arXiv subjects

Ambar Ghosal

Publications and source records attributed to Ambar Ghosal.

At least 19 recordsLinked to original sources

arXiv2025

Complementarity between Cosmic String Gravitational Waves and long-lived particle searches in a laboratory

Cosmic strings are powerful witnesses to cosmic events including any period of early matter domination. If such a period of matter domination was catalysed by metastable, long-lived particles, then there will be complementary signals to ascertain the nature of dark sector in experiments detecting primordial features in the gravitational wave (GW) power spectrum and laboratory searches for long-lived particles. We give explicit examples of global and local U(1) gauge extended dark sectors to demonstrate such a complementarity as the union of the two experiments reveals more information about the dark sector than either experiment. Demanding that Higgs-portal long-lived scalar be looked for, in various experiments such as DUNE, FASER, FASER-II, MATHUSLA, SHiP, we identify the parameter space which leads to complementary observables for GW detectors such as LISA and ET.

hep-ph↗

arXiv2020

Baryogenesis from ultralight primordial black holes and strong gravitational waves from cosmic strings

Ultralight primordial black holes (PBHs)($\lesssim10^9$g) completely evaporate via Hawking radiation (HR) and produce all the particles in a given theory regardless of their other interactions. If the right handed (RH) neutrinos are produced from PBH evaporation, successful baryogenesis via leptogenesis predicts mass scale of RH neutrinos as well as black holes. We show that, given the lepton number violation (generation of RH neutrino masses) in the theory is a consequence of a gauged $U(1)$ breaking which is then followed by the formation of PBHs, a network of cosmic strings naturally gives rise to strong stochastic gravitational wave (GW) signal at the sensitivity level of pulsar timing arrays (PTA) and LIGO5. Besides, due to a transient period of black hole domination in the early universe, for which baryon asymmetry is independent of initial PBH density, a break in the GW spectra occurs around MHz frequency. Therefore, to observe the break along with the usual GW signal by the emission of gravitons via HR, GW detectors at higher frequencies are called for. The recent finding by the NANOGrav PTA of a stochastic common spectrum process (interpreted as GWs) across many pulsars is in tension with PBH baryogenesis for large cosmic string loops ($\alpha\simeq 0.1$).

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arXiv2020

Predictive $S_4$ flavon model with $\text{TM}_1$ mixing and baryogenesis through leptogenesis

We use $S_4$ discrete group to construct a neutrino flavour model which leads to $TM_1$ mixing and is consistent with the neutrino oscillation data. Using the model's constrained parameter space, we predict the values of Dirac $CP$ phase and the light neutrino mass as $-1<\sin \delta <-0.9$ and $1.7<m_1 (\text{meV})<5.5$ respectively. We thoroughly examine the usefulness of this model in explaining the observed baryon asymmetry of the Universe. Near-maximal breaking of CP symmetry (arising due to the $\text{TM}_1$ constraint) helps us in generating adequate baryon asymmetry through leptogenesis. We study the evolution of the asymmetry (generated due to the decay of the heavy Majorana neutrinos) starting from the primordial Universe in two different ways (i)explicitly solving network of Boltzmann equations, (ii) using approximate analytic solution and we have shown the extent of their equivalence. Nearly accurate analytical fits are used thereafter to evaluate baryon asymmetry for the whole parameter space allowed by $3\sigma$ global fit of oscillation data and to impose a constraint on the yet unbounded mass scale parameter of Dirac neutrino mass matrix. Furthermore, significant contribution of $N_2$ decay in the context of flavoured leptogenesis is also estimated.

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arXiv2019

Importance of generalized $μτ$ symmetry and its CP extension on neutrino mixing and leptogenesis

