Left right symmetric models (LRSM) are extensions of the standard model by an enlarged gauge grou... more Left right symmetric models (LRSM) are extensions of the standard model by an enlarged gauge group SU (2) L × SU (2) R × U (1) B−L where automatic inclusion of right handed fermions as SU (2) R doublets guarantees a natural seesaw origin of neutrino masses. Apart from the extended gauge symmetry, LRSM also has an in-built global discrete symmetry, called D-parity which ensures equal gauge couplings for left and right sectors. Motivated by the fact that global symmetries are expected to be explicitly broken by theories of quantum gravity, here we study the effects of such gravity or Planck scale physics on neutrino masses and mixings by introducing explict Dparity breaking Planck scale suppressed higher dimensional operators. Although such Planck scale suppressd operators have dimension at least six in generic LRSM, dimension five operators can also arise in the presence of additional scalar fields which can be naturally accommodated within SO(10) grand unified theory (GUT) multiplets. We show that, such corrections can give rise to significant changes in the predictions for neutrino mixing parameters from the ones predicted by tree level seesaw formula if the left right symmetry breaking scale is lower than 10 14 GeV.
We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of... more We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of XMM-Newton telescope data and the galactic center gamma ray excess observed by the Fermi gamma ray space telescope within an abelian gauge extension of standard model. We consider a two component dark matter scenario with tree level mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant $Z_2$ symmetry into which the abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within $31-40$ GeV, then this model can also explain the galactic center gamma ray excess if the dark matter annihilation into $b\bar{b}$ pairs has a cross section of $\langle \sigma v \rangle \simeq (1.4-2.0) \times 10^{-26} \; \text{cm}^3/\text{s}$. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV X-ray line flux and galactic center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and gauge coupling. We also briefly discuss how this model can give rise to sub-eV neutrino masses at tree level as well as one-loop level while keeping the dark matter mass at few tens of GeV. We also constrain the model parameters from the requirement of keeping the one-loop mass difference between two dark matter particles below a keV. We find that the constraints from light neutrino mass and keV mass splitting between two dark matter components show more preference for opposite $CP$ eigenvalues of the two fermion singlet dark matter candidates in the model
We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of... more We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of XMM-Newton telescope data and the galactic center gamma ray excess observed by the Fermi gamma ray space telescope within an abelian gauge extension of standard model. We consider a two component dark matter scenario with a mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant Z 2 symmetry into which the abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within 31 − 40 GeV, then this model can also explain the galactic center gamma ray excess if the dark matter annihilation into bb pairs has a cross section of σv (1.4 − 2.0) × 10 −26 cm 3 /s. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV X-ray line flux and galactic center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and coupling. We also briefly discuss how this model can give rise to sub-eV neutrino masses at tree level as well as one loop level while keeping the dark matter mass at few tens of GeV. We also show the natural origin of keV mass splitting between two electroweak scale dark matter particles at one loop level in this model.
We discuss the possible origin of non-zero reactor mixing angle θ 13 and Dirac CP phase δ CP in t... more We discuss the possible origin of non-zero reactor mixing angle θ 13 and Dirac CP phase δ CP in the leptonic sector from a combination of type I and type II seesaw mechanisms. Type I seesaw contribution to neutrino mass matrix is of tri-bimaximal (TBM) type which gives rise to vanishing θ 13 leaving the Dirac CP phase undetermined. If the Dirac neutrino mass matrix is assumed to take the diagonal charged lepton type structure, such a TBM type neutrino mass matrix originating from type I seesaw corresponds to real values of Dirac Yukawa couplings in the terms Y ijLi HN j .
We study the possibility of connecting leptonic Dirac CP phase δ, lightest neutrino mass and bary... more We study the possibility of connecting leptonic Dirac CP phase δ, lightest neutrino mass and baryon asymmetry of the Universe within the framework of a model where both type I and type II seesaw mechanisms contribute to neutrino mass. Type I seesaw gives rise to Tri-Bimaximal (TBM) type neutrino mixing whereas type II seesaw acts as a correction in order to generate non-zero θ 13 .
