arXiv subjects

Srubabati Goswami

Publications and source records attributed to Srubabati Goswami.

At least 73 records · Page 4Linked to original sources

arXiv2016

New look at the degeneracies in the neutrino oscillation parameters, and their resolution by T2K, NO$ν$A and ICAL

At present the three major unknowns in neutrino oscillation parameters are the mass hierarchy, the octant of $θ_{23}$ and the CP phase $δ_{CP}$. It is well known that the presence of hierarchy$-δ_{CP}$ and octant degeneracies affects the unambiguous determination of these parameters. In this paper we show that a comprehensive way to study the remaining parameter degeneracies is in the form of a generalized hierarchy-$θ_{23}$ - $δ_{CP}$ degeneracy. We show that the wrong-hierarchy and/or wrong-octant solutions can be further classified into eight different solutions depending on whether they occur with the wrong or right value of $δ_{CP}$. These eight solutions are different from the original eightfold degenerate solutions and can exist, in principle, even if $θ_{13}$ is known. These multiple solutions, apart from affecting the determination of the true hierarchy and octant, also affect the accurate estimation of $δ_{CP}$. We identify which of these eight different degenerate solutions can occur in the test ($θ_{23} - δ_{CP}$) parameter space, taking the long-baseline experiment NO$ν$A running in the neutrino mode as an example. The inclusion of the NO$ν$A antineutrino run removes the wrong-octant solutions appearing with both right and wrong hierarchy. Adding T2K data to this resolves the wrong hierarchy -- right octant solutions to a large extent. The remaining wrong hierarchy solutions can be removed by combining NO$ν$A + T2K with atmospheric neutrino data. We demonstrate this using ICAL@INO as the prototype atmospheric neutrino detector. We find that the degeneracies can be resolved at the $2σ$ level by the combined data set, for the true parameter space considered in the study.

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arXiv2015

Can the hint of $δ_{CP}$ from T2K also indicate the hierarchy and octant ?

The T2K neutrino data has already given a hint for the best-fit value of the leptonic CP phase $δ_{CP}$ as $-90^\circ$. In this paper we ask the question that if this hint is confirmed by the subsequent neutrino and anti-neutrino runs of T2K, then can it also give any information about the other two remaining unknown oscillation parameters - the neutrino mass hierarchy and octant of $θ_{23}$. We find that if T2K runs in only neutrino mode with its full targeted exposure, then $δ_{CP} = -90^\circ$ would indicate the true hierarchy as normal and the true octant as higher. On the other hand if T2K runs in equal neutrino and anti-neutrino mode then the true hierarchy can be confirmed as normal but the octant will remain undetermined. We have also studied the effect of anti-neutrino runs on CP sensitivity of T2K.

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arXiv2015

Looking for hints of a reconstructible seesaw model at the Large Hadron Collider

We study the production of heavy neutrinos at the Large Hadron Collider (LHC) through the dominant s-channel production mode as well as the vector boson fusion (VBF) process. We consider the TeV scale minimal linear seesaw model containing two heavy singlets with opposite lepton number. This model is fully reconstructible from oscillation data apart from an overall normalization constant which can be constrained from meta-stability of the electroweak vacuum and bounds coming from lepton flavor violation (LFV) searches. Dirac nature of heavy neutrinos in this model implies suppression of the conventional same-sign-dilepton signal at the LHC. We analyze the collider signatures with tri-lepton final state and missing transverse energy as well as VBF type signals which are characterized by two additional forward tagged jets. Our investigation reveals that due to stringent constraints on light-heavy mixing coming from LFV and meta-stability bounds, the model can be explored only for light to moderate mass range of heavy neutrinos. We also note that in case of a positive signal, flavor counting of the final tri-lepton channel can give information about the mass hierarchy of the light neutrinos.

