arXiv subjects

Biswajit Karmakar

Publications and source records attributed to Biswajit Karmakar.

34 records · Page 2Linked to original sources

arXiv2018

Linear Seesaw for Dirac Neutrinos with $A_4$ Flavour Symmetry

We propose a linear seesaw model to realise light Dirac neutrinos within the framework of $A_4$ 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 $-π/2\lesssim δ\lesssim -π/5$ and $π/5\lesssim δ\lesssim π/2$ 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.

hep-ph↗

arXiv2018

Common Origin of Dirac Neutrino Mass and Freeze-in Massive Particle Dark Matter

Motivated by the fact that the origin of tiny Dirac neutrino masses via the standard model Higgs field and non-thermal dark matter populating the Universe via freeze-in mechanism require tiny dimensionless couplings of similar order of magnitudes $(\sim 10^{-12})$, we propose a framework that can dynamically generate such couplings in a unified manner. Adopting a flavour symmetric approach based on $A_4$ group, we construct a model where Dirac neutrino coupling to the standard model Higgs and dark matter coupling to its mother particle occur at dimension six level involving the same flavon fields, thereby generating the effective Yukawa coupling of same order of magnitudes. The mother particle for dark matter, a complex scalar singlet, gets thermally produced in the early Universe through Higgs portal couplings followed by its thermal freeze-out and then decay into the dark matter candidates giving rise to the freeze-in dark matter scenario. Some parts of the Higgs portal couplings of the mother particle can also be excluded by collider constraints on invisible decay rate of the standard model like Higgs boson. We show that the correct neutrino oscillation data can be successfully produced in the model which predicts normal hierarchical neutrino mass. The model also predicts the atmospheric angle to be in the lower octant if the Dirac CP phase lies close to the presently preferred maximal value.

hep-ph↗

arXiv2018

$A_4$ Flavour Model for Dirac Neutrinos: Type I and Inverse Seesaw

We propose two different seesaw models namely, type I and inverse seesaw to realise light Dirac neutrinos within the framework of $A_4$ 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.

hep-ph↗

arXiv2017

An $A_4$ realization of inverse seesaw: neutrino masses, $θ_{13}$ and leptonic non-unitarity

We provide an $A_4$ based flavor symmetric scenario to accommodate the inverse seesaw mechanism for explaining light neutrino masses and mixing. We find that the lepton mixing, in particular the tri-bimaximal mixing pattern and its deviation through nonzero $θ_{13}$, is originated solely from the flavor structure of the lepton number violating contribution of the neutral lepton mass matrix. Here we discuss in detail how a nonzero value of $θ_{13}$ is correlated with the other parameters in the framework and its impact on the Dirac CP phase $δ$. We also analyze the non-unitarity effects on lepton mixing matrix and its implication in terms of the lepton flavor violating decays, etc..

hep-ph↗

arXiv2017

Flavor origin of dark matter and its relation with leptonic nonzero $θ_{13}$ and Dirac CP phase $δ$

We propose a minimal extension of the standard model by including a $U(1)$ flavor symmetry to establish a correlation between the relic abundance of dark matter, measured by WMAP and PLANCK satellite experiments and non-zero value of $\sin θ_{13}$ observed at DOUBLE CHOOZ, Daya Bay, RENO and T2K. The flavour symmetry is allowed to be broken at a high scale to a remnant $\mathcal{Z}_2$ symmetry, which not only ensures the stability to the dark matter, but also gives rise to a modification to the existing $A_4$-based tri-bimaximal neutrino mixing. This deviation in turn suggests the required non-zero value of $\sin θ_{13}$. We assume the dark matter to be neutral under the existing $A_4$ symmetry while charged under the $U(1)$ flavor symmetry. Hence in this set-up, the non-zero value of $\sin θ_{13}$ predicts the dark matter charge under $U(1)$, which can be tested at various ongoing and future direct and collider dark matter search experiments. We also point out the involvement of nonzero leptonic CP phase $δ$, which plays an important role in the analysis.