The nondetection of neutrinos coming from Gamma Ray Bursts (GRBs) by the IceCube experiment has r... more The nondetection of neutrinos coming from Gamma Ray Bursts (GRBs) by the IceCube experiment has raised serious questions on our understanding of GRB's and the mechanism of neutrino flux production in them. Motivated by this and the need for a precise calculation for GRB neutrino flux, here we study the effects of beyond standard model physics on the GRB neutrino flux. In the internal shock model of GRB, high energy neutrinos are expected from muon, pion and kaon decays. Using the latest best fit neutrino oscillation parameters, we compute the expected flux on earth for standard as well as non-standard oscillation scenarios. Among the non-standard scenarios, we consider neutrino decay, pseudo-dirac nature of neutrinos and presence of one eV scale light sterile neutrino. Incorporating other experimental bounds on these new physics scenarios, we show that neutrino decay scenario can significantly alter the neutrino flux on earth from the expected ones whereas the corresponding chan...
Unified models incorporating the right handed neutrino in a symmetric way generically possess par... more Unified models incorporating the right handed neutrino in a symmetric way generically possess parity symmetry. If this is broken spontaneously it results in the formation of domain walls in the early Universe, whose persistence is unwanted. A generic mechanism for destabilisation of such walls is a small pressure difference signalled by difference in the free energy across the walls. It is interesting to explore the possibility of such effects in conjunction with the effects that break supersymmetry in a phenomenologically acceptable way. Realising this possibility in the context of several scenarios of supersymmetry breaking results in an upper bound on the scale of spontaneous parity breaking, often much lower than the GUT scale. In the left-right symmetric models studied, the upper bound is no higher than $10^{11}$GeV but a scale as low as $10^5$GeV is acceptable.
We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetri... more We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetric Standard Model (MSSM) by considering a μ–τμ–τ symmetric mass matrix at high energy scale giving rise to Tri-Bi-Maximal (TBM) type mixing. We outline a flavor symmetry model based on A4A4 symmetry giving rise to the desired neutrino mass matrix at high energy scale. We take the three neutrino mass eigenvalues at high energy scale as input parameters and compute the neutrino parameters at low energy by taking into account of renormalization group effects. We observe that the correct output values of neutrino parameters at low energy are obtained only when the input mass eigenvalues are large |m1,2,3|=0.08–0.12 eV|m1,2,3|=0.08–0.12 eV with a very mild hierarchy of either inverted or normal type. A large inverted or normal hierarchical pattern of neutrino masses is disfavored within our framework. We also find a preference towards higher values of tan β, the ratio of vacuum expectation val...
We study all possible texture zeros in the Majorana neutrino mass matrix which are allowed from n... more We study all possible texture zeros in the Majorana neutrino mass matrix which are allowed from neutrino oscillation as well as cosmology data when the charged lepton mass matrix is assumed to take the diagonal form. Considering two different possible values of the lightest neutrino mass giving rise to quasi-degenerate and hierarchical light neutrino mass spectrum respectively, we write down the Majorana CP phases as a function of the Dirac CP phase using the constraints coming from vanishing or equality of elements in a particular texture zero mass matrix. We constrain texture zero mass matrices from the requirement of producing correct baryon asymmetry through the mechanism of leptogenesis. Adopting a type I seesaw framework, we consider the CP violating out of equilibrium decay of the lightest right handed neutrino as the source of lepton asymmetry. Apart from discriminating between the texture zero mass matrices and light neutrino mass hierarchy, we also constrain the Dirac CP phase so that the observed baryon asymmetry can be produced.
