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S. Uma Sankar

Publications and source records attributed to S. Uma Sankar.

At least 55 records · Page 3Linked to original sources
arXiv2013

Exploring the three flavor effects with future superbeams using liquid argon detectors

Recent measurement of a moderately large value of theta13 signifies an important breakthrough in establishing the standard three flavor oscillation picture of neutrinos. It has provided an opportunity to explore the sub-dominant three flavor effects in present and future long-baseline experiments. In this paper, we perform a comparative study of the physics reach of two future superbeam facilities, LBNE and LBNO in their first phases of run, to resolve the issues of neutrino mass hierarchy, octant of theta23, and leptonic CP violation. We also find that the sensitivity of these future facilities can be improved significantly by adding the projected data from T2K and NOvA. Stand-alone LBNO setup with a 10 kt detector has a mass hierarchy discovery reach of more than 7 sigma, for the lowest allowed value of sin^2theta23(true) = 0.34. This result is valid for any choice of true deltaCP and hierarchy. LBNE10, in combination with T2K and NOvA, can achieve 3 sigma hierarchy discrimination for any choice of deltaCP, sin^2theta23, and hierarchy. The same combination can provide a 3 sigma octant resolution for sin^2theta23(true) leq 0.44 or for sin^2theta23(true) geq 0.58 for all values of deltaCP(true). LBNO can give similar results with 10 kt detector mass. In their first phases, both LBNE10 and LBNO with 20 kt detector can establish leptonic CP violation for around 50% values of true deltaCP at 2 sigma confidence level. In case of LBNE10, CP coverage at 3 sigma can be enhanced from 3% to 43% by combining T2K and NOvA data, assuming sin^2theta23(true) = 0.5. For LBNO setup, CP violation discovery at 3 sigma is possible for 46% values of true deltaCP if we add the data from T2K and NOvA.

hep-ph↗
arXiv2013

Resolving the octant of theta23 with T2K and NOvA

Preliminary results of MINOS experiment indicate that theta23 is not maximal. Global fits to world neutrino data suggest two nearly degenerate solutions for theta23: one in the lower octant (LO: theta23 < 45 degree) and the other in the higher octant (HO: theta23 > 45 degree). numu to nue oscillations in superbeam experiments are sensitive to the octant and are capable of resolving this degeneracy. We study the prospects of this resolution by the current T2K and upcoming NOvA experiments. Because of the hierarchy-deltacp degeneracy and the octant-deltacp degeneracy, the impact of hierarchy on octant resolution has to be taken into account. As in the case of hierarchy determination, there exist favorable (unfavorable) values of deltacp for which octant resolution is easy (challenging). However, for octant resolution the unfavorable deltacp values of the neutrino data are favorable for the anti-neutrino data and vice-verse. This is in contrast to the case of hierarchy determination. In this paper, we compute the combined sensitivity of T2K and NOvA to resolve the octant ambiguity. If sin^2\theta23 =0.41, then NOvA can rule out all the values of theta23 in HO at 2 sigma C.L., irrespective of the hierarchy and deltacp. Addition of T2K data improves the octant sensitivity. If T2K were to have equal neutrino and anti-neutrino runs of 2.5 years each, a 2 sigma resolution of the octant becomes possible provided sin^2\theta23 \leq 0.43 or \geq 0.58 for any value of deltacp.

hep-ph↗
arXiv2012

Potential of optimized NOvA for large theta(13) & combined performance with a LArTPC & T2K

NOvA experiment has reoptimized its event selection criteria in light of the recently measured moderately large value of theta(13). We study the improvement in the sensitivity to the neutrino mass hierarchy and to leptonic CP violation due to these new features. For favourable values of deltacp, NOvA sensitivity to mass hierarchy and leptonic CP violation is increased by 20%. Addition of 5 years of neutrino data from T2K to NOvA more than doubles the range of deltacp for which the leptonic CP violation can be discovered, compared to stand alone NOvA. But for unfavourable values of deltacp, the combination of NOvA and T2K are not enough to provide even a 90% C.L. hint of hierarchy discovery. Therefore, we further explore the improvement in the hierarchy and CP violation sensitivities due to the addition of a 10 kt liquid argon detector placed close to NOvA site. The capabilities of such a detector are equivalent to those of NOvA in all respects. We find that combined data from 10 kt liquid argon detector (3 years of nu + 3 years of nubar run), NOvA (6 years of nu + 6 years of nubar run) and T2K (5 years of nu run) can give a close to 2 sigma hint of hierarchy discovery for all values of deltacp. With this combined data, we can achieve CP violation discovery at 95% C.L. for roughly 60% values of deltacp.

