arXiv subjects

S. Uma Sankar

Publications and source records attributed to S. Uma Sankar.

At least 37 records · Page 2Linked to original sources

arXiv2019

A global fit to $b\rightarrow cτ\barν$ anomalies after Moriond 2019

At Moriond 2019, Belle collaboration announced their new measurements on $R_D$ and $R_{D^*}$ which are in agreement with their Standard Model (SM) predictions within $1.2σ$. After inclusion of these measurements, the discrepancy between the world averages and the SM predictions of $R_D$-$R_{D^*}$ comes down from $4.1σ$ to $3.1σ$. Here we do a global fit by taking all measurements in $b\rightarrow cτ\barν$ transition. We find that there are seven allowed new physics solutions, each with different Lorentz structure. Further we show that it is possible to distinguish between them by means of the five observables: the $τ$ polarization fraction in the decay $B \rightarrow D τ\barν$, the precision measurement of $R_D$, the forward-backward asymmetries in the decays $B \rightarrow (D,D^*) τ\barν$ and the branching ratio of $B_c\rightarrow τ\barν$.

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arXiv2019

A quantum information theoretic quantity sensitive to the neutrino mass-hierarchy

In this work, we derive a quantum information theoretic quantity similar to the Leggett-Garg inequality, which can be defined in terms of neutrino transition probabilities. For the case of $ν_μ\to ν_e/\barν_μ\to\barν_e$ transitions, this quantity is sensitive to CP violating effects as well as the neutrino mass-hierarchy, namely which neutrino mass eigenstate is heavier than the other ones. The violation of the inequality for this quantity shows an interesting dependence on mass-hierarchy. For normal (inverted) mass-hierarchy, it is significant for $ν_μ\to ν_e$ ($\barν_μ\to \barν_e$) transitions. This is applied to the two ongoing accelerator experiments T$2$K and NO$ν$A as well as the future experiment DUNE.

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arXiv2019

Hadron energy estimation from Atmospheric neutrino events

The ICAL at INO is designed to mainly observe the muons produced in the charged current interactions of atmospheric muon neutrinos and anti-neutrinos. The track of the muon is reconstructed using the hits they produce in the detector. From this track, the charge, the energy and the direction of the muon are estimated, which are used to do oscillation physics analysis. In a large fraction of events, a number of hadrons are also produced in addition to the muons. The charged hadrons also leave hits in the detector which can be utilized to estimate the hadron energy. In this work, we study the relation between hadron hits, defined to be the difference between the total number of hits and the muon track hits, and the hadron energy. We find that a non-negligible number of baryons are produced in atmospheric neutrino interactions. For E(had) < 5 GeV, almost all the hadron energy is carried by these baryons. Finally, we formulate a procedure by which the hadron energy can be estimated from the number of hadron hits.

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arXiv2019

Leggett-Garg inequality in the context of three flavour neutrino oscillation

The present work is devoted to the characterization of the Leggett-Garg inequality for three-flavoured neutrino oscillations in presence of both matter and Charge-Conjugation and Parity violating (CP) effects. This study complements and completes the recent one put forward in [17] by relaxing the stationary condition. At variance with the latter case, the LGI contains interference terms which cannot be expressed in terms of experimentally measurable quantities, thus drawing a clear-cut distinction between the two scenarios, as well as highlighting the role of the stationary assumption on such systems. We find that the additional terms are small for high energy neutrino beam compared to the maximum value attained by the Leggett-Garg parameter.