We propose an abelian extension of the Standard Model which can explain the origin of eV scale ma... more We propose an abelian extension of the Standard Model which can explain the origin of eV scale masses and mixing for active and sterile neutrinos and at the same time providing a natural cold dark matter candidate. One of the three active neutrinos acquires mass at tree level through seesaw mechanism whereas the other two active neutrinos and one sterile neutrino acquire eV scale masses at one-loop level. The model also allows non-trivial mixing between active and sterile neutrinos at one-loop level which could have interesting signatures at neutrino experiments. After the abelian gauge symmetry gets spontaneously broken down to a Z2 symmetry, the lightest Z2 odd particle can naturally give rise to the cold dark matter of the Universe. The phenomenology of both fermionic and scalar dark matter is briefly discussed by incorporating latest experimental constraints.
ABSTRACT We study a very specific type of neutrino mass and mixing structure based on the idea of... more ABSTRACT 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 $\theta_{13}$ and inverted hierarchy with vanishing lightest neutrino mass. Motivated by several recent attempts to explain non-zero $\theta_{13}$ by incorporating corrections to a leading order neutrino mass or mixing matrix giving $\theta_{13}=0$, here we study the origin of non-zero $\theta_{13}$ as well as leptonic Dirac CP phase $\delta_{CP}$ 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.
We study the possibility of generating tiny neutrino mass through a combination of type I and typ... more 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.
The Fermi gamma ray space telescope data have pointed towards an excess of gamma rays with a peak... more 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 cm 3 /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 N R and two triplet fermions Σ 1R , Σ 2R with suitable Higgs scalars. The spontaneous gauge symmetry breaking is achieved in such a way which results in a residual Z 2 symmetry and 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 while satisfying the constraints on relic density, direct detection scattering as well as collider constraints at the same time. We also discuss the compatibility of such a light fermion singlet dark matter with light neutrino mass.
The baryon to photon ratio in the present Universe is very accurately measured to be (6.065 ± 0.0... more The baryon to photon ratio in the present Universe is very accurately measured to be (6.065 ± 0.090) × 10 −10 . We study the possible origin of this baryon asymmetry in the neutrino sector through the generic mechanism of baryogenesis through leptogenesis. We consider both type I and type II seesaw origin of neutrino masses within the framework of left right symmetric models (LRSM). Using the latest best fit global neutrino oscillation data of mass squared differences, mixing angles and Dirac CP phase, we compute the predictions for baryon to photon ratio keeping the Majorana CP phases as free parameters for two different choices of lightest neutrino mass eigenvalue for both normal and inverted hierarchical patterns of neutrino masses. We do our calculation with and without lepton flavor effects being taken into account. We choose different diagonal Dirac neutrino mass matrix for different flavor effects in such a way that the lightest right handed neutrino mass is in the appropriate range. We also study the predictions for baryon asymmetry when the neutrino masses arise from a combination of both type I and type II seesaw (with dominating type I term) and discriminate between several combinations of Dirac and Majorana CP phases by demanding successful predictions for baryon asymmetry.
We study the possibility of generating non-zero reactor mixing angle θ 13 by perturbing the µ − τ... more We study the possibility of generating non-zero reactor mixing angle θ 13 by perturbing the µ − τ symmetric neutrino mass matrix. The leading order µ − τ symmetric neutrino mass matrix originates from type I seesaw mechanism whereas the perturbations to µ − τ symmetry originate from type II seesaw term. We consider four different realizations of µ − τ symmetry: Bimaximal Mixing(BM), Tri-bimaximal Mixing (TBM), Hexagonal Mixing (HM) and Golden Ratio Mixing (GRM) all giving rise to θ 13 = 0, θ 23 = π 4 but different non-zero values of solar mixing angle θ 12 . We assume a minimal µ − τ symmetry breaking type II seesaw mass matrix as a perturbation and calculate the neutrino oscillation parameters as a function of type II seesaw strength. We then consider the origin of non-trivial leptonic CP phase in the charged lepton sector and calculate the lepton asymmetry arising from the lightest right handed neutrino decay by incorporating the presence of both type I and type II seesaw. We constrain the type II seesaw strength as well as leptonic CP phase (and hence the charged lepton sector) by comparing our results with experimental neutrino oscillation parameters as well as Planck bound on baryon to photon ratio.
