arXiv subjects

S Uma Sankar

Publications and source records attributed to S Uma Sankar.

13 recordsLinked to original sources

arXiv2021

New physics in $b\rightarrow se^+e^-$: A model independent analysis

The lepton universality violating flavor ratios $R_K/R_{K^*}$ indicate new physics either in $b \to s μ^+ μ^-$ or in $b \to s e^+ e^-$ or in both. If the new physics is only $b \to s e^+ e^-$ transition, the corresponding new physics operators, in principle, can have any Lorentz structure. In this work, we perform a model independent analysis of new physics only in $b \to se^+e^-$ decay by considering effective operators either one at a time or two similar operators at a time. We include all the measurements in $b\rightarrow se^+e^-$ sector along with $R_K/R_{K^*}$ in our analysis. We show that various new physics scenarios with vector/axial-vector operators can account for $R_K/R_{K^*}$ data but those with scalar/pseudoscalar operators and with tensor operators can not. We also show that the azimuthal angular observable $P_1$ in $B \to K^* e^+ e^-$ decay is most suited to discriminate between the different allowed solutions.

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arXiv2019

Solutions to $R_D$-$R_{D^*}$ in light of Belle 2019 data

Earlier this year, the Belle collaboration presented their new measurements of $R_D$ and $R_{D^*}$ using a new method. These measurements are consistent with the Standard Model predictions, whereas the global averages of the earlier measurements had a $4.1σ$ discrepancy. With the inclusion of the new data in the global averages, the discrepancy comes down to $3.1σ$. In this work, we study the study the new physics solutions to the $R_D$-$R_{D^*}$ anomaly allowed by the reduction in the discrepancy. Among the four fermion operators, which arise through a single particle exchange, only the $(V-A)$ operator solution survives. We found three additional solutions with two dis-similar operators. The branching ratio of $B_c\rightarrow τ\,\barν$ is powerful discriminant between these four allowed solutions.

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arXiv2019

Resolving $R_D$ and $R_{D^*}$ anomalies

The current world averages of the ratios $R_{D^{(*)}}$ are about $4σ$ away from their Standard Model prediction. These measurements indicate towards the violation of lepton flavor universality in $b\rightarrow c\,l\,\barν$ decay. The different new physics operators, which can explain the $R_{D^{(*)}}$ measurements, have been identified previously. We show that a simultaneous measurement of the polarization fractions of $τ$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*τ\barν$ decay can distinguish all the new physics amplitudes and hence uniquely identify the Lorentz structure of new physics.

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arXiv2018

New physics solutions for $R_D$ and $R_{D^*}$

Recent measurements of $R_{D^*}$ have reduced tension with the Standard Model prediction. Taking all the present data into account, we obtain the values of the Wilson coefficients of each new physics four-fermion operator of a given Lorentz structure. We find that the combined data rule out most of the solutions based on scalar/pseudoscalar operators. By studying the inter-relations between different solutions, we find that there are only four allowed solutions, which are based on operators with $(V-A)$, linear combination of $(V-A)$ and $(V+A)$, tensor and linear combination of scalar/pseudoscalar and tensor structure. We demonstrate that the need for new physics is driven by those measurement of $R_D$ and $R_{D^*}$ where the $τ$ lepton is not studied. Further, we show that new physics only in $b\rightarrow c\,μ\,\barν$ is not compatible with the full set of observables in the decays $B\rightarrow Dl\barν$ and $B\rightarrow D^*l\barν$.

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arXiv2018

Resolution of $R_D$/$R_{D^*}$ puzzle

One of the exciting results in flavor physics in recent times is the $R_D$/$R_{D^*}$ puzzle. The measurements of these flavor ratios performed by the B-factory experiments, BaBar and Belle, and the LHCb experiment are about $4σ$ away from the Standard Model expectation. These measurements indicate that the mechanism of $b\rightarrow cτ\barν$ decay is not identical to that of $b\rightarrow c(μ/e)\barν$. This charge lepton universality violation is particularly intriguing because these decays occur at tree level in the Standard Model. In particular, we expect a moderately large new physics contribution to $b\rightarrow cτ\barν$. The different types of new physics amplitudes, which can explain the $R_D$/$R_{D^*}$ puzzle, have been identified previously. In this letter, we show that the polarization fractions of $τ$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*τ\barν$ decay have the capability to uniquely identify the Lorentz structure of the new physics. A measurement of these four observables will lead to such an identification.

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arXiv2017

D* polarization as a probe to discriminate new physics in B --> D* tau nubar

The confirmation of excess in $R_{D^*}$ at the LHCb is an indication of lepton flavor non-universality. Various different new physics operators and their coupling strengths, which provide a good fit to $R_D$, $R_{D^*}$ and $q^2$ spectra, were identfied previously. In this work, we try to find angular observables in $\bar{B} \to D^* τ\barν$ which enable us to distinguish between these new physics operators. We find that the $D^*$ polarization fraction $f_L(q^2)$ is a good discriminant of scalar and tensor new physics operators. The change in $\langle f_L(q^2) \rangle$, induced by scalar and tensor operators, is about three times larger than the expected uncertainty in the upcoming Belle measurement.

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arXiv2009

Mass Hierarchy Determination for $θ_{13}=0$ and Atmospheric Neutrinos

We examine the possibility of determining the neutrino mass hierarchy in the limit θ_{13} = 0 using atmospheric neutrinos as the source. In this limit, in which θ_{13} driven matter effects are absent, independent measurements of Δ_{31} and Δ_{32} can, in principle, lead to hierarchy determination. Since the difference between these two is Δ_{21}, one needs an experimental arrangement where Δ_{21} L/E \gtrsim 1 can be achieved. This condition can be satisfied by atmospheric neutrinos since they have a large range of energies and baselines. In spite of this, we find that hierarchy determination in the θ_{13}=0 limit with atmospheric neutrinos is not a realistic possibility, even in conjunction with an apparently synergistic beam experiment like T2K or NOvA. We discuss the reasons for this, and also in the process clarify the conditions that must be satisfied in general for hierarchy determination if θ_{13} = 0.