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arXiv2018

Understanding the degeneracies in NO$ν$A data

The combined analysis of $ν_μ$ disappearance and $ν_e$ appearance data of NO$ν$A experiment leads to three nearly degenerate solutions. This degeneracy can be understood in terms of deviations in $ν_e$ appearance signal, caused by unknown effects, with respect to the signal expected for a reference set of oscillations parameters. We define the reference set to be vacuum oscillations in the limit of maximal $θ_{23}$ and no CP-violation. We then calculate the deviations induced in the $ν_e$ appearance signal event rate by three unknown effects: (a) matter effects, due to normal or inverted hierarchy (b) octant effects, due to $θ_{23}$ being in higher or lower octant and (c) CP-violation, whether $δ_{CP} \sim - π/2$ or $δ_{CP} \sim π/2$. We find that the deviation caused by each of these effects is the same for NO$ν$A. The observed number of $ν_e$ events in NO$ν$A is equivalent to the increase caused by one of the effects. Therefore, the observed number of $ν_e$ appearance events of NO$ν$A is the net result of the increase caused by two of the unknown effects and the decrease caused by the third. Thus we get the three degenerate solutions. We also find that further data by NO$ν$A can not distinguish between these degenerate solutions but addition of one year of neutrino run of DUNE can make a distinction between all three solutions. The distinction between the two NH solutions and the IH solution becomes possible because of the larger matter effect in DUNE. The distinction between the two NH solutions with different octants is a result of the synergy between the anti-neutrino data of NO$ν$A and the neutrino data of DUNE.

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arXiv2018

Precision neutrino data confronts $μ\leftrightarrowτ$ symmetry

Neutrino oscillation data indicate that $θ_{23}$ is close to $π/4$ and $θ_{13}$ is very small. A simple $μ\leftrightarrowτ$ exchange symmetry of the neutrino mass matrix predicts $θ_{23}=-π/4$ and $θ_{13}=0$. Since the experimental measurements differ from these predictions, this symmetry is obviously broken. This breaking is given by two parameters: $\varepsilon_1$ parametrizing the inequality bewteen $12$ and $13$ elements and $\varepsilon_2$ parametrizing the inequality bewteen $22$ and $33$ elements. We show that the magnitude of $θ_{13}$ is essentially controlled by $\varepsilon_1$ whereas the deviation of $θ_{23}$ from maximality is controlled by $\varepsilon_2$. The measured value of $θ_{13}$ requires $μ\leftrightarrowτ$ symmetry to be badly broken for both normal hierarchy and inverted hierarchy, though the level of breaking depends sensitively on the hierarchy. In this paper we obtain constraints on the parameters of neutrino mass matrix, including the symmetry breaking parameters, using the precision oscillation data. We find that this precision data constrains all elements of neutrino mass matrix to be in very narrow ranges. We also consider $μ\leftrightarrow -τ$ exchange symmetry in the case of inverted hierarchy and find that it provides an explanation of neutrino mixing angles with some fine-tuning.

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arXiv2017

New Physics in $b \rightarrow s μ^+ μ^-$: Distinguishing Models through CP-Violating Effects

At present, there are several measurements of $B$ decays that exhibit discrepancies with the predictions of the SM, and suggest the presence of new physics (NP) in $b \to s μ^+ μ^-$ transitions. Many NP models have been proposed as explanations. These involve the tree-level exchange of a leptoquark (LQ) or a flavor-changing $Z'$ boson. In this paper we examine whether it is possible to distinguish the various models via CP-violating effects in $B \to K^{(*)} μ^+ μ^-$. Using fits to the data, we find the following results. Of all possible LQ models, only three can explain the data, and these are all equivalent as far as $b \to s μ^+ μ^-$ processes are concerned. In this single LQ model, the weak phase of the coupling can be large, leading to some sizeable CP asymmetries in $B \to K^{(*)} μ^+ μ^-$. There is a spectrum of $Z'$ models; the key parameter is $g_L^{μμ}$, which describes the strength of the $Z'$ coupling to $μ^+μ^-$. If $g_L^{μμ}$ is small (large), the constraints from $B^0_s$-${\bar B}^0_s$ mixing are stringent (weak), leading to a small (large) value of the NP weak phase, and corresponding small (large) CP asymmetries. We therefore find that the measurement of CP-violating asymmetries in $B \to K^{(*)} μ^+ μ^-$ can indeed distinguish among NP $b \to s μ^+ μ^-$ models

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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.