hep-ph↗
arXiv2012

Getting the best out of T2K and NOvA

We explore the combined physics potential of T2K and NOvA in light of the moderately large measured value of theta13. For sin^2 2*theta13 = 0.1, which is close to the best fit value, a 90% C.L. evidence for the hierarchy can be obtained only for the combinations (Normal hierarchy, -170 <= deltaCP <= 0) and (Inverted hierarchy, 0 <= deltaCP <= 170), with the currently planned runs of NOvA and T2K. However, the hierarchy can essentially be determined for any value of deltaCP, if the statistics of NOvA are increased by 50% and those of T2K are doubled. Such an increase will also give an allowed region of deltaCP around its true value, except for the CP conserving cases deltaCP = 0 or 180. We demonstrate that any measurement of deltaCP is not possible without first determining hierarchy. We find that comparable data from a shorter baseline (L ~ 130 km) experiment will not lead to any significant improvement.

hep-ph↗
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.

hep-ph↗
arXiv2011

Physics Potential of a 2540 Km Baseline Superbeam Experiment

We study the physics potential of a neutrino superbeam experiment with a 2540 km baseline. We assume a neutrino beam similar to the NuMI beam in medium energy configuration. We consider a 100 kton totally active scintillator detector at a 7 mr off-axis location. We find that such a configuration has outstanding hierarchy discriminating capability. In conjunction with the data from the present reactor neutrino experiments, it can determine the neutrino mass hierarchy at 3 sigma level in less than 5 years, if sin^2(2*theta13) > 0.01, running in the neutrino mode alone. As a stand alone experiment, with a 5 year neutrino run and a 5 year anti-neutrino run, it can determine non-zero theta13 at 3 sigma level if sin^2(2*theta13) > 7*10^{-3} and hierarchy at 3 sigma level if sin^2(2*theta13) > 8*10^{-3}. This data can also distinguish deltaCP = pi/2 from the CP conserving values of 0 and pi, for sin^2(2*theta13) > 0.02.

hep-ph↗
arXiv2011

Magical properties of 2540 Km baseline Superbeam Experiment

Lack of any information on the CP violating phase $\dcp$ weakens our ability to determine neutrino mass hierarchy. Magic baseline of 7500 km was proposed to overcome this problem. However, to obtain large enough fluxes, at this very long baseline, one needs new techniques of generating high intensity neutrino beams. In this letter, we highlight the {\it magical} properties of a 2540 km baseline. At such a baseline, using a narrow band neutrino superbeam whose flux peaks around the energy 3.5 GeV, we can determine neutrino mass hierarchy {\bf independent} of the CP phase. For $\sin^2 2 θ_{13} \geq 0.05$, a very modest exposure of 10 Kiloton-years is sufficient to determine the hierarchy. For $0.02 \leq \sin^2 2 θ_{13} \leq 0.05$, an exposure of about 100 Kiloton-years is needed.

hep-ph↗
arXiv2009

Determining the Sign of Δ_{31} by Future Long Baseline and Reactor Experiments

We study the determination of neutrino mass hierarchy, through neutrino experiments within the next ten years. The T2K neutrino oscillation experiment will start in 2009. In the experiment the high intensity ν_μ beam from JHF is directed to Super-Kamiokande (SK) detector 295 km away. The NOνA (off axies neutrino oscillation) experiment is being planned, with the ν_μ beam from Fermi-Lab directed to a site 610km away, which is 0,7,14 milliradian off-axies. Both the above experiments will measure ν_μ\toν_{e} oscillation probability. The double-CHOOZ experiment under construction detects \bar{ν_{e}}s emitted by nuclear reactors both through a near detector (150m) and a far detector (1.05 Km) and measure \bar{ν_{e}}\to\bar{ν_{e}} survival probability. In this paper, we outline a procedure to determine the sign of Δ_{31} from the simulated data of the above experiments.

hep-ex↗
arXiv2009

Tension between scalar/pseudoscalar new physics contribution to B_s --> mu+ mu- and B --> K mu+ mu-

New physics in the form of scalar/pseudoscalar operators cannot lower the semileptonic branching ratio B(B --> K mu+ mu-) below its standard model value. In addition, we show that the upper bound on the leptonic branching ratio B(B_s --> mu+ mu-) sets a strong constraint on the maximum value of B(B --> K mu+ mu-) in models with multiple Higgs doublets: with the current bound, B(B --> K mu+ mu-) cannot exceed the standard model prediction by more than 2.5%. The conclusions hold true even if the new physics couplings are complex. However these constraints can be used to restrict new physics couplings only if the theoretical and experimental errors in B(B --> K mu+ mu-) are reduced to a few per cent. The constraints become relaxed in a general class of models with scalar/pesudoscalar operators.

