arXiv subjects

Sandhya Choubey

Publications and source records attributed to Sandhya Choubey.

At least 73 records · Page 4Linked to original sources

arXiv2010

Collective Flavor Oscillations Of Supernova Neutrinos and r-Process Nucleosynthesis

Neutrino-neutrino interactions inside core-collapse supernovae may give rise to collective flavor oscillations resulting in swap between flavors. These oscillations depend on the initial energy spectra, and relative fluxes or relative luminosities of the neutrinos. It has been observed that departure from energy equipartition among different flavors can give rise to one or more sharp spectral swap over energy, termed as splits. We study the occurrence of splits in the neutrino and antineutrino spectra, varying the initial relative fluxes for different models of initial energy spectrum, in both normal and inverted hierarchy. These initial relative flux variations give rise to several possible split patterns whereas variation over different models of energy spectra give similar results. We explore the effect of these spectral splits on the electron fraction, $Y_e$, that governs r-process nucleosynthesis inside supernovae. Since spectral splits modify the electron neutrino and antineutrino spectra in the region where r-process is postulated to happen, and since the pattern of spectral splits depends on the initial conditions of the spectra and the neutrino mass hierarchy, we show that the condition $Y_e < 0.5$ required for successful r-process nucleosynthesis will lead to constraints on the initial spectral conditions, for a given neutrino mass hierarchy.

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arXiv2010

Ultra-high neutrino fluxes as a probe for non-standard physics

We examine how light neutrinos coming from distant active galactic nuclei (AGN) and similar high energy sources may be used as tools to probe non-standard physics. In particular we discuss how studying the energy spectra of each neutrino flavour coming from such distant sources and their distortion relative to each other may serve as pointers to exotic physics such as neutrino decay, Lorentz symmetry violation, pseudo-Dirac effects, CP and CPT violation and quantum decoherence. This allows us to probe hitherto unexplored ranges of parameters for the above cases, for example lifetimes in the range $ 10^{-3}-10^{4} $ s/eV for the case of neutrino decay. We show that standard neutrino oscillations ensure that the different flavours arrive at the earth with similar shapes even if their flavour spectra at source may differ strongly in both shape and magnitude. As a result, observed differences between the spectra of various flavours at the detector would be signatures of non-standard physics altering neutrino fluxes during propagation rather than those arising during their production at source. Since detection of ultra-high energy (UHE) neutrinos is perhaps imminent, it is possible that such differences in spectral shapes will be tested in neutrino detectors in the near future. To that end, using the IceCube detector as an example, we show how our results translate to observable shower and muon-track event rates.

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arXiv2010

Diffuse Ultra-High Energy Neutrino Fluxes and Physics Beyond the Standard Model

We study spectral distortions of diffuse ultra-high energy (UHE) neutrino flavour fluxes resulting due to physics beyond the Standard Model (SM). Even large spectral differences between flavours at the source are massaged into a common shape at earth by SM oscillations, thus, any significant observed spectral differences are an indicator of new physics present in the oscillation probability during propagation. Neutrino decay and Lorentz symmetry violation (LV) are examples, and result in significant distortion of the fluxes and the well-known bounds on them, which may allow UHE detectors to probe lifetimes, the mass hierarchy and LV parameters over a broad range.

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arXiv2010

Spontaneous R-parity violating type III seesaw

We present a model where neutrino masses are generated by a combination of spontaneous R-parity violation and Type III seesaw. In addition to the usual MSSM particle content, our model consists of one extra triplet matter chiral superfield containing heavy SU(2) triplet fermions and its superpartners. R-parity is broken spontaneously when the sneutrinos associated with the one heavy neutrino as well as the three light neutrinos get vacuum expectation values, giving rise to the mixed $8\times 8$ neutralino-neutrino mass matrix. We show that our model can comfortably explain all the existing neutrino oscillation data. Due to the presence of the triplet fermion, we have a pair of additional heavy charged leptons which mix with the standard model charged leptons and the charginos. This gives rise to a $6\times 6$ chargino-charged lepton mass matrix, with 6 massive eigenstates. Finally we discuss about the different R-parity violating possible decay modes and the distinctive collider signatures which our model offers.