Left right symmetric models (LRSM) are extensions of the standard model by an enlarged gauge grou... more Left right symmetric models (LRSM) are extensions of the standard model by an enlarged gauge group SU (2) L × SU (2) R × U (1) B−L where automatic inclusion of right handed fermions as SU (2) R doublets guarantees a natural seesaw origin of neutrino masses. Apart from the extended gauge symmetry, LRSM also has an in-built global discrete symmetry, called D-parity which ensures equal gauge couplings for left and right sectors. Motivated by the fact that global symmetries are expected to be explicitly broken by theories of quantum gravity, here we study the effects of such gravity or Planck scale physics on neutrino masses and mixings by introducing explict Dparity breaking Planck scale suppressed higher dimensional operators. Although such Planck scale suppressd operators have dimension at least six in generic LRSM, dimension five operators can also arise in the presence of additional scalar fields which can be naturally accommodated within SO(10) grand unified theory (GUT) multiplets. We show that, such corrections can give rise to significant changes in the predictions for neutrino mixing parameters from the ones predicted by tree level seesaw formula if the left right symmetry breaking scale is lower than 10 14 GeV.
We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of... more We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of XMM-Newton telescope data and the galactic center gamma ray excess observed by the Fermi gamma ray space telescope within an abelian gauge extension of standard model. We consider a two component dark matter scenario with tree level mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant $Z_2$ symmetry into which the abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within $31-40$ GeV, then this model can also explain the galactic center gamma ray excess if the dark matter annihilation into $b\bar{b}$ pairs has a cross section of $\langle \sigma v \rangle \simeq (1.4-2.0) \times 10^{-26} \; \text{cm}^3/\text{s}$. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV X-ray line flux and galactic center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and gauge coupling. We also briefly discuss how this model can give rise to sub-eV neutrino masses at tree level as well as one-loop level while keeping the dark matter mass at few tens of GeV. We also constrain the model parameters from the requirement of keeping the one-loop mass difference between two dark matter particles below a keV. We find that the constraints from light neutrino mass and keV mass splitting between two dark matter components show more preference for opposite $CP$ eigenvalues of the two fermion singlet dark matter candidates in the model
We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of... more We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of XMM-Newton telescope data and the galactic center gamma ray excess observed by the Fermi gamma ray space telescope within an abelian gauge extension of standard model. We consider a two component dark matter scenario with a mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant Z 2 symmetry into which the abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within 31 − 40 GeV, then this model can also explain the galactic center gamma ray excess if the dark matter annihilation into bb pairs has a cross section of σv (1.4 − 2.0) × 10 −26 cm 3 /s. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV X-ray line flux and galactic center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and coupling. We also briefly discuss how this model can give rise to sub-eV neutrino masses at tree level as well as one loop level while keeping the dark matter mass at few tens of GeV. We also show the natural origin of keV mass splitting between two electroweak scale dark matter particles at one loop level in this model.
We discuss the possible origin of non-zero reactor mixing angle θ 13 and Dirac CP phase δ CP in t... more We discuss the possible origin of non-zero reactor mixing angle θ 13 and Dirac CP phase δ CP in the leptonic sector from a combination of type I and type II seesaw mechanisms. Type I seesaw contribution to neutrino mass matrix is of tri-bimaximal (TBM) type which gives rise to vanishing θ 13 leaving the Dirac CP phase undetermined. If the Dirac neutrino mass matrix is assumed to take the diagonal charged lepton type structure, such a TBM type neutrino mass matrix originating from type I seesaw corresponds to real values of Dirac Yukawa couplings in the terms Y ijLi HN j .
We study the possibility of connecting leptonic Dirac CP phase δ, lightest neutrino mass and bary... more We study the possibility of connecting leptonic Dirac CP phase δ, lightest neutrino mass and baryon asymmetry of the Universe within the framework of a model where both type I and type II seesaw mechanisms contribute to neutrino mass. Type I seesaw gives rise to Tri-Bimaximal (TBM) type neutrino mixing whereas type II seesaw acts as a correction in order to generate non-zero θ 13 .
