arXiv subjects

S. Uma Sankar

Publications and source records attributed to S. Uma Sankar.

72 records · Page 4Linked to original sources

arXiv2006

New physics upper bound on the branching ratio of B_s--> l+ l- and B_s--> l+ l- gamma

We consider the most general new physics effective Lagrangian for b--> s l+ l-. We derive the upper limit on the branching ratio for the processes B_s--> l^+ l- where l=e, mu, subject to the current experimental bounds on related processes, B--> (K,K*) l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B--> (K,K*) l+ l- constrain B_s--> l+ l to be of the same order of magnitude as their respective Standard Model (SM) predictions. On the other hand, if the new physics interactions are of scalar/pseudoscalar form, B--> (K,K*) l+ l- rates do not impose any useful constraint on B_s--> l+ l- and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B_s--> l+ l- to be > 10^{-8} then the new physics giving rise to these decays has to be of the scalar/pseudoscalar form. We also consider the effect of new physics on B_s--> l+ l- gamma subject to the present experimental constraints on B--> (K,K*) l+ l- and B--> K* gamma. New physics in form scalar/pseudoscalar, which makes a very large contribution to B_s--> l+ l-, makes no contribution at all to B_s--> l+ l- gamma due to angular momentum conservation. New Physics in the form of vector/axial-vector operators is constrained by the data on B--> (K,K*) l+ l- and new physics in the form of tensor/pseudo-tensor is constrained by the data on B--> K* gamma. In both cases, enhancement of B_s--> l+ l- gamma much beyond the SM expectation is impossible. In conclusion, present data on B-->(K,K*) transitions allow for large B(B_s--> l+ l-) but do not allow B(B_s--> l+ l- gamma) to be much larger than its SM expectation.

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arXiv2006

New physics upper bound on the branching ratio of B_s --> l+ l- gamma

We consider the effect of new physics on the branching ratio of B_s--> l+ l- gamma where l=e,mu. If the new physics is of the form scalar/pseudoscalar, then it makes no contribution to B_s--> l+ l- gamma, unlike in the case of B_s--> l+ l-, where it can potentially make a very large contribution. If the new physics is in the form of vector/axial-vector operators, then present data on B-->(K,K^*)l+ l-, does not allow a large enhancement for B_s--> l+ l- gamma. If the new physics is in the form of tensor/pseudotensor operators, then the data on B-->(K,K^*)l+ l- gives no useful constraint but the data on B-->K^* gamma does. Here again, a large enhancement of B(B_s--> l+ l- gamma), much beyond the Standard Model expectation, is not possible. Hence, we conclude that the present data on b-->s transitions allow a large boost in B_s--> l+ l- but not in B_s--> l+ l- gamma.

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arXiv2005

Heavy neutrino mass hierarchy from leptogenesis in left-right symmetric models with spontaneous CP-violation

We consider left-right symmetric model with spontaneous CP-violation. The Lagrangian of this model is CP invariant and the Yukawa couplings are real. Due to spontaneous breaking of the gauge symmetry, some of the neutral Higgses acquire complex vacuum expectation values, which lead to CP-violation. In the model considered here, we identify the neutrino Dirac mass matrix with that of Fritzsch type charged lepton mass matrix. We assume a hierarchical spectrum of the right handed neutrino masses and derive a bound on this hierarchy by assuming that the decay of the lightest right handed neutrino produces the baryon asymmetry via the leptogenesis route. It is shown that the mass hierarchy we obtain is compatible with the current neutrino oscillation data.

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arXiv2005

New physics upper bound on the branching ratio of B_s --> l+ l-

We consider the most general new physics effective Lagrangian for b --> s l+ l-. We derive the upper limit on the branching ratio for the processes B_s --> l+ l- where l=e, mu, subject to the current experimental bounds on related processes, B --> K l+ l- and B --> K* l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B --> (K,K*) l+ l- constrain B(B_s --> l+ l-) to be of the same order of magnitude as their respective Standard Model predictions. On the other hand, if the new physics interactions are of scalar/pseudo-scalar form, B --> (K,K*) l+ l- rates do not impose any constraint on B_s --> l+ l- and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B(B_s --> l+ l-) to be > 10^{-8} then the new physics giving rise to these decays has to be of the scalar/pseudo-scalar form.

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arXiv2005

Do two flavour oscillations explain both KamLAND data and the Solar Neutrino Spectrum?