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arXiv2010

On the Vanishing of the CP Asymmetry in Leptogenesis due to Form Dominance

We emphasize that the vanishing of the CP asymmetry in leptogenesis, previously observed for models with tri-bimaximal mixing and family symmetry, may be traced to a property of the type I see-saw mechanism satisfied by such models known as Form Dominance, corresponding to the case of a diagonal Casas-Ibarra R-matrix. Form Dominance leads to vanishing flavour-dependent CP asymmetries irrespective of whether one has tri-bimaximal mixing or a family symmetry. Successful leptogenesis requires violation of Form Dominance, but not necessarily violation of tri bimaximal mixing. This may be achieved in models where the family symmetry responsible for tri-bimaximal mixing is implemented indirectly and a strong neutrino mass hierarchy is present with the Form Dominance broken only softly by the right-handed neutrino responsible for the lightest neutrino mass, as in constrained sequential dominance.

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arXiv2010

Flavor Composition of UHE Neutrinos at Source and at Neutrino Telescopes

We parameterize the initial flux composition of high energy astrophysical neutrinos as (Phi_e^0 : Phi_mu^0 : Phi_tau^0) = (1 : n : 0), where n characterizes the source. All usually assumed neutrino sources appear as limits of this simple parametrization. We investigate how precise neutrino telescopes can pin down the value of n. We furthermore show that there is a neutrino mixing scenario in which the ratio of muon neutrinos to the other neutrinos takes a constant value regardless of the initial flux composition. This occurs when the muon neutrino survival probability takes its minimal allowed value. The phenomenological consequences of this very predictive neutrino mixing scenario are given.

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arXiv2009

Optimized Two-Baseline Beta-Beam Experiment

We propose a realistic Beta-Beam experiment with four source ions and two baselines for the best possible sensitivity to theta_{13}, CP violation and mass hierarchy. Neutrinos from 18Ne and 6He with Lorentz boost gamma=350 are detected in a 500 kton water Cerenkov detector at a distance L=650 km (first oscillation peak) from the source. Neutrinos from 8B and 8Li are detected in a 50 kton magnetized iron detector at a distance L=7000 km (magic baseline) from the source. Since the decay ring requires a tilt angle of 34.5 degrees to send the beam to the magic baseline, the far end of the ring has a maximum depth of d=2132 m for magnetic field strength of 8.3 T, if one demands that the fraction of ions that decay along the straight sections of the racetrack geometry decay ring (called livetime) is 0.3. We alleviate this problem by proposing to trade reduction of the livetime of the decay ring with the increase in the boost factor of the ions, such that the number of events at the detector remains almost the same. This allows to substantially reduce the maximum depth of the decay ring at the far end, without significantly compromising the sensitivity of the experiment to the oscillation parameters. We take 8B and 8Li with gamma=390 and 656 respectively, as these are the largest possible boost factors possible with the envisaged upgrades of the SPS at CERN. This allows us to reduce d of the decay ring by a factor of 1.7 for 8.3 T magnetic field. Increase of magnetic field to 15 T would further reduce d to 738 m only. We study the sensitivity reach of this two baseline two storage ring Beta-Beam experiment, and compare it with the corresponding reach of the other proposed facilities.

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arXiv2009

Two Higgs Doublet Type III Seesaw with mu-tau symmetry at LHC

We propose a two Higgs doublet Type III seesaw model with $μ$-$τ$ flavor symmetry. We add an additional SU(2) Higgs doublet and three SU(2) fermion triplets in our model. The presence of two Higgs doublets allows for natural explanation of small neutrino masses with triplet fermions in the 100 GeV mass range, without fine tuning of the Yukawa couplings to extremely small values. The triplet fermions couple to the gauge bosons and can be thus produced at the LHC. We study in detail the effective cross-sections for the production and subsequent decays of these heavy exotic fermions. We show for the first time that the $μ$-$τ$ flavor symmetry in the low energy neutrino mass matrix results in mixing matrices for the neutral and charged heavy fermions that are not unity and which carry the flavor symmetry pattern. This flavor structure can be observed in the decays of the heavy fermions at LHC. The large Yukawa couplings in our model result in the decay of the heavy fermions into lighter leptons and Higgs with a decay rate which is about $10^{11}$ times larger than what is expected for the one Higgs Type III seesaw model with 100 GeV triplet fermions. The smallness of neutrino masses constrains the neutral Higgs mixing angle $\sinα$ in our model in such a way that the heavy fermions decay into the lighter neutral CP even Higgs $h^0$, CP odd Higgs $A^0$ and the charged Higgs $H^\pm$, but almost never to the heavier neutral CP even Higgs $H^0$. The small value for $\sinα$ also results in a very long lifetime for $h^0$. This displaced decay vertex should be visible at LHC. We provide an exhaustive list of collider signature channels for our model and identify those that have very large effective cross-sections at LHC and almost no standard model background.