Within the framework of residual symmetry, two $\mathbb{Z}_2$ type associate $μτ$ interchange symmetries robustly constrain the Dirac CP phase $δ$ in a model independent way. Both of them predict simultaneous maximality of $δ$ and the atmospheric mixing angle $θ_{23}$. We show how these well known correlations will be changed if we generalize the $μτ$ interchange symmetry to a $μτ$ mixing symmetry. In particular, we show that the stringent condition of simultaneous maximality could be relaxed even with a very small departure from the exact $μτ$ interchange. In addition, the present neutrino data on $δ$ and $θ_{23}$ can be explained better by the mixing symmetry. After discussing the impact of the $μτ$ mixing in some realistic neutrino mass models, we show how the proposed mixing could be realized with two simultaneous CP transformations which also lead to novel and testable correlations between $δ$ and the mixing angles $θ_{ij}$. Next we discuss in particular, the `three flavour regime' of leptogenesis within the CP extended framework and show, unlike the ordinary CP extended $μτ$ interchange symmetry, a resonant leptogenesis is possible due the generalization of $μτ$ interchange to the $μτ$ mixing and the resulting baryon asymmetry always requires a nonmaximal $θ_{23}$ owing to the fact that the baryon to photon ratio $η_B$ vanishes in the exact limit of $θ_{23}=π/4$. This is one of the robust predictions of this framework. The CP extended $μτ$ mixing is also a novel example of a low energy effective model that provides an important insight to the off-diagonal terms of the flavour coupling matrix which have usually been neglected in literature to compute the final baryon asymmetry, in particular in the models with flavour symmetries.

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arXiv2019

A radiative Type-II seesaw model with broken Symmetry Ansatz

Parametrization of the neutrino mass matrix in terms of well-known measured quantities is an attractive way to obtain a phenomenologically viable form. We propose a model of neutrino mass matrix based on type-II seesaw mechanism adhering the concept of badly broken symmetry. Two of the mixing angles are coming out as $\theta_{23}\sim46.08^{\circ}$, $\theta_{13}\sim8.69^{\circ}$. However, to accommodate the other oscillation parameters, we further add an extra doublet and the neutrino masses are generated through the Zee mechanism at the one loop level, and the modified model can admit all the extant data for a suitable choice of model parameters.

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arXiv2019

CP transformed mixed $μτ$ antisymmetry for neutrinos and its consequences

We propose a complex extension of mixed $μτ$ antisymmetry in the neutrino Majorana mass matrix $M_ν$. This can be implemented in the Lagrangian by a generalized CP transformation (labeled by a mixing parameter $θ$) on the left-chiral flavor neutrino fields. We investigate its implications for leptonic CP violation and neutrino phenomenology in general. Interestingly, the $μτ$ mixing parameter $θ$ gets correlated with the Dirac CP phase $δ$ and the atmospheric mixing angle $θ_{23}$ through an analytical relation. In general, for arbitrary $θ$, both $θ_{23}$ and $δ$ are nonmaximal. We discuss the corresponding results for the CP asymmetry parameter $A_{μe}$ in neutrino oscillation experiments. For a nonmaximal $δ$, one of the two Majorana phases is different from $0$ or $π$, thereby leading to nonvanishing Majorana CP violation with observable consequences for the neutrinoless double beta ($β\beta0ν$) decay process. We numerically work out in detail the predictions for that process in relation to various ongoing and forthcoming experiments. We also work out the predictions of our scheme on flavor flux ratios at neutrino telescopes. While exact CP transformed $μτ$ interchange antisymmetry ($θ=π/2$) leads to an exact equality among those ratios, taking a value $0.5$, a tiny deviation can cause a drastic change in them. Careful measurement of these flux ratios in future will further constrain the parameter $θ$.

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arXiv2018

Consequences of minimal seesaw with complex $μτ$ antisymmetry of neutrinos

We propose a complex extension of $μτ$ permutation antisymmetry in the neutrino Majorana matrix $M_ν$. The latter can be realized for the Lagrangian by appropriate CP transformations on the neutrino fields. The resultant form of $M_ν$ is shown to be simply related to that with a complex (CP) extension of $μτ$ permutation symmetry, with identical phenomenological consequences, though their group theoretic origins are quite different. We investigate those consequences in detail for the minimal seesaw induced by two strongly hierarchical right-chiral neutrinos $N_1$ and $N_2$ with the result that the Dirac phase is maximal while the two Majorana phases are either 0 or $π$. We further provide an uptodate discussion of the $β\beta0ν$ process vis-a-vis ongoing and forthcoming experiments. Finally, a thorough treatment is given of baryogenesis via leptogenesis in this scenario, primarily with the assumption that the lepton asymmetry produced by the decays of $N_1$ only matters here with the asymmetry produced by $N_2$ being washed out. Tight upper and lower bounds on the mass of $N_1$ are obtained from the constraint of obtaining the correct observed range of the baryon asymmetry parameter and the role played by $N_2$ is elucidated thereafter. The mildly hierarchical right-chiral neutrino case (including the quasidegenerate possibility) is discussed in an Appendix.