hep-ph↗

arXiv2016

Unifying the flavor origin of dark matter with leptonic nonzero $θ_{13}$

We propose a flavor symmetric approach to unify the origin of dark matter (DM) with the non-zero $θ_{13}$ in the lepton sector. In this framework, breaking of a $U(1)$ flavor symmetry to a remnant $Z_2$ ensures the stability of the DM and gives rise to a modification to the existing $A_4$-based tri-bimaximal neutrino mixing to attain the required non-zero $\sin θ_{13}$. This results in a range of Higgs portal coupling of the DM which can be potentially accessible at various ongoing and future direct and collider search experiments.

hep-ph↗

arXiv2016

Quantum Hall realization of polarized intensity interferometry

We combine the ideas of intensity interferometry, polarization optics and Bell's measurement into an experimental proposal which is hosted in a $ν\,$=$\,2$ quantum Hall (QH) edge state. Our interferometer comprises of a single gate, that separates the spin resolved edge states of $ν$\,=\,$2$ state. An analog of waveplate (from polarization optics) is realized by exposing the individual edges to nano-magnets over a finite length which facilitates coherent manipulation of the electron spin on the edge. We show that the in-plane rotation of magnetization direction of the nano-magnets results in oscillations observed solely in the cross-correlated noise which arises from two particle interference. Through numerical simulations we demonstrate that our proposal is well within the reach of recent experimental developments of spin manipulations on the QH edge.

cond-mat.mes-hall↗

arXiv2015

Nanoscale Mach-Zehnder interferometer with spin-resolved quantum Hall edge states

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.

cond-mat.mes-hall↗

arXiv2015

Spontaneous CP Violation in Lepton-sector: a common origin for $θ_{13}$, Dirac CP phase and leptogenesis

A possible interplay between the two terms of the general type-II seesaw formula is exercised which leads to the generation of nonzero $θ_{13}$. The specific flavor structure of the model, guided by the $A_4 \times Z_4 \times Z_3$ symmetry and accompanied with the Standard Model singlet flavons, yields the conventional seesaw contribution to produce the tribimaximal lepton mixing which is further corrected by the presence of the $SU(2)_L$ triplet contribution to accommodate $θ_{13}$. We consider the CP symmetry to be spontaneously broken by the complex vacuum expectation value (vev) of a singlet field $S$. While the magnitude of its complex vev is responsible for generating $θ_{13}$, its phase part induces the low energy CP violating phase ($δ$) and the CP violation required for leptogenesis. Hence the triplet contribution, although sub-dominant, plays crucial role in providing a common source for non-zero $θ_{13}$, $δ$ and CP-violation required for leptogenesis. We find that the recent hint for $δ$ close to $3π/2$ is somewhat favored in this set-up though it excludes the exact equality with $3π/2$. We also discuss the generation of lepton asymmetry in this scenario.

hep-ph↗

arXiv2015

Nonzero $θ_{13}$ and Leptogenesis in a Type-I See-saw Model with $A_4$ Symmetry

In the light of recent discovery of nonzero $θ_{13}$, we have analyzed the Altarelli-Feruglio $A_4$ flavor symmetry model extended with additional flavon. The inclusion of the new field leads to the deviation from exact tri-bimaximal neutrino mixing pattern in the context of type-I see-saw by producing a nonzero $θ_{13}$ consistent with the recent experimental results at the leading order. A sum rule for light neutrino masses is also obtained in this context. The set-up constraints the two Majorana phases involved in the lepton mixing matrix in terms of $A_4$ parameter space. We have shown that a nonzero lepton asymmetry can be generated while next-to-leading order contributions to the neutrino Yukawa couplings are considered. The two Majorana phases play crucial role in CP-asymmetry parameter and the involvement of $θ_{13}$ in it, is exercised.

hep-ph↗

arXiv2012

Evidence of inter-layer interaction in magneto-luminescence spectra of electron bilayers