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arXiv2008

Resolving the Mass Hierarchy with Atmospheric Neutrinos using a Liquid Argon Detector

We explore the potential offered by large-mass Liquid Argon detectors for determination of the sign of Delta m_{31}^2, or the neutrino mass hierarchy, through interactions of atmospheric neutrinos. We give results for a 100 kT sized magnetized detector which provides separate sensitivity to ν_μ, \barν_μand, over a limited energy range, to ν_e, \barν_e.We also discuss the sensitivity for the unmagnetized version of such a detector. After including the effect of smearing in neutrino energy and direction and incorporating the relevant statistical,theoretical and systematic errors, we perform a binned χ^2 analysis of simulated data. The χ^2 is marginalized over the presently allowed ranges of neutrino parameters and determined as a function of θ_{13}. We find that such a detector offers superior capabilities for hierarchy resolution, allowing a > 4σdetermination for a 100 kT detector over a 10 year running period for values of \sin^2 2θ_{13} \ge 0.05. For an unmagnetized detector, a 2.5σhierarchy sensitivity is possible for \sin^2 2θ_{13} = 0.04.

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arXiv2007

Mass Hierarchy Determination via future Atmospheric Neutrino Detectors

We study the problem of determination of the sign of Delta m^2_{31}, or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types : (a) Megaton sized water Cerenkov detectors, which can measure the survival rates of nu_μ+ \barν_μand nu_e + \barν_e and (b) 100 kton sized magnetized iron detectors, which can measure the survival rates of ν_μand \barν_μ. For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from P_{μe}, P_{μμ} and P_{ee}, leading to an appreciable sensitivity to the hierarchy. We do a binned χ^2 analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the χ^2 over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis a vis the hierarchy determination.

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arXiv2006

Possible CPT Violation from Planck Scale Effects

At present there is good agreement between the neutrino mass-squared difference determined from the solar neutrino data and the anti-neutrino mass-squared difference determined from the KamLAND reactor anti-neutrino experiment. However, the central values of the two cases differ from each other by about $10^{-5}$ eV$^2$. An improvement in the accuracy of both the solar neutrino experiments and reactor anti-neutrino experiments can establish the existence of a non-zero difference between neutrino and anti-neutrino mass-squared differences and provide a signal for CPT violation. In this paper, we show how such a difference can arise through the CPT violating neutrino mass terms from Planck scale physics.

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arXiv2006

Earth Matter Effects at Very Long Baselines and the Neutrino Mass Hierarchy

We study matter effects which arise in the muon neutrino oscillation and survival probabilities relevant to atmospheric neutrino and very long baseline beam experiments. The inter-relations between the three probabilities P_{μe}, P_{μτ} and P_{μμ} are examined. It is shown that large and observable sensitivity to the neutrino mass hierarchy can be present in P_{μμ} and P_{μτ}. We emphasize that at baselines of > 7000 Km, matter effects in P_{μτ} can be large under certain conditions. The muon survival rates in experiments with very long baselines thus depend on matter effects in both P_{μτ} and P_{μe}. We indicate where these effects are sensitive to θ_{13}, and identify ranges of E and L where the event rates increase with decreasing θ_{13}, providing a handle to probe small θ_{13}. The effect of parameter degeneracies in the three probabilities at these baselines and energies is studied in detail. Realistic event rate calculations are performed for a charge discriminating 100 kT iron calorimeter which demonstrate the possibility of realising the goal of determining the neutrino mass hierarchy using atmospheric neutrinos. It is shown that a careful selection of energy and baseline ranges is necessary in order to obtain a statistically significant signal, and that the effects are largest in bins where matter effects in both P_{μe} and P_{μτ} combine constructively. Under these conditions, upto a 4σsignal for matter effects is possible (for Δ_{31}>0) within a timescale appreciably shorter than the one anticipated for neutrino factories.

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arXiv2005

Probing the Neutrino Mass Hierarchy via Atmospheric $ν_μ+ \bar ν_μ$ Survival Rates in Megaton Water Cerenkov Detectors

The neutrino mass hierarchy, presently unknown, is a powerful discriminator among various classes of unification theories. We show that the $ν_μ + \barν_μ$ survival rate in atmospheric events can provide a novel method of determining the hierarchy in megaton water Cerenkov detectors. For pathlength and energy ranges relevant to atmospheric neutrinos,this rate obtains significant matter sensitive variations not only from resonant matter effects in $P_{μe}$ but also from those in $P_{μτ}$. We calculate the expected muon event rates in the case of matter oscillations with both natural and inverted hierarchy.We identify the energy and pathlength ranges for which resonant matter effects can lead to observable differences between the above two cases. We also estimate the exposure time required to observe this difference and determine the sign of $Δ_{31}$ in a statistically significant manner.

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arXiv2004

Atmospheric Neutrinos as a Probe of CPT Violation

We show that atmospheric neutrinos can provide a sensitive and robust probe of CPT violation (CPTV). We perform realistic event-rate calculations and study the variations of the ratio of total muon to antimuon survival rates with $L/E$ and $L$ ($L$ $\equiv$ baseline length, $E$ $\equiv$ neutrino energy) in a detector capable of identifying the muon charge. We demonstrate that measurements of these ratios when coupled with the significant $L$ and $E$ range which characterizes the atmospheric neutrino spectrum provides a method of both detecting the presence of such violations and putting bounds on them which compare very favourably with those possible from a future neutrino factory.

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