The recent measurement of Delta_{sol} by the KamLAND experiment with very small errors, makes definitive predictions for the energy dependence of the solar neutrino survival probability P_{ee}. We fix Delta_{sol} to be the KamLAND best fit value of 8*10^{-5} eV^2 and study the energy dependence of P_{ee} for solar neutrinos in the framework of two flavour oscillations and also of three flavour oscillations. For the case of two flavour oscillations, P_{ee} has a measurable slope in the 5-8 MeV range but the solar spectrum measurements in this range find P_{ee} to be flat. The predicted values of P_{ee}, even for the best fit value of theta_{sol}, differ by 2 to 3 sigma from the Super-K measured values in each of the three energy bins of the 5-8 MeV range. If future measurements of solar neutrinos by Super-K and SNO find a flat spectrum with reduced error bars (by a factor of 2), it will imply that two flavour oscillations can no longer explain both KamLAND data and the solar spectrum. However a flat solar neutrino spectrum and the Delta_{sol} measured by KamLAND can be reconciled in a three flavour oscillation framework with a moderate value of theta_{13} approx 13 degrees.

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arXiv2005

Bounds on neutrino masses from leptogenesis in type-II see-saw models

The presence of the triplet $Δ_{L}$ in left-right symmetric theories leads to type-II see-saw mechanism for the neutrino masses. In these models, assuming a normal mass hierarchy for the heavy Majorana neutrinos, we derive a lower bound on the mass of the lightest of heavy Majorana neutrino from the leptogenesis constraint. From this bound we establish a consistent picture for the hierarchy of heavy Majorana neutrinos in a class of left right symmetric models in which we identify the neutrino Dirac mass matrix with that of Fritzsch type charged lepton mass matrix. It is shown that these values are compatible with the current neutrino oscillation data.

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arXiv2004

Large Matter Effects in ${ν_μ\to ν_τ}$ Oscillations

We show that matter effects change the ${\rm {ν_μ\to ν_τ}}$ oscillation probability by as much as 70% for certain ranges of energies and pathlengths. Consequently, the ${\rm {ν_μ\to ν_μ}}$ survival probability also undergoes large changes. A proper understanding of $ν_μ$ survival rates must consider matter effects in $P_{μτ}$ as well as $P_{μe}$. We comment on a) how these matter effects may be observed and the sign of $Δ_{31}$ determined in atmospheric neutrino measurements and at neutrino factories and b) how they lead to heightened sensitivity for small $θ_{13}$.

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arXiv2004

Working Group Report: Neutrino and Astroparticle Physics

This is the report of neutrino and astroparticle physics working group at WHEPP-8. We present the discussions carried out during the workshop on selected topics in the above fields and also indicate progress made subsequently. The neutrino physics subgroup studied the possibilites of constraining neutrino masses, mixing and CPT violation in lepton sector from future experiments. Neutrino mass models in the context of abelian horizontal symmetries, warped extra dimensions and in presence of triplet Higgs were studied. Effect of threshold corrections on radiative magnification of mixing angles was investigated. The astroparticle physics subgroup focused on how various particle physics inputs affect the CMBR fluctuation spectrum, and on brane cosmology. This report also contains an introduction on how to use the publicly available code CMBFAST to calculate the CMBR fluctuations.

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arXiv2001

Analytical calculation of matter effects in two mass-scale neutrino oscillations

We consider three active flavor neutrino oscillations where both the mass-square differences play a role in atmospheric neutrino problem. We calculate the matter effects arising due to propagation through earth. We demonstrate that these effects improve the fit to the electron data vis-a-vis vacuum oscillations but make the fit to the muon data far worse, thus worsening the overall fit. The results of our analytical calculation verify the numerical investigations of this scheme presented earlier by Fogli et al.

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arXiv2001

Determining the sign of $Δ_{31}$ at long baseline neutrino experiments

Recently it is advocated that high intensity and low energy $(E_ν\sim 2 GeV)$ neutrino beams should be built to probe the $(13)$ mixing angle $ϕ$ to a level of a few parts in $10^4$. Experiments using such beams will have better signal to background ratio in searches for $ν_μ\to ν_e$ oscillations. We propose that such experiments can also determine the sign of $Δ_{31}$ even if the beam consists of {\it neutrinos} only. By measuring the $ν_μ\to ν_e$ transitions in two different energy ranges, the effects due to propagation of neutrinos through earth's crust can be isolated and the sign of $Δ_{31}$ can be determined. If the sensitivity of an experiment to $ϕ$ is $ε$, then the same experiment is automatically sensitive to matter effects and the sign of $Δ_{31}$ for values of $ϕ\geq 2 ε$.

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arXiv2000

Testing LSND at long-baseline neutrino experiments

Recently it was suggested that two very different mass-squared differences play a role in atmospheric neutrino oscillations. The larger of these also accounts for the LSND result and the smaller of these also drives the solar neutrino oscillations. We consider the predictions of this scheme for long-baseline experiments. We find that high statistics experiments, such as MINOS, can observe a clean signal for this scheme, which is clearly distinguishable from the usual scheme of atmospheric neutrino oscillations driven by a single mass-squared difference.