Magneto-luminescence studies in electron bilayers reveal the hallmarks of the even-denominator and other quantum Hall states in the intensities and energies of the inter-band optical recombination lines. In the presence of a small tunneling gap between the layers the magneto-optical emission from the lowest anti-symmetric subband, not populated in a single-electron picture, displays maxima at filling factors 1 and 2/3. These findings uncover a loss of pseudospin polarization, where the pseudospin describes the layer index degree of freedom, that is linked to an anomalous population of the anti-symmetric level due to excitonic correlations. The results demonstrate a new realm to probe the impact of inter-layer Coulomb interaction in quantum Hall bilayers.

cond-mat.mes-hall↗

arXiv2011

Controlled coupling of spin-resolved quantum Hall edge states

Topologically-protected edge states are dissipationless conducting surface states immune to impurity scattering and geometrical defects that occur in electronic systems characterized by a bulk insulating gap. One example can be found in a two-dimensional electron gas (2DEG) under high magnetic field in the quantum Hall regime. Based on the coherent control of the coupling between these protected states, several theoretical proposals for the implementation of information processing architectures were proposed. Here we introduce and experimentally demonstrate a new method that allows us to controllably couple co-propagating spin-resolved edge states of a QH insulator. The scheme exploits a spatially-periodic in-plane magnetic field that is created by an array of Cobalt nano-magnets placed at the boundary of the 2DEG. A maximum charge/spin transfer of about 28% is achieved at 250 mK. This result may open the way to the realization of scalable quantum-information architectures exploiting the spin degree of freedom of topologically-protected states.

cond-mat.mes-hall↗

arXiv2009

A Phase Diagram for Quantum Hall Bilayers with Strong Inter-layer Correlation

Inter-layer excitonic coherence in a quantum Hall bilayer with negligible tunneling is monitored by measurements of low-lying spin excitations. At $ν_T =1$ new quasiparticle excitations are observed above a transition temperature revealing a competing metallic phase. For magnetic fields above an onset Zeeman energy this metallic phase has full spin polarization. A phase diagram in the parameter space of temperature and Zeeman energy reveals that the transition temperature increases at higher fields. This unexpected result suggests intriguing impacts of spin polarization in the highly correlated phases.

cond-mat.other↗

arXiv2008

Optical Anisotropy of Electronic Excitations in Elliptical Quantum Dots

The authors report that anisotropic confining potentials in laterally-coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of inter-shell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared inter-shell optical transitions in quantum dots.

cond-mat.mes-hall↗

arXiv2008

Evidence of a first-order quantum phase transition of excitons in electron double layers

Complexity in many-particle systems occurs through processes of qualitative differentiation. These are described by concepts such as emerging states with specific symmetries that are linked to order parameters. In quantum Hall phases of electrons in semiconductor double layers with large inter-layer electron correlation there is an emergent many body exciton phase with an order parameter that measures the condensate fraction of excitons across the tunneling gap. As the inter-layer coupling is reduced by application of an in-plane magnetic field, this excitonic insulating state is brought in competition with a Fermi-metal phase of composite fermions (loosely, electrons with two magnetic flux quanta attached) stabilized by intra-layer electron correlation. Here we show that the quantum phase transformation between metallic and excitonic insulating states in the coupled bilayers becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder. The evidence is based on precise determinations of the excitonic order parameter by inelastic light scattering measurements close to the phase boundary. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing orders linked to quasi-particle correlations in removing the divergence of quantum fluctuations.

cond-mat.mes-hall↗

arXiv2007

Metamorphosis of a Quantum Hall Bilayer State into a Composite Fermion Metal

Composite fermion metal states emerge in quantum Hall bilayers at total Landau level filling factor $ν_T$=1 when the tunneling gap collapses by application of in-plane components of the external magnetic field. Evidence of this transformation is found in the continua of spin excitations observed by inelastic light scattering below the spin-wave mode at the Zeeman energy. The low-lying spin modes are interpreted as quasiparticle excitations with simultaneous changes in spin orientation and composite fermion Landau level index.

cond-mat.mes-hall↗