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arXiv2016

Quantum correlations in terms of neutrino oscillation probabilities

Neutrino oscillations provide evidence for the mode entanglement of neutrino mass eigenstates in a given flavour eigenstate. Given this mode entanglement, it is pertinent to consider the relation between the oscillation probabilities and other quantum correlations. In this work, we show that all the well-known quantum correlations, such as the Bell's inequality, are directly related to the neutrino oscillation probabilities. The results of the neutrino oscillation experiments, which measure the neutrino survival probability to be less than unity, imply Bell's inequality violation.

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arXiv2016

Flavor signatures of isosinglet vector-like down quark model

We consider a model where the standard model is extended by the addition of a vector-like isosinglet down-type quark b'. We perform a chi square fit to the flavor physics data and obtain the preferred central values along with errors of all the elements of the measurable 3 X 4 quark mixing matrix. The fit indicates that all the new-physics parameters are consistent with zero and the mixing of the b' quark with the other three is constrained to be small. The current flavor physics data rules out possibility of detectable new physics signals in most of the flavor physics observables. We also investigate possible deviations in the standard model Wtb couplings and bottom quark coupling to Higgs boson. We find that these deviations are less than a percent level which is too small to be observed at the LHC with current precision.

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arXiv2016

Hierarchy sensitivity of NO$ν$A in light of T2K $ν_e$ appearance data

The $ν_e$ appearance data of T2K experiment has given a glimpse of the allowed parameters in the hierarchy-$δ_{CP}$ parameter space. In this paper, we explore how this data affects our expectations regarding the hierarchy sensitivity of the NO$ν$A experiment. For the favourable combinations of hierarchy and $δ_{CP}$, the hierarchy sensitivity of NO$ν$A is unaffected by the addition of T2K data. For the unfavourable combinations, NO$ν$A data gives degenerate solutions. Among these degenerate solutions, T2K data prefers IH and $δ_{CP}$ in the lower half plane over NH and $δ_{CP}$ in the upper half plane. Hence, addition of the T2K data to NO$ν$A creates a bias towards IH and $δ_{CP}$ in the lower half plane irrespective of what the true combination is.

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arXiv2015

Improving the hierarchy sensitivity of ICAL using neural network

Atmospheric neutrino experiments can determine the neutrino mass hierarchy for any value of $δ_{CP}$. The Iron Calorimeter (ICAL) detector at the India-based Neutrino Observatory can distinguish between the charged current interactions of $ν_μ$ and $\barν_μ$ by determining the charge of the produced muon. Hence it is particularly well suited to determine the hierarchy. The hierarchy signature is more prominent in neutrinos with energy of a few GeV and with pathlength of a few thousand kilometers, $\textit{i.e.}$ neutrinos whose direction is not close to horizontal. We use adaptive neural networks to identify such events with good efficiency and good purity. The hierarchy sensitivity, calculated from these selected events, reaches a $3 σ$ level, with a $Δχ^2$ of 9.

physics.ins-det↗

arXiv2015

Re-examining sin(2beta) and Delta m(d) from evolution of B(d) mesons with decoherence

In the time evolution of neutral meson systems, a perfect quantum coherence is usually assumed. The important quantities of the B(d) system, such as sin (2beta) and Delta m(d), are determined under this assumption. However, the meson system interacts with its environment. This interaction can lead to decoherence in the mesons even before they decay. In our formalism this decoherence is modelled by a single parameter lambda. It is desirable to re-examine the procedures of determination of sin(2beta) and Delta m(d) in meson systems with decoherence. We find that the present values of these two quantities are modulated by lambda. Re-analysis of B(d) data from B-factories and LHCb can lead to a clean determination of lambda, sin(2beta) and Delta m(d).