hep-ph↗
arXiv2009

Bounds on Tensor operator contribution to B_s --> l+ l- gamma

We consider the effect of new physics interactions in the form of tensor operators on the branching ratio of B_s --> l+ l- gamma where l=e, mu. We find that the present data on the inclusive branching ratio B --> X_s l+ l- puts a strong constraint on the new physics tensor couplings and does not allow a large enhancement in the branching ratio of B_s --> l+ l- gamma beyond its standard model predictions. Large enhancement in the branching ratio of B_s --> l+ l- gamma due to new physics in the form of vector/axial-vector, scalar/pseudoscalar and magnetic dipole operators has already been ruled out. Thus we conclude that no new physics can provide a large enhancement in the branching ratio of B_s --> l+ l- gamma and hence it is expected to be observed in future experiments with a branching ratio close to its standard model expectation.

hep-ph↗
arXiv2008

Large forward-backward asymmetry in B --> K mu+ mu- from new physics tensor operators

We study the constraints on possible new physics contribution to the forward-backward asymmetry of muons, A_{FB}(q^2), in B --> K mu+ mu-. New physics in the form of vector/axial-vector operators does not contribute to A_{FB}(q^2) whereas new physics in the form of scalar/pseudoscalar operators can enhance A_{FB}(q^2) only by a few per cent. However new physics the form of tensor operators can take the peak value of A_{FB}(q^2) to as high as 40% near the high-q^2 end point. In addition, if both scalar/pseudoscalar and tensor operators are present, then A_{FB}(q^2) can be more than 15% for the entire high-q^2 region q^2 > 15 GeV^2. The observation of significant A_{FB} would imply the presence of new physics tensor operators, whereas its q^2 dependence could further indicate the presence of new scalar/pseudoscalar physics.

hep-ph↗
arXiv2008

Probing extended Higgs sector through rare b --> s mu+ mu- transitions

We study the constraints on the contribution of new physics in the form of scalar/pseudoscalar operators to the average forward backward asymmetry of muons in B --> K mu+ mu- and the longitudinal polarization asymmetry A_{LP} of muons in B_s --> mu+ mu-. We find that the maximum possible value of allowed by the present upper bound on B(B_s --> mu+ mu-) is about 1% at 95% C.L. and hence will be very difficult to measure. On the other hand, the present bound on B(B_s --> mu+ mu-) fails to put any constraints on A_{LP}, which can be as high as 100% even if B(B_s --> mu+ mu-) is close to its standard model prediction. The measurement of A_{LP} will be a direct evidence for an extended Higgs sector, and combined with the branching ratio B(B_s --> mu+ mu-) it can even separate the new physics scalar and pseudoscalar contributions.

hep-ph↗
arXiv2007

New Jarlskog determinant from Physics above the GUT Scal

We study the Planck scale effects on Jarlskog determinant. Quantum gravitational (Planck scale) effects lead to an effective SU(2)_{L}\times U(1) invariant dimension-5 Lagrangian involving neutrino and Higgs fields, which gives rise to additional terms in neutrino mass matrix on electroweak symmetry breaking. We assume that gravitational interaction is flavor blind and compute the Jarlskog determinant due to the Planck scale effects. In the case of neutrino sector, the strength of CP violation is measured by Jarlskog determinant. In this paper, we assume CP violation arise from Planck scale effects. We applied our approach to study Jarlskog determinant due to the Planck scale effects.

hep-ph↗
arXiv2007

Does the present data on B_s - bar B_s mixing rule out a large enhancement in the branching ratio of B_s --> mu+ mu- ?

In this letter, we consider the constraints imposed by the recent measurement of B_s - bar B_s mixing on the new physics contribution to the rare decay B_s --> mu+ mu-. New physics in the form vector and axial-vector couplings is already severely constrained by the data on B --> (K,K*) mu+ mu-. Here, we show that B_s - bar B_s mixing data, together with the data on K0 - bar K0 mixing and K_L --> mu+ mu- decay rate, strongly constrain the scalar-pseudoscalar contribution to B_s --> mu+ mu-. We conclude that new physics can at best lead to a factor of 2 increase in the branching ratio of B_s --> mu+ mu- compared to its Standard Model expectation.

hep-ph↗
arXiv2006

Deviation from Bimaximality due to Planck scale effects

We consider the effect of Planck scale operators on neutrino mixing. We assume that GUT scale operators give rise to degenerate neutrino masses with bimaximal mixing. Quantum gravity (Planck scale) effects lead to an effective SU(2)L*U(1) invariant dimension-5 Lagrangian involving neutrino and Higgs fields. This gives rise to additional terms in the neutrino mass matrix on electroweak symmetry breaking. These additional terms can be considered as a perturbation to the GUT scale bi-maximal neutrino mass matrix. We assume that the gravitational interaction is flavour blind and compute the deviations of the three neutrino mixing angles due to the Planck scale effects. We find that the changes in theta13, and theta23 are very small but the change in solar mixing angle theta12 can be as large as 3.5 degrees.

hep-ph↗