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arXiv2014

TeV Scale Left-Right Symmetry and Large Mixing Effects in Neutrinoless Double Beta Decay

We analyze various contributions to neutrinoless double beta decay ($0νββ$) in a TeV-scale Left-Right Symmetric Model (LRSM) for type-I seesaw dominance. We find that the momentum-dependent effects due to $W_L-W_R$ exchange ($λ$-diagram) and $W_L-W_R$ mixing ($η$-diagram) could give dominant contributions to the $0νββ$ amplitude in a wide range of the LRSM parameter space. In particular, for a relatively large $W_L-W_R$ mixing, the $η$-contribution by itself could saturate the current experimental limit on the $0νββ$ half-life, thereby providing stringent constraints on the relevant LRSM parameters, complementary to the indirect constraints derived from lepton flavor violating observables. In a simplified scenario parametrized by a single light-heavy neutrino mixing, the inclusion of the $λ$ and $η$ contributions leads to significantly improved $0νββ$ constraints on the light-heavy neutrino mixing as well as on the $W_L-W_R$ mixing parameters. We also present a concrete TeV-scale LRSM setup, where the mixing effects are manifestly enhanced, and discuss the interplay between $0νββ$, lepton flavor violation and electric dipole moment constraints.

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arXiv2014

Evidence for leptonic CP phase from NO$ν$A, T2K and ICAL: A chronological progression

We study the synergy between the long-baseline (LBL) experiments NO$ν$A and T2K and the atmospheric neutrino experiment ICAL@INO for obtaining the first hint of CP violation in the lepton sector. We also discuss how precisely the leptonic CP phase ($δ_{CP}$) can be measured by these experiments. The CP sensitivity is first described at the level of oscillation probabilities, discussing its dependence on the parameters -- $θ_{13}$, mass hierarchy and $θ_{23}$. In particular, we discuss how the precise knowledge or lack thereof of these parameters can affect the CP sensitivity of LBL experiments. We follow a staged approach and analyze the $δ_{CP}$ sensitivity that can be achieved at different points of time over the next 15 years from these LBL experiments alone and/or in conjunction with ICAL@INO. We find that the CP sensitivity of NO$ν$A/T2K is enhanced due to the synergies between the different channels and between the two experiments. On the other hand the lack of knowledge of hierarchy and octant makes the CP sensitivity poorer for some parameter ranges. Addition of ICAL data to T2K and NO$ν$A can exclude these spurious wrong-hierarchy and/or wrong-octant solutions and cause a significant increase in the range of $δ_{CP}$ values for which a hint of CP violation can be achieved. In fact in parameter regions unfavourable for NO$ν$A/T2K, we may get the first evidence of CP violation by adding the ICAL data to these. Similarly the precision with which $δ_{CP}$ can be measured also improves with inclusion of ICAL data.

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arXiv2014

Vacuum Stability Constraints on the Minimal Singlet TeV Seesaw Model

We consider the minimal seesaw model in which two gauge singlet right handed neutrinos with opposite lepton numbers are added to the Standard Model. In this model, the smallness of the neutrino mass is explained by the tiny lepton number violating coupling between one of the singlets with the standard left-handed neutrinos. This allows one to have the right handed neutrino mass at the TeV scale as well as appreciable mixing between the light and heavy states. This model is fully reconstructible in terms of the neutrino oscillation parameters apart from the overall coupling strengths. We show that the overall coupling strength $y_ν$ for the Dirac type coupling between the left handed neutrino and one of the singlets can be restricted by consideration of the (meta)stability bounds on the electroweak vacuum. In this scenario the lepton flavor violating decays of charged leptons can be appreciable which can put further constraint on $y_ν$, for right-handed neutrinos at TeV scale. We discuss the combined constraints on $y_ν$ for this scenario from the process $μ\rightarrow e γ$ and from the consideration of vacuum (meta)stability constraints on the Higgs self coupling. We also briefly discuss the implications for neutrinoless double beta decay and possible signatures of the model that can be expected at colliders.