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arXiv2015

Non-Zero $θ_{13}$ and $δ_{CP}$ in a Neutrino Mass Model with $A_4$ Symmetry

In this paper, we consider a neutrino mass model based on $A_4$ symmetry. The spontaneous symmetry breaking in this model is chosen to obtain tribimaximal mixing in the neutrino sector. We introduce $Z_2 \times Z_2$ invariant perturbations in this model which can give rise to acceptable values of $θ_{13}$ and $δ_{CP}$. Perturbation in the charged lepton sector alone can lead to viable values of $θ_{13}$, but cannot generate $δ_{CP}$. Perturbation in the neutrino sector alone can lead to acceptable $θ_{13}$ and maximal CP violation. By adjusting the magnitudes of perturbations in both sectors, it is possible to obtain any value of $δ_{CP}$.

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arXiv2015

Energy and Direction Estimation of Neutrinos in muonless events at ICAL

In this paper, we study events without identifiable muon tracks in the Iron Calorimeter detector at the India-based Neutrino Observatory. Such events are dominated by high energy (E$_ν>$1 GeV) $ν_e$ charged current interactions, which have been studied only in a few experiments so far. The charged particles, produced in these neutrino interactions, give rise to a set of hits in the detector. We attempt to reconstruct the energy and the direction of the neutrino in such events. We study the energy distribution for a given pattern of hits of these events and find that the Landau distribution provides a good fit. % The parameters of the fit can be correlated to the energy of the neutrino. We define two kinematic variables based on the hit distribution and use them to determine the cosine of the polar angle of the neutrino direction ($\cos θ$). There is a moderate correlation between these variables and the $\cos θ$. These provide us enough information to prepare calibration charts for looking up the energy and direction of the incident neutrino.

physics.ins-det↗

arXiv2015

New-physics signals of a model with a vector-singlet up-type quark

The VuQ model involves the addition of a vector isosinglet up-type quark to the standard model. In this model the full CKM quark mixing matrix is 4 X 3. Using present flavor-physics data, we perform a fit to this full CKM matrix, looking for signals of new physics (NP). We find that the VuQ model is very strongly constrained. There are no hints of NP in the CKM matrix, and any VuQ contributions to loop-level flavor-changing b -> s, b -> d and s -> d transitions are very small. There can be significant enhancements of the branching ratios of the flavor-changing decays t -> u Z and t -> c Z, but these are still below present detection levels.

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arXiv2015

Muonless Events in ICAL at INO

The primary physics signal events in the ICAL at INO are the $ν_μ$ charged current (CC) interactions with a well defined muon track. Apart from these events, ICAL can also detect other types of neutrino interactions, i.e. the electron neutrino charged current interactions and the neutral current events. It is possible to have a dataset containing mostly $ν_e$CC events, by imposing appropriate selection cuts on the events. The $ν_μ$ CC and the neutral current events form the background to these events. This study uses the Monte Carlo generated neutrino events, to design the necessary selection cuts to obtain a $ν_e$ CC rich dataset. An optimized set of constraints are developed which balance the need for improving the purity of the sample and having a large enough event sample. Depending on the constraints used, one can obtain a neutrino data sample, with the purity of $ν_e$ events varying between 55% to 70%.

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arXiv2014

The need for an early anti-neutrino run of NOvA

The moderately large value of $\ty$, measured recently by reactor experiments, is very welcome news for the future neutrino experiments. In particular, the \nova experiment, with 3 years each of $ν$ and $\anu$ runs, will be able to determine the mass hierarchy if one of the following two favourable combinations is true: normal hierarchy with $-180^\circ \leq \dcp \leq 0$ or inverted hierarchy with $0\leq \dcp \leq 180^\circ$. In this report, we study the hierarchy reach of the first 3 years of \nova data. Since $\sin^2 2 \tz$ is measured to be non-maximal, $\tz$ can be either in the lower or higher octant. Pure $ν$ data is affected by $\ty$-hierarchy and octant-hierarchy degeneracies, which limit the hierarchy sensitivity of such data. A combination of $ν$ and $\anu$ data is not subject to these degeneracies and hence has much better hierarchy discrimination capability. We find that, with a 3 year $ν$ run, hierarchy determination is possible for only two of the four octant-hierarchy combinations. Equal 1.5 year runs in $ν$ and $\anu$ modes give good hierarchy sensitivity for all the four combinations.

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