The nondetection of neutrinos coming from Gamma Ray Bursts (GRBs) by the IceCube experiment has r... more The nondetection of neutrinos coming from Gamma Ray Bursts (GRBs) by the IceCube experiment has raised serious questions on our understanding of GRB's and the mechanism of neutrino flux production in them. Motivated by this and the need for a precise calculation for GRB neutrino flux, here we study the effects of beyond standard model physics on the GRB neutrino flux. In the internal shock model of GRB, high energy neutrinos are expected from muon, pion and kaon decays. Using the latest best fit neutrino oscillation parameters, we compute the expected flux on earth for standard as well as non-standard oscillation scenarios. Among the non-standard scenarios, we consider neutrino decay, pseudo-dirac nature of neutrinos and presence of one eV scale light sterile neutrino. Incorporating other experimental bounds on these new physics scenarios, we show that neutrino decay scenario can significantly alter the neutrino flux on earth from the expected ones whereas the corresponding chan...
Unified models incorporating the right handed neutrino in a symmetric way generically possess par... more Unified models incorporating the right handed neutrino in a symmetric way generically possess parity symmetry. If this is broken spontaneously it results in the formation of domain walls in the early Universe, whose persistence is unwanted. A generic mechanism for destabilisation of such walls is a small pressure difference signalled by difference in the free energy across the walls. It is interesting to explore the possibility of such effects in conjunction with the effects that break supersymmetry in a phenomenologically acceptable way. Realising this possibility in the context of several scenarios of supersymmetry breaking results in an upper bound on the scale of spontaneous parity breaking, often much lower than the GUT scale. In the left-right symmetric models studied, the upper bound is no higher than $10^{11}$GeV but a scale as low as $10^5$GeV is acceptable.
We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetri... more We study the renormalization group effects on neutrino masses and mixing in Minimal Supersymmetric Standard Model (MSSM) by considering a μ–τμ–τ symmetric mass matrix at high energy scale giving rise to Tri-Bi-Maximal (TBM) type mixing. We outline a flavor symmetry model based on A4A4 symmetry giving rise to the desired neutrino mass matrix at high energy scale. We take the three neutrino mass eigenvalues at high energy scale as input parameters and compute the neutrino parameters at low energy by taking into account of renormalization group effects. We observe that the correct output values of neutrino parameters at low energy are obtained only when the input mass eigenvalues are large |m1,2,3|=0.08–0.12 eV|m1,2,3|=0.08–0.12 eV with a very mild hierarchy of either inverted or normal type. A large inverted or normal hierarchical pattern of neutrino masses is disfavored within our framework. We also find a preference towards higher values of tan β, the ratio of vacuum expectation val...
We study all possible texture zeros in the Majorana neutrino mass matrix which are allowed from n... more We study all possible texture zeros in the Majorana neutrino mass matrix which are allowed from neutrino oscillation as well as cosmology data when the charged lepton mass matrix is assumed to take the diagonal form. Considering two different possible values of the lightest neutrino mass giving rise to quasi-degenerate and hierarchical light neutrino mass spectrum respectively, we write down the Majorana CP phases as a function of the Dirac CP phase using the constraints coming from vanishing or equality of elements in a particular texture zero mass matrix. We constrain texture zero mass matrices from the requirement of producing correct baryon asymmetry through the mechanism of leptogenesis. Adopting a type I seesaw framework, we consider the CP violating out of equilibrium decay of the lightest right handed neutrino as the source of lepton asymmetry. Apart from discriminating between the texture zero mass matrices and light neutrino mass hierarchy, we also constrain the Dirac CP phase so that the observed baryon asymmetry can be produced.