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arXiv2009

Effect of Collective Flavor Oscillations on the Diffuse Supernova Neutrino Background

Collective flavor oscillations driven by neutrino-neutrino self interaction inside core-collapse supernovae have now been shown to bring drastic changes in the resultant neutrino fluxes. This would in turn significantly affect the diffuse supernova neutrino background (DSNB), created by all core-collapse supernovae that have exploded in the past. In view of these collective effects, we re-analyze the potential of detecting the DSNB in currently running and planned large-scale detectors meant for detecting both electron neutrinos and antineutrinos. The next generation detectors should be able to observe DSNB fluxes. Under certain conducive conditions, one could learn about neutrino parameters. For instance, it might be possible to determine the neutrino mass hierarchy, even if theta_{13} is almost zero.

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arXiv2009

Phenomenological consequences of four zero neutrino Yukawa textures

For type I seesaw and in the basis where the charged lepton and heavy right-handed neutrino mass matrices are real and diagonal, four has been shown to be the maximum number of zeros allowed in the neutrino Yukawa coupling matrix $Y_ν$. These four zero textures have been classified into two distinct categories. We investigate certain phenomenological consequences of these textures within a supersymmetric framework. This is done by using conditions implied on elements of the neutrino Majorana mass matrix for textures of each category in $Y_ν$. These conditions turn out to be stable under radiative corrections. Including the effective mass, which appears in neutrinoless double beta decay, along with the usual neutrino masses, mixing angles and phases, it is shown analytically and through scatter plots how restricted regions in the seesaw parameter space are selected by these conditions. We also make consequential statements on the yet unobserved radiative lepton flavor violating decays such as $μ\to e γ$. All these decay amplitudes are proportional to the moduli of entries of the neutrino Majorana mass matrix. We also show under which conditions the low energy CP violation, showing up in neutrino oscillations, is directly linked to the CP violation required for producing successful flavor dependent and flavor independent lepton asymmetries during leptogenesis.

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arXiv2008

Lepton Masses in a Minimal Model with Triplet Higgs Bosons and $S_3$ Flavor Symmetry

Viable neutrino and charged lepton masses and mixings are obtained by imposing a $S_3 \times Z_4 \times Z_3$ flavor symmetry in a model with a few additional Higgs. We use two $SU(2)_L$ triplet Higgs which are arranged as a doublet of $S_3$, and standard model singlet Higgs which are also put as doublets of $S_3$. We break the $S_3$ symmetry in this minimal model by giving vacuum expectation values (VEV) to the additional Higgs fields. Dictated by the minimum condition for the scalar potential, we obtain certain VEV alignments which allow us to maintain $μ-τ$ symmetry in the neutrino sector, while breaking it maximally for the charged leptons. This helps us to simultaneously explain the hierarchical charged lepton masses, and the neutrino masses and mixings. In particular, we obtain maximal $θ_{23}$ and zero $θ_{13}$. We allow for a mild breaking of the $μ-τ$ symmetry for the neutrinos and study the phenomenology. We give predictions for $θ_{13}$ and the CP violating Jarlskog invariant $J_{CP}$, as a function of the $μ-τ$ symmetry breaking parameter. We also discuss possible collider signatures and phenomenology associated with lepton flavor violating processes.

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arXiv2008

CERN-INO magical Beta-beam experiment: A high precision probe for neutrino parameters

This talk is an attempt to underscore in detail the physics reach of an experimental set-up where neutrinos produced in a beta-beam facility at CERN would be observed in the proposed large magnetized iron calorimeter detector (ICAL) at the India-based Neutrino Observatory (INO). The "magical" CERN-INO beta-beam set-up offers an excellent avenue to use the "Golden" channel ($ν_e \to ν_μ$) oscillation probability for a simultaneous determination of the neutrino mass ordering and $θ_{13}$ avoiding the impact of the CP phase $δ_{CP}$ on these measurements. With Lorentz boost $γ=650$ and irrespective of the true value of $δ_{CP}$, the neutrino mass hierarchy could be determined at $3σ$ C.L. if $\sin^22θ_{13}{\rm {(true)}} > 5.6 \times 10^{-4}$ and we can expect an unambiguous signal for $θ_{13}$ at $3σ$ C.L. if $\sin^22θ_{13}{\rm {(true)}} > 5.1 \times 10^{-4}$ independent of the true neutrino mass hierarchy.