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arXiv2017

Generalized $\mathbb{Z}_2\times \mathbb{Z}_2$ in Scaling neutrino Majorana mass matrix and baryogenesis via flavored leptogenesis

We investigate the consequences of a generalized $\mathbb{Z}_2\times\mathbb{Z}_2$ symmetry on a scaling neutrino Majorana mass matrix. It enables us to determine definite analytical relations between the mixing angles $θ_{12}$ and $θ_{13}$, maximal CP violation for the Dirac type and vanishing for the Majorana type. Beside the other testable predictions on the low energy neutrino parameters such as $ββ_{0ν}$ decay matrix element $|M_{ee}|$ and the light neutrino masses $m_{1,2,3}$, the model also has intriguing consequences from the perspective of leptogenesis. With the assumption that the required CP violation for leptogenesis is created by the decay of lightest ($N_1$) of the heavy Majorana neutrinos, only $τ$-flavored leptogenesis scenario is found to be allowed in this model. For a normal (inverted) ordering of light neutrino masses, $θ_{23}$ is found be less (greater) than its maximal value, for the final baryon asymmetry $Y_B$ to be in the observed range. Besides, an upper and a lower bound on the mass of $N_1$ have also been estimated. Effect of the heavier neutrinos $N_{2,3}$ on final $Y_B$ has been worked out subsequently. The predictions of this model will be tested in the experiments such as nEXO, LEGEND, GERDA-II, T2K, NO$ν$A, DUNE etc.

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arXiv2017

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

physics.ins-det↗

arXiv2017

Baryon asymmetry via leptogenesis in a neutrino mass model with complex scaling

Baryogenesis via leptogenesis is investigated in a specific model of light neutrino masses and mixing angles. The latter was proposed on the basis of an assumed complex-extended scaling property of the neutrino Majorana mass matrix $M_ν$, derived with a type-1 seesaw from a Dirac mass matrix $m_D$ and a heavy singlet neutrino Majorana mass matrix $M_R$. One of its important features, highlighted here, is that there is a common source of the origin of a nonzero $θ_{13}$ and the CP violating lepton asymmetry through the imaginary part of $m_D$. The model predicted CP violation to be maximal for the Dirac type and vanishing for the Majorana type. We assume strongly hierarchical mass eigenvalues for $M_R$. The leptonic CP asymmetry parameter $\varepsilon^α_{1}\hspace{1mm}$ with lepton flavor $α$, originating from the decays of the lightest of the heavy neutrinos $N_1$ (of mass $M_1$) at a temperature $T\sim M_1$, is what matters here with $\varepsilon^α_{2,3}$, originating from the decays of $N_{2,3}$, being washed out. The light leptonic and heavy neutrino number densities (normalized to the entropy density) are evolved via Boltzmann equations down to electroweak temperatures to yield a baryon asymmetry through sphaleronic transitions. The effect of flavored vs. unflavored leptogenesis in the three mass regimes (1) $M_1<10^{9}$ GeV, (2) $10^9$ GeV $ 10^{12}$ GeV are numerically worked out for both a normal and an inverted mass ordering of the light neutrinos. Corresponding results on the baryon asymmetry of the universe are obtained, displayed and discussed.

hep-ph↗

arXiv2016

Extended scaling and residual flavor symmetry in the neutrino Majorana mass matrix

The residual symmetry approach, along with a complex extension for some flavor invariance, is a powerful tool to uncover the flavor structure of the $3\times3$ neutrino Majorana mass matrix $M_ν$ towards gaining insights into neutrino mixing. We utilize this to propose a complex extension of the real scaling ansatz for $M_ν$ which was introduced some years ago. Unlike the latter, our proposal allows a nonzero mass for each of the three light neutrinos as well as a nonvanishing $θ_{13}$. A major result of this scheme is that leptonic Dirac CP-violation must be maximal while atmospheric neutrino mixing need not be exactly maximal. Moreover, each of the two allowed Majorana phases, to be probed by the search for nuclear $0νββ$ decay, has to be at one of its two CP-conserving values. There are other interesting consequences such as the allowed occurrence of a normal mass ordering which is not favored by the real scaling ansatz. Our predictions will be tested in ongoing and future neutrino oscillation experiments at T2K, NO$ν$A and DUNE.