We propose an abelian extension of the Standard Model which can explain the origin of eV scale ma... more We propose an abelian extension of the Standard Model which can explain the origin of eV scale masses and mixing for active and sterile neutrinos and at the same time providing a natural cold dark matter candidate. One of the three active neutrinos acquires mass at tree level through seesaw mechanism whereas the other two active neutrinos and one sterile neutrino acquire eV scale masses at one-loop level. The model also allows non-trivial mixing between active and sterile neutrinos at one-loop level which could have interesting signatures at neutrino experiments. After the abelian gauge symmetry gets spontaneously broken down to a Z2 symmetry, the lightest Z2 odd particle can naturally give rise to the cold dark matter of the Universe. The phenomenology of both fermionic and scalar dark matter is briefly discussed by incorporating latest experimental constraints.
ABSTRACT We study a very specific type of neutrino mass and mixing structure based on the idea of... more ABSTRACT 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 $\theta_{13}$ and inverted hierarchy with vanishing lightest neutrino mass. Motivated by several recent attempts to explain non-zero $\theta_{13}$ by incorporating corrections to a leading order neutrino mass or mixing matrix giving $\theta_{13}=0$, here we study the origin of non-zero $\theta_{13}$ as well as leptonic Dirac CP phase $\delta_{CP}$ 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.
We study the possibility of generating tiny neutrino mass through a combination of type I and typ... more 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.
The Fermi gamma ray space telescope data have pointed towards an excess of gamma rays with a peak... more 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 cm 3 /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 N R and two triplet fermions Σ 1R , Σ 2R with suitable Higgs scalars. The spontaneous gauge symmetry breaking is achieved in such a way which results in a residual Z 2 symmetry and 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 while satisfying the constraints on relic density, direct detection scattering as well as collider constraints at the same time. We also discuss the compatibility of such a light fermion singlet dark matter with light neutrino mass.
The baryon to photon ratio in the present Universe is very accurately measured to be (6.065 ± 0.0... more The baryon to photon ratio in the present Universe is very accurately measured to be (6.065 ± 0.090) × 10 −10 . We study the possible origin of this baryon asymmetry in the neutrino sector through the generic mechanism of baryogenesis through leptogenesis. We consider both type I and type II seesaw origin of neutrino masses within the framework of left right symmetric models (LRSM). Using the latest best fit global neutrino oscillation data of mass squared differences, mixing angles and Dirac CP phase, we compute the predictions for baryon to photon ratio keeping the Majorana CP phases as free parameters for two different choices of lightest neutrino mass eigenvalue for both normal and inverted hierarchical patterns of neutrino masses. We do our calculation with and without lepton flavor effects being taken into account. We choose different diagonal Dirac neutrino mass matrix for different flavor effects in such a way that the lightest right handed neutrino mass is in the appropriate range. We also study the predictions for baryon asymmetry when the neutrino masses arise from a combination of both type I and type II seesaw (with dominating type I term) and discriminate between several combinations of Dirac and Majorana CP phases by demanding successful predictions for baryon asymmetry.
We study the possibility of generating non-zero reactor mixing angle θ 13 by perturbing the µ − τ... more We study the possibility of generating non-zero reactor mixing angle θ 13 by perturbing the µ − τ symmetric neutrino mass matrix. The leading order µ − τ symmetric neutrino mass matrix originates from type I seesaw mechanism whereas the perturbations to µ − τ symmetry originate from type II seesaw term. We consider four different realizations of µ − τ symmetry: Bimaximal Mixing(BM), Tri-bimaximal Mixing (TBM), Hexagonal Mixing (HM) and Golden Ratio Mixing (GRM) all giving rise to θ 13 = 0, θ 23 = π 4 but different non-zero values of solar mixing angle θ 12 . We assume a minimal µ − τ symmetry breaking type II seesaw mass matrix as a perturbation and calculate the neutrino oscillation parameters as a function of type II seesaw strength. We then consider the origin of non-trivial leptonic CP phase in the charged lepton sector and calculate the lepton asymmetry arising from the lightest right handed neutrino decay by incorporating the presence of both type I and type II seesaw. We constrain the type II seesaw strength as well as leptonic CP phase (and hence the charged lepton sector) by comparing our results with experimental neutrino oscillation parameters as well as Planck bound on baryon to photon ratio.
Uploads
Papers by Debasish Borah