attempts to explain non-zero θ13 by incorporating corrections to a leading order neutrino mass or mixing matrix giving θ13=0, here we study the origin of non-zero θ13 as well as leptonic Dirac CP phase δCP by incorporating two different corrections to scaling neutrino mass and mixing: one, where type II seesaw acts as a correction to scaling neutrino mass matrix and the other, with charged lepton correction to scaling neutrino mixing. Although scaling neutrino mass matrix originating from type I seesaw predicts inverted hierarchy, the total neutrino mass matrix after type II seesaw correction can give rise to either normal or inverted hierarchy. However, charged lepton corrections do not disturb the inverted hierarchy prediction of scaling neutrino mass matrix. We further discriminate between neutrino hierarchies, different choices of lightest neutrino mass and Dirac CP phase by calculating baryon asymmetry and comparing with the observations made by the Planck experiment.

The Fermi gamma ray space telescope data have pointed towards an excess of gamma rays with a peak around 1–3 GeV in the region surrounding the galactic center. This anomalous excess can be described well by a dark matter candidate having mass in the range 31–40 GeV annihilating into bb¯ pairs with a cross section of 〈σv〉≃(1.4–2.0)×10−26 cm3/s. In this work we explore the possibility of having such a dark matter candidate within the framework of a radiative neutrino mass model. The model is a simple extension of the standard model by an additional U(1)X gauge symmetry where the standard model neutrino masses arise both at tree level as well as radiatively by the anomaly free addition of one singlet fermion NR and two triplet fermions Σ1R, Σ2R with suitable Higgs scalars. The spontaneous gauge symmetry breaking is achieved in such a way which results in a residual Z2 symmetry hence providing a stable cold dark matter candidate. We show that the singlet fermionic dark matter candidate in our model can give rise to the galactic center gamma ray excess. The parameter space which simultaneously satisfies the constraints on relic density, direct detection scattering as well as collider bounds essentially corresponds to an s-wave resonance where the gauge boson mass mX is approximately twice that of dark matter mass mχ. We also discuss the compatibility of such a light fermion singlet dark matter with light neutrino mass.

We study the possibility of generating tiny neutrino mass through a combination of type I and type II seesaw mechanism within the framework of an abelian extension of standard model. The model also provides a naturally stable dark matter candidate in terms of the lightest neutral component of a scalar doublet. We compute the relic abundance of such a dark matter candidate and also point out how the strength of type II seesaw term can affect the relic abundance of dark matter. Such a model which connects neutrino mass and dark matter abundance has the potential of being verified or ruled out in the ongoing neutrino, dark matter, as well as accelerator experiments.