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arXiv2014

Synergies between neutrino oscillation experiments: An `adequate' configuration for LBNO

Determination of the neutrino mass hierarchy, octant of the mixing angle theta_{23} and the CP violating phase delta_{CP} are the unsolved problems in neutrino oscillation physics today. In this paper our aim is to obtain the minimum exposure required for the proposed Long Baseline Neutrino Oscillation (LBNO) experiment to determine the above unknowns. We emphasize on the advantage of exploiting the synergies offered by the existing and upcoming long-baseline and atmospheric neutrino experiments in economising the LBNO configuration. In particular, we do a combined analysis for LBNO, T2K, NOvA and INO. We consider three prospective LBNO setups -- CERN-Pyhasalmi (2290 km), CERN-Slanic (1500 km) and CERN-Frejus (130 km) and evaluate the adequate exposure required in each case. Our analysis shows that the exposure required from LBNO can be reduced considerably due to the synergies arising from the inclusion of the other experiments.

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arXiv2013

Can atmospheric neutrino experiments provide the first hint of leptonic CP violation?

The measurement of a non-zero value of the 1-3 mixing angle has paved the way for the determination of leptonic CP violation. However the current generation long-baseline experiments T2K and NOvA have limited sensitivity to delta_{CP}. In this paper we show for the first time, the significance of that atmospheric neutrino experiments in providing the first hint of CP violation in conjunction with T2K and NOvA. In particular, we find that adding atmospheric neutrino data from the ICAL detector at the India-based Neutrino Observatory (INO) to T2K and NOvA results in a two-fold increase in the range of delta_{CP} values for which a 2 sigma hint of CP violation can be obtained. In fact in the parameter region unfavorable for the latter experiments, the first signature of CP violation may well come from the inclusion of atmospheric neutrino data.

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arXiv2013

Threshold effects and renormalization group evolution of neutrino parameters in TeV scale seesaw models

We consider the threshold effect on the renormalization group (RG) evolution of the neutrino masses and mixing angles in TeV scale seesaw models. We obtain the analytic expressions using the factorization method in presence of threshold effects. We also perform numerical study of RG effects in two specific low scale seesaw models following the bottom-up approach and ascertain the role of seesaw thresholds in altering the values of masses and mixing angles during RG evolution.

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arXiv2013

Implication of a vanishing element in 3+1 Scenario

In this paper we study the phenomenological implications of the one zero textures of low energy neutrino mass matrices in presence of a sterile neutrino. We consider the 3+1 scheme and use the results from global fit for short baseline neutrino oscillation data which provides the bounds on the three additional mixing angles. We find that the mass matrix elements $m_{αβ}$ ($α, β= e, μ, τ$) involving only the active states can assume vanishing values in the allowed parameter space for all the mass spectrum. Among the mass matrix elements connecting the active and sterile states, $m_{es}$ and $m_{μs}$ can become small only for the quasi-degenerate neutrinos. The element $m_{τs}$ on the other hand can vanish even for lower values of masses since the 3-4 mixing angle only has an upper bound from current data. The mass matrix element ($m_{ss}$) involving only the sterile state stays $\sim$ $\mathcal{O}$(1) eV in the whole parameter region. We study the possible correlations between the sterile mixing angles and the Majorana phases to give a zero element in the mass matrix.