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arXiv2016

Probing maximal zero textures with broken cyclic symmetry in inverse seesaw

Within the framework of inverse seesaw mechanism we investigate neutrino mass matrices invariant under cyclic symmetry ($Z_3$) with maximal zero texture (6 zero textures). We explore two different approaches to obtain the cyclic symmetry invariant form of the constituent matrices. In the first one we consider explicit cyclic symmetry in the neutrino sector of the Lagrangian which dictates the emerged effective neutrino mass matrix ($m_ν$) to be symmetry invariant and hence leads to a degeneracy in masses. We then consider explicit breaking of the symmetry through a dimensionless parameter $ε^{\prime}$ to remove the degeneracy. It is seen that the method doesn't support the current neutrino oscillation global fit data even after considering the correction from cyclic symmetry invariant charged lepton mass matrix ($m_l$) unless the breaking parameter is too large. In the second method, we assume the same forms of the neutrino mass matrices, however, symmetry is broken in the charged lepton sector. All the structures of the mass matrices are now dictated by an effective residual symmetry of some larger symmetry group in the Lagrangian. For illustration, we exemplify a toy model based on softly broken $A_4$ symmetry group which leads to one of the combination of $m_l$, $m_D$, $M_{RS}$ and $μ$ to generate effective $m_ν$. All the emerged mass matrices predict a constraint range of the CP violating phases and atmospheric mixing angle along with an inverted hierarchical structure of the neutrino masses. Further, significant predictions on $β\beta0ν$ decay parameter $|m_{11}|$ and the sum of the three light neutrino masses ($Σ_im_i$) are also obtained.

hep-ph↗

arXiv2016

Maximal Zero Textures in Linear and Inverse Seesaw

We investigate Linear and Inverse seesaw mechanisms with maximal zero textures of the constituent matrices subjected to the assumption of non-zero eigenvalues for the neutrino mass matrix $m_ν$ and charged lepton mass matrix $m_e$. If we restrict to the minimally parametrized non-singular `$m_e$' (i.e., with maximum number of zeros) it gives rise to only 6 possible textures of $m_e$. Non-zero determinant of $m_ν$ dictates six possible textures of the constituent matrices. We ask in this minimalistic approach, what are the phenomenologically allowed maximum zero textures are possible. It turns out that Inverse seesaw leads to 7 allowed two-zero textures while the Linear seesaw leads to only one. In Inverse seesaw, we show that 2 is the maximum number of independent zeros that can be inserted into $μ_S$ to obtain all 7 viable two-zero textures of $m_ν$. On the other hand, in Linear seesaw mechanism, the minimal scheme allows maximum 5 zeros to be accommodated in `$m$' so as to obtain viable effective neutrino mass matrices ($m_ν$). Interestingly, we find that our minimalistic approach in Inverse seesaw leads to a realization of all the phenomenologically allowed two-zero textures whereas in Linear seesaw only one such texture is viable. Next our numerical analysis shows that none of the two-zero textures give rise to enough CP violation or significant $δ_{CP}$. Therefore, if $δ_{CP}=π/2$ is established, our minimalistic scheme may still be viable provided we allow more number of parameters in `$m_e$'.

hep-ph↗

arXiv2016

Flavored leptogenesis with quasi degenerate neutrinos in a broken cyclic symmetric model

Cyclic symmetry in the neutrino sector with the type-I seesaw mechanism in the mass basis of charged leptons and right chiral neutrinos ($N_{iR}$, $i=e,μ,τ$) generates two fold degenerate light neutrino and three fold degenerate heavy neutrino mass spectrum. Consequently, such scheme, produces vanishing one light neutrino mass squared difference and lepton asymmetry. To circumvent such unphysical outcome, we break cyclic symmetry in the diagonal right chiral neutrino mass term by a small breaking parameter. Nonzero mass squared differences and mixing angles are generated with the help of the small breaking parameter. Smallness of the breaking parameter opens up a possibility of resonant leptogenesis. Assuming complex Yukawa couplings, we derive generalized expressions flavor dependent CP asymmetry parameters ($\varepsilon^α_i$) which are valid for quasi degenerate as well as hierarchical mass spectrum of right handed neutrinos. There after we set up the chain of coupled Boltzmann equations (which are flavor dependent too) which have to be solved in order to get the final lepton asymmetries. Depending upon the temperature regime the CP asymmetries and the Boltzmann equations may also be flavor independent. As our goal is to study the enhancement of CP asymmetry due to quasi degeneracy of right handed neutrinos, we select only the lowest allowed (by neutrino oscillation data) value of breaking parameter (and other corresponding Lagrangian parameters) and estimate the baryon asymmetry parameter $Y_B$. Experimental constraint of $Y_B$ introduces a bound on right handed neutrino mass which remained unrestricted by neutrino oscillation data.