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arXiv2000

Matter effects for `just so' oscillations

We study the effect of the matter term on the evolution of the solar neutrinos when the neutrino parameters are those of the `just-so' case. The extreme non-adiabatic effects at the edge of the sun reduce the expression for the survival probability in the just-so case to that of the vacuum case. This conclusion is independent of the width of the extreme non-adiabatic region, which is a function of the density profile of the sun beyond $r > 0.9 R_s$. However, in its propagation, neutrino encounters regions of moderate (non-extreme) non-adiabticity. Neutrino traversal through these regions give corrections to the survival probability which are profile dependent.

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arXiv1999

Probing the matter term at long baseline experiments

We consider (ν_μ--> ν_e) oscillations in long baseline experiments within a three flavor framework. A non-zero measurement of this oscillation probability implies that the (13) mixing angle `phi' is non-zero. We consider the effect of neutrino propagation through the matter of earth's crust and show that, given the constraints from solar neutrino and CHOOZ data, matter effects enhance the mixing for neutrinos rather than for anti-neutrinos. We need data from two different experiments with different baseline lengths (such as K2K and MINOS) to distinguish matter effects unambiguously.

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arXiv1998

One pion events by atmospheric neutrinos: A three flavor analysis

We study the one-pion events produced via neutral current (NC) and charged current (CC) interactions by the atmospheric neutrinos. We analyze the ratios of these events in the framework of oscillations between three neutrino flavors. The ratios of the CC events induced by $ν_e$ to that of the NC events and a similar ratio defined with $ν_μ$ help us in distinguishing the different regions of the neutrino parameter space.

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arXiv1997

Three flavor implications of CHOOZ result

We analyze the recent result of the CHOOZ collaboration in the context of mixing and oscillations between all the three neutrino flavors. If one assumes a hierarchy among the vacuum mass eigenvalues (δ_{21} \ll δ_{31}), then the CHOOZ result limits the (13) mixing angle ϕto quite small values. This in turn limits the contribution of the ν_e <--> ν_μoscillation channel to the atmospheric neutrino anomaly to less than 10 percent. It also implies that the solution to the solar neutrino problem is effectively ν_e <--> ν_μoscillation and that of the atmospheric neutrino problem is effectively ν_μ<--> ν_τ.

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arXiv1997

Atmospheric neutrinos with three flavor mixing

We analyze the atmospheric neutrino data in the context of three flavor neutrino oscillations taking account of the matter effects in the earth. With the hierarchy among the vacuum mass eigenvalues $μ_3^2 \gg μ_2^2 \geq μ_1^2$, the solution of the atmospheric neutrino problem depends on $δ_{31}=μ_3^2 - μ_1^2$ and the $13$ and $23$ mixing angles $ϕ$ and $ψ$. Whereas the sub-GeV atmospheric neutrino data imposes only a lower limit on $δ_{31} > 10^{-3} eV^2$, the zenith angle dependent suppression observed in the multi-GeV data limits $δ_{31}$ from above also. The allowed regions of the parameter space are strongly constrained by the multi-GeV data. Combined with our earlier solution to the solar neutrino problem which depends on $δ_{21}= μ_2^2-μ_1^2$ and the $12$ and $13$ mixing angles $ω$ and $ϕ$, we have obtained the ranges of values of the five neutrino parameters which solve both the solar and the atmospheric neutrino problems simultaneously.

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arXiv1995

Solar and atmospheric neutrino oscillations with three flavours

We analyze the solar and the atmospheric neutrino problems in the context of three flavour neutrino oscillations. We assume a mass hierarchy in the vacuum mass eigenvalues $μ_3^2 \gg μ_2^2 \geq μ_1^2$, but make no approximation regarding the magnitudes of the mixing angles. We find that there are small but continuous bands in the parameter space where the constraints imposed by the current measurements of $ \ {}^{71} Ga$, ${}^{37} Cl$ and Kamiokande experiments are satisfied at $1 σ$ level. The allowed parameter space increases dramatically if the error bars are enlarged to $1.6 σ$. The electron neutrino survival probability has different energy dependence in different regions of the parameter space. Measurement of the recoil electron energy spectrum in detectors that use $ν- e$ scattering may distinguish between some of the allowed regions of parameter space. Finally we use the results for the parameter space admitted by the solar neutrinos as an input for the atmospheric neutrino problem and show that there exists a substantial region of parameter space in which both problems can be solved.

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arXiv1992

CP-violating Phenomena in an $ SU(2)_L \times SU(2)_R \times U(1)_{B-L} \times SU(3)_H^{VL}$ Horizontal Symmetric Model

We consider an $SU(2)_L \times SU(2)_R \times U(1)_{B-L} \times SU(3)_H^{VL}$ gauge model with natural flavour conservation in the Higgs sector, in which CP-violation occurs due to the horizontal interactions only. We calculate the CP-violating observables $ε$ and $ε'$ of the neutral kaon sector and $d_n$, the electric dipole moment of the neutron. The regions of the parameter space which yield a value of $ε$ that is in agreement with the experiment, lead to predictions for $ε'$ and $d_n$ which are at least five orders of magnitude smaller than the current experimental upper bounds.

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