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arXiv2013

Constraining Neutrino Mass from Neutrinoless Double Beta Decay

We re-analyze the compatibility of the claimed observation of neutrinoless double beta decay ($0νββ$) in $^{76}$Ge with the new limits on the half-life of $^{136}$Xe from EXO-200 and KamLAND-Zen. Including recent calculations of the nuclear matrix elements (NMEs), we show that while the claim in $^{76}$Ge is still compatible with the individual limits from $^{136}$Xe, it is inconsistent with the KamLAND-Zen+EXO-200 combined limit for all but one NME calculations. After imposing the most stringent upper limit on the sum of light neutrino masses from Planck, we find that the canonical light neutrino contribution cannot satisfy the claimed $0νββ$ signature or saturate the current limit, irrespective of the NME uncertainties. However, inclusion of the heavy neutrino contributions, arising naturally in TeV-scale Left-Right symmetric models, can saturate the current limit of $0νββ$. In a type-II seesaw framework, this imposes a lower limit on the lightest neutrino mass. Depending on the mass hierarchy, we obtain this limit to be in the range of 0.07 - 4 meV for a typical choice of the right-handed (RH) gauge boson and RH neutrino masses relevant for their collider searches. Using the $0νββ$ bounds, we also derive correlated constraints in the RH sector, complimentary to those from the LHC.

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arXiv2013

Octant sensitivity for large theta(13) in atmospheric and long baseline neutrino experiments

One of the unknown parameters in neutrino oscillations is the octant of the mixing angle theta_{23}. In this paper, we discuss the possibility of determining the octant of theta_{23} in the long baseline experiments T2K and NOvA in conjunction with future atmospheric neutrino detectors, in light of non-zero value of theta_{13} measured by reactor experiments. We consider two detector technologies for atmospheric neutrinos - magnetized iron calorimeter and non-magnetized Liquid Argon Time Projection Chamber. We present the octant sensitivity for T2K/NOvA and atmospheric neutrino experiments separately as well as combined. For the long baseline experiments, a precise measurement of theta_{13}, which can exclude degenerate solutions in the wrong octant, increases the sensitivity drastically. For theta_{23} = 39^o and sin^2 2 theta_{13} = 0.1, at least ~2 sigma sensitivity can be achieved by T2K+NOvA for all values of delta_{CP} for both normal and inverted hierarchy. For atmospheric neutrinos, the moderately large value of theta_{13} measured in the reactor experiments is conducive to octant sensitivity because of enhanced matter effects. A magnetized iron detector can give a 2 sigma octant sensitivity for 500 kT yr exposure for theta_{23} = 39^o, delta_{CP} = 0 and normal hierarchy. This increases to 3 sigma for both hierarchies by combining with T2K+NOvA. This is due to a preference of different theta_{23} values at the minimum chi^2 by T2K/NOvA and atmospheric neutrino experiments. A Liquid Argon detector for atmospheric neutrinos with the same exposure can give higher octant sensitivity, due to the interplay of muon and electron contributions and superior resolutions. We obtain a ~3 sigma sensitivity for theta_{23} = 39^o for normal hierarchy. This increases to > ~4 sigma for all values of delta_{CP} if combined with T2K+NOvA. For inverted hierarchy the combined sensitivity is ~3 sigma.

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arXiv2013

Generation of Neutrino mass from new physics at TeV scale and Multi-lepton Signatures at the LHC

In this paper we consider generation of naturally small neutrino masses from a dimension-7 operator. Such a term can arise in presence of a scalar quadruplet and a pair of vector-like fermion triplets and enables one to obtain small neutrino masses through TeV scale linear seesaw mechanism. We study the phenomenology of the charged scalars of this model, in particular, the multi-lepton signatures at the Large Hadron Collider. Of special importance are the presence of the same-sign-tri-lepton signatures originating from the triply-charged scalars. The Standard Model background for such processes is small and hence this is considered as smoking gun signal of new physics. We also looked for events with three, four, five and six-leptons which have negligible contamination from the Standard Model. We further point out the spectacular lepton flavour violating four-lepton signal which can be the hallmark for these type of models. We also compute the added contributions in the rate for the Standard Model Higgs decaying to two photons via the charged scalars in this model.