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arXiv2016

Complex Scaling in Neutrino Mass Matrix

Using the residual symmetry approach, we propose a complex extension of the scaling ansatz on $M_ν$ which allows a nonzero mass for each of the three light neutrinos as well as a nonvanishing $θ_{13}$. Leptonic Dirac CP violation must be maximal while atmospheric neutrino mixing need not to be exactly maximal. Each of the two Majorana phases, to be probed by the search for $0νββ$ decay, has to be zero or $π$ and a normal neutrino mass hierarchy is allowed.

hep-ph↗

arXiv2016

Evaluation of the Majorana Phases of a General Majorana Neutrino Mass Matrix: Testability of hierarchical Flavour Models

We evaluate the Majorana phases for a general $3\times3$ complex symmetric neutrino mass matrix on the basis of Mohapatra-Rodejohann's phase convention using the three rephasing invariant quantities $I_{12}$, $I_{13}$ and $I_{23}$ proposed by Sarkar and Singh. We find them interesting as they allow us to evaluate each Majorana phase in a model independent way even if one eigenvalue is zero. Utilizing the solution of a general complex symmetric mass matrix for eigenvalues and mixing angles we determine the Majorana phases for both the hierarchies, normal and inverted, taking into account the constraints from neutrino oscillation global fit data as well as bound on the sum of the three light neutrino masses ($Σ_im_i$) and the neutrinoless double beta decay ($ββ_{0ν}$) parameter $|m_{11}|$. This methodology of finding the Majorana phases is applied thereafter in some predictive models for both the hierarchical cases (normal and inverted) to evaluate the corresponding Majorana phases and it is shown that all the sub cases presented in inverted hierarchy section can be realized in a model with texture zeros and scaling ansatz within the framework of inverse seesaw although one of the sub case following the normal hierarchy is yet to be established. Except the case of quasi degenerate neutrinos, the methodology obtained in this work is able to evaluate the corresponding Majorana phases, given any model of neutrino masses.

hep-ph↗

arXiv2015

Probing texture zeros with scaling ansatz in inverse seesaw

We investigate neutrino mass matrix phenomenology involving scaling ansatz and texture zeros adhering inverse seesaw mechanism. It is seen that four is the maximum number of zeros in $m_D$ and $μ$ to obtain viable phenomenology. Depending upon the generic nature of the effective neutrino mass matrices we classify all the emerged matrices in four categories. One of them is ruled out phenomenologically due to inappropriate value of reactor mixing angle after breaking of the scaling ansatz. The mass ordering is inverted in all cases. One of the distinguishable feature of all these categories is the vanishingly small value of CP violation measure $J_{CP}$ due to small value of $δ_{CP}$. Thus those categories will be ruled out if CP violation is observed in the leptonic sector in future experiments.

hep-ph↗

arXiv2015

Scaling ansatz with texture zeros in linear seesaw

We investigate scaling ansatz with texture zeros within the framework of linear seesaw mechanism. In this variant of seesaw mechanism a simplified expression of effective neutrino mass matrix $m_ν$ containing two Dirac type matrices ($m_D$ and $m_{DS}$) and one Majorana type matrix ($m_{RS}$) is obtained by virtue of neglecting the global $U(1)_L$ symmetry breaking term in the mass term of the Lagrangian. Along with the charged lepton mass matrix, the matrix $m_{RS}$ too, is chosen in a diagonal basis whereas a scaling relation is incorporated in $m_D$ and $m_{DS}$ with different scale factors. Our goal in this work is to achieve a completely phenomenologically acceptable $m_ν$ generated by combinations of $m_D$ and $m_{DS}$ containing least number of independent parameters or maximum number of zeros. At the end of the numerical analysis it is found that number of zeros in any of the constituent Dirac type matrices ($m_D$ and $m_{DS}$) of $m_ν$ cannot be greater than six in order to meet the phenomenological requirements. The hierarchy obtained here is normal and also the values of the two parameters sum mass ($\sum m_i$) and $|m_{ν_{ee}}|$ are below the present experimental lower limit.

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