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arXiv2008

Probing neutrino parameters with a Two-Baseline Beta-beam set-up

We discuss the prospects of exploring the neutrino mass parameters with a CERN based Beta-beam experiment using two different detectors at two different baselines. The proposed set-up consists of a 50 kton iron calorimeter (ICAL) at a baseline of around 7150 km which is roughly the magic baseline, e.g., ICAL@INO, and a 50 kton Totally Active Scintillator Detector at a distance of 730 km, e.g., at Gran Sasso. We take 8B and 8Li source ions with a boost factor $γ$ of 650 for the magic baseline while for the closer detector we consider 18Ne and 6He ions with a range of Lorentz boosts. We find that the locations of the two detectors complement each other leading to an exceptional high sensitivity. With $γ=650$ for 8B/8Li and $γ=575$ for 18Ne/6He and total luminosity corresponding to $5\times (1.1\times 10^{19})$ and $5\times (2.9\times 10^{19})$ useful ion decays in neutrino and antineutrino modes respectively, we find that the two-detector set-up can probe maximal CP violation and establish the neutrino mass ordering if $\sin^22θ_{13}$ is $1.8 \times 10^{-5}$ and $4.6 \times 10^{-5}$, respectively, or more. The sensitivity reach for $\sin^22θ_{13}$ itself is $5.3 \times 10^{-5}$. CP violation can be discovered for 64% of the possible $δ_{CP}$ values for $\sin^22θ_{13} \geq 8\times 10^{-5}$.

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arXiv2008

Exceptional Sensitivity to Neutrino Parameters with a Two Baseline Beta-Beam Set-up

We examine the reach of a Beta-beam experiment with two detectors at carefully chosen baselines for exploring neutrino mass parameters. Locating the source at CERN, the two detectors and baselines are: (a) a 50 kton iron calorimeter (ICAL) at a baseline of around 7150 km which is roughly the magic baseline, e.g., ICAL@INO, and (b) a 50 kton Totally Active Scintillator Detector at a distance of 730 km, e.g., at Gran Sasso. We choose 8B/8Li source ions with a boost factor γof 650 for the magic baseline while for the closer detector we consider 18Ne/6He ions with a range of Lorentz boosts. We find that the locations of the two detectors complement each other leading to an exceptional high sensitivity. With γ=650 for 8B/8Li and γ=575 for 18Ne/6He and total luminosity corresponding to 5\times (1.1 \times 10^{18}) and 5\times (2.9\times 10^{18}) useful ion decays in neutrino and antineutrino modes respectively, we find that our two detector set-up can probe maximal CP violation and establish the neutrino mass ordering if \sin^22θ_{13} is 1.4\times 10^{-4} and 2.7\times 10^{-4}, respectively, or more. The sensitivity reach for \sin^22θ_{13} itself is 5.5 \times 10^{-4}. With a factor of 10 higher luminosity, the corresponding \sin^22θ_{13} reach of this set-up would be 1.8\times 10^{-5}, 4.6\times 10^{-5} and 5.3\times 10^{-5} respectively for the above three performance indicators. CP violation can be discovered for 64% of the possible δ_{CP} values for \sin^22θ_{13} \geq 10^{-3} (\geq 8\times 10^{-5}), for the standard luminosity (10 times enhanced luminosity). Comparable physics performance can be achieved in a set-up where data from CERN to INO@ICAL is combined with that from CERN to the Boulby mine in United Kingdom, a baseline of 1050 km.