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arXiv2012

Two Zero Mass Matrices and Sterile Neutrinos

Recent experimental data is indicative of the existence of sterile neutrinos. The minimal scheme that can account for the data and is consistent with cosmological observations is the 3+1 picture which consists of three predominantly active and one predominantly sterile neutrino with the fourth neutrino being heavier than the other three. Within this scheme there are two possibilities depending on whether the three light states obey normal or inverted hierarchy. In this paper we consider the two zero textures of the low energy neutrino mass matrix in presence of one additional sterile neutrino. We find that among 45 possible two zero textures for this case, 15 are consistent with all current observations. Remarkably, these correspond to the two-zero textures of a three active neutrino mass matrix. We discuss the mass spectrum and the parameter correlations that we find in the various textures. We also present the effective mass governing neutrinoless double beta decay as a function of the lowest mass.

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arXiv2012

Optimization of the baseline and the parent muon energy for a low energy neutrino factory

We discuss the optimal setup for a low energy neutrino factory in order to achieve a 5σ-discovery of a nonzero mixing angle θ_{13}, a nonzero CP phase δ_{CP}, and the mass hierarchy. We explore parent muon energies in the range 5--16 GeV, and baselines in the range 500--5000 km. We present the results in terms of the reach in sin^2θ_{13}, emphasizing the dependence of the optimal baseline on the true value of δ_{CP}. We show that the sensitivity of a given setup typically increases with parent muon energy, reaching saturation for higher energies. The saturation energy is larger for longer baselines; we present an estimate of this dependence. In the light of the recent indications of a large θ_{13}, we also determine how these preferences would change if indeed a large θ_{13} is confirmed. In such a case, the baselines ~2500 km (~1500 km) may be expected to lead to hierarchy determination (δ_{CP} discovery) with the minimum exposure.

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arXiv2012

Neutrino mass hierarchy and octant determination with atmospheric neutrinos

The recent discovery by the Daya-Bay and RENO experiments, that θ_{13} is nonzero and relatively large, significantly impacts existing experiments and the planning of future facilities. In many scenarios, the nonzero value of θ_{13} implies that θ_{23} is likely to be different from π/4. Additionally, large detectors will be sensitive to matter effects on the oscillations of atmospheric neutrinos, making it possible to determine the neutrino mass hierarchy and the octant of θ_{23}. We show that a 50 kT magnetized liquid argon neutrino detector can ascertain the mass hierarchy with a significance larger than 4 sigma with moderate exposure times, and the octant at the level of 2-3 sigma with greater exposure.

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arXiv2012

Neutrinoless double-$β$ decay in TeV scale Left-Right symmetric models

In this paper we study in detail the neutrinoless double beta decay in left-right symmetric models with right-handed gauge bosons at TeV scale which is within the presently accessible reach of colliders. We discuss the different diagrams that can contribute to this process and identify the dominant ones for the case where the right-handed neutrino is also at the TeV scale. We calculate the contribution to the effective mass governing neutrinoless double beta decay assuming type-I, and type-II dominance and discuss what are the changes in the effective mass due to the additional contributions. We also discuss the effect of the recent Daya-Bay and RENO measurements on $\sin^2θ_{13}$ on the effective mass in different scenarios.

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arXiv2010

An SO(10) model with adjoint fermions for double seesaw neutrino masses

An $SO(10)$ model where the $10_H$ and $120_H$ representations are used for generating fermion masses is quite predictive, though due to the absence of $SU(2)_{L,R}$ triplet/singlet fields it cannot give rise to neutrino masses through the usual type-I or type-II seesaw mechanisms. In this paper for neutrino masses we propose an extension of such an $SO(10)$ model by adding fermions in the adjoint representation (${45}_F$) and a symmetry breaking scalar $\bar{16}_H$. The $\bar{16}_H$ couples the adjoint fermions to the standard fermions in ${16}_F$ and induces neutrino masses through the `double seesaw' mechanism. In order to enhance the predictivity of the model we impose $μ-τ$ flavour symmetry on the Yukawa matrices for $10_H$ and $\bar{16}_H$ whereas for the $120_H$ it is assumed to be antisymmetric. We discuss the conditions that the mass matrices must obey so that the model can reproduce the tri-bimaximal mixing pattern.

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