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arXiv2008

Optimizing the greenfield Beta-beam

We perform a comprehensive and detailed comparison of the physics reach of Beta-beam neutrino experiments between two pairs of plausible source ions, (8B, 8Li) and (18Ne, 6He). We study the optimal choices for the baseline, boost factor, and luminosity. We take a 50 kton iron calorimeter, a la ICAL@INO, as the far detector. We follow two complementary approaches for our study: (i) Fixing the number of useful ion decays and boost factor of the beam, and optimizing for the sensitivity reach between the two pairs of ions as a function of the baseline. (ii) Matching the shape of the spectrum between the two pairs of ions, and studying the requirements for baseline, boost factor, and luminosity. We find that for each pair of ions there are two baselines with very good sensitivity reaches: a short baseline with $L [km]/ γ\simeq 2.6$ (8B+8Li) and $L [km]/γ\simeq 0.8$ (18Ne+6He), and a long ``magic'' baseline. For $γ\sim 500$, one would optimally use 18Ne and 6He at the short baseline for CP violation, 8B and 8Li at the magic baseline for the mass hierarchy, and either 18Ne and 6He at the short baseline or 8B and 8Li at the magic baseline for the $\sin^22θ_{13}$ discovery.

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arXiv2008

Neutrino Oscillation Parameters After High Statistics KamLAND Results

We do a re-analysis to asses the impact of the results of the Borexino experiment and the recent 2.8 KTy KamLAND data on the solar neutrino oscillation parameters. The current Borexino results are found to have no impact on the allowed solar neutrino parameter space. The new KamLAND data causes a significant reduction of the allowed range of $Δm^2_{21}$, determining it with an unprecedented precision of 8.3% at 3$σ$. The precision of $Δm^2_{21}$ is controlled practically by the KamLAND data alone. Inclusion of new KamLAND results also improves the upper bound on $\sin^2θ_{12}$, but the precision of this parameter continues to be controlled by the solar data. The third mixing angle is constrained to be $\sin^2θ_{13} < 0.063$ at $3σ$ from a combined fit to the solar, KamLAND, atmospheric and CHOOZ results. We also address the issue of how much further reduction of allowed range of $Δm^2_{21}$ and $\sin^2θ_{12}$ is possible with increased statistics from KamLAND. We find that there is a sharp reduction of the $3σ$ ``spread'' with enhanced statistics till about 10 KTy after which the spread tends to flatten out reaching to less than 4% with 15 KTy data. For $\sin^2θ_{12}$ however, the spread is more than 25% even after 20 KTy exposure and assuming $θ_{12} < π/4$, as dictated by the solar data. We show that with a KamLAND like reactor ``SPMIN'' experiment at a distance of $\sim$ 60 km, the spread of $\sin^2θ_{12}$ could be reduced to about 5% at $3σ$ level while $Δm_{21}^2$ could be determined to within 4%, with just 3 KTy exposure.

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arXiv2008

On Probing theta_{23} in Neutrino Telescopes

Among all neutrino mixing parameters, the atmospheric neutrino mixing angle theta_{23} introduces the strongest variation on the flux ratios of ultra high energy neutrinos. We investigate the potential of these flux ratio measurements at neutrino telescopes to constrain theta_{23}. We consider astrophysical neutrinos originating from pion, muon-damped and neutron sources and make a comparative study of their sensitivity reach to theta_{23}. It is found that neutron sources are most favorable for testing deviations from maximal theta_{23}. Using a chi^2 analysis, we show in particular the power of combining (i) different flux ratios from the same type of source, and also (ii) combining flux ratios from different astrophysical sources. We include in our analysis ``impure'' sources, i.e., deviations from the usually assumed initial (1 : 2 : 0), (0 : 1 : 0) or (1 : 0 : 0) flux compositions.

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arXiv2008

The A4 flavor symmetry and neutrino phenomenology

It has been shown that tribimaximal mixing can be obtained by some particular breaking pattern of the $A_4$ symmetry, wherein the extra $A_4$ triplet Higgs scalars pick up certain fixed vacuum expectation value (VEV) alignments. We have performed a detailed analysis of the different possible neutrino mass matrices within the framework of the $A_4$ model. We take into account all possible singlet and triplet Higgs scalars which leave the Lagrangian invariant under $A_4$. We break $A_4$ spontaneously, allowing the Higgs to take any VEV in general. We show that the neutrino mixing matrix deviates from tribimaximal, both due to the presence of the extra Higgs singlets, as well as from the deviation of the triplet Higgs VEV from its desired alignment, taken previously. We solve the eigenvalue problem for a variety of these illustrative cases and identify the ones where one obtains exact tribimaximal mixing. All such cases require fine-tuning. We show which neutrino mass matrices would be strongly disfavored by the current neutrino data. Finally, we study in detail the phenomenology of the remaining viable mass matrices and establish the deviation of the neutrino mixing from tribimaximal, both analytically as well as numerically.

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