arXiv subjects

Amol Dighe

Publications and source records attributed to Amol Dighe.

At least 73 records · Page 4Linked to original sources

arXiv2010

Enhanced $B_s$--$\bar{B}_s$ lifetime difference and anomalous like-sign dimuon charge asymmetry from new physics in $B_s \to τ^+ τ^-$

New physics models that increase the decay rate of $B_s \to τ^+ τ^-$ contribute to the absorptive part of $B_s$--$\bar{B}_s$ mixing, and may enhance $ΔΓ_s$ all the way up to its current experimental bound. In particular, the model with a scalar leptoquark can lead to a significant violation of the expectation $ΔΓ_s \leq ΔΓ_s$ (SM). It can even allow regions in the $ΔΓ_s$-$β_s$ parameter space that are close to the best fit obtained by CDF and D\{O} through $B_s \to J/ψϕ$. In addition, it can help explain the anomalous like-sign dimuon charge asymmetry observed recently by DØ. A measurement of $BR(B_s \to τ^+ τ^-)$ is thus crucial for a better understanding of new physics involved in $B_s$--$\bar{B}_s$ mixing.

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arXiv2010

Collective three-flavor oscillations of supernova neutrinos

Neutrinos and antineutrinos emitted from a core collapse supernova interact among themselves, giving rise to collective flavor conversion effects that are significant near the neutrinosphere. We develop a formalism to analyze these collective effects in the complete three-flavor framework. It naturally generalizes the spin-precession analogy to three flavors and is capable of analytically describing phenomena like vacuum/MSW oscillations, synchronized oscillations, bipolar oscillations and spectral split. Using the formalism, we demonstrate that the flavor conversions may be "factorized" into two-flavor oscillations with hierarchical frequencies. We explicitly show how the three-flavor solution may be constructed by combining two-flavor solutions. For a typical supernova density profile, we identify an approximate separation of regions where distinctly different flavor conversion mechanisms operate, and demonstrate the interplay between collective and MSW effects. We pictorialize our results in terms of the "e_3 - e_8 triangle" diagram, which is a tool that can be used to visualize three-neutrino flavor conversions in general, and offers insights into the analysis of the collective effects in particular.

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arXiv2009

Supernova neutrino oscillations: what do we understand?

We summarize our current understanding of the neutrino flavor conversions inside a core collapse supernova, clarifying the important role played by the "collective effects" in determining flavor conversion probabilities. The potentially observable $ν_e$ and $\barν_e$ spectra may help us identify the neutrino mixing scenario, distinguish between primary flux models, and learn more about the supernova explosion.

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arXiv2009

Physics potential of future supernova neutrino observations

We point out possible features of neutrino spectra from a future galactic core collapse supernova that will enhance our understanding of neutrino mixing as well as supernova astrophysics. We describe the neutrino flavor conversions inside the star, emphasizing the role of "collective effects" that has been appreciated and understood only very recently. These collective effects change the traditional predictions of flavor conversion substantially, and enable the identification of neutrino mixing scenarios through signatures like Earth matter effects.

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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.

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arXiv2009

Multiple Spectral Splits of Supernova Neutrinos

Collective oscillations of supernova neutrinos swap the electron neutrino and antineutrino spectra with those of another flavor in certain energy intervals bounded by sharp spectral splits. This phenomenon is far more general than previously appreciated: typically one finds one or more swaps and accompanying splits in the neutrino and antineutrino channels for both inverted and normal neutrino mass hierarchies. Depending on an instability condition, swaps develop around spectral crossings (energies where the electron neutrino or antineutrino fluxes are equal to that of another flavor, as well as infinite E where all fluxes vanish), and the widths of swaps are determined by the spectra and fluxes. Wash-out by multi-angle decoherence varies across the spectrum and splits can survive as sharp spectral features.

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arXiv2009

Renormalization group evolution of neutrino masses and mixing in the Type-III seesaw mechanism

We consider the standard model extended by heavy right handed fermions transforming as triplets under SU(2)$_L$, which generate neutrino masses through the Type-III seesaw mechanism. At energies below their respective mass scales, the heavy fields get sequentially decoupled to give an effective dimension-5 operator. Above their mass thresholds, these fields also participate in the renormalization of the wavefunctions, masses and coupling constants. We compute the renormalization group evolution of the effective neutrino mass matrix in this model, with particular emphasis on the threshold effects. The evolution equations are obtained in a basis of neutrino parameters where all the quantities are well-defined everywhere, including at $θ_{13} = 0$. We also point out the important role of the threshold effects and Majorana phases in the evolution of mixing angles through illustrative examples.

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arXiv2009

CP asymmetry in the decays B --> (X_s, X_d) mu+ mu- with four generations

We estimate the CP asymmetry A_{CP}(q^2) in the decays B --> X_s mu+ mu- and B --> X_d mu+ mu- in the standard model (SM) with an additional fourth generation. We use a parametrization that allows us to explore the complete parameter space of the 4X4 quark mixing matrix, and constrain these parameters from the current data on B decays. We find that the enhancement in A_{CP}(q^2) depends strongly on the mass of the t', the up-type quark in the fourth generation. For m_t' around 400 GeV, the CP asymmetry in the high-q^2 region (q^2 > 14.4 GeV^2) can be enhanced by more than an order of magnitude for B --> X_s mu+ mu- and up to a factor of 6 for B --> X_d mu+ mu-. There is no enhancement in the low-q^2 region (1< q^2 <6 GeV^2). With increasing m_t', the A_{CP}(q^2) in the high-q^2 (low-q^2) region first decreases (increases) and then saturates at a value a few times the SM prediction. In the high-q^2 region of B --> X_s mu+ mu-, this saturation value may be up to 25 times the SM expectation.

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arXiv2009

Texture zeroes and discrete flavor symmetries in light and heavy Majorana neutrino mass matrices: a bottom-up approach

Texture zeroes in neutrino mass matrix $M_ν$ may give us hints about the symmetries involved in neutrino mass generation. We examine the viability of such texture zeroes in a model independent way through a bottom-up approach. Using constraints from the neutrino oscillation data, we develop an analytic framework that can identify these symmetries and quantify deviations from them. We analyze the textures of $M_ν$ as well as those of $M_M$, the mass matrix of heavy Majorana neutrinos in the context of Type-I seesaw. We point out how the viability of textures depends on the absolute neutrino mass scale, the neutrino mass ordering and the mixing angle $θ_{13}$. We also examine the compatibility of discrete flavor symmetries like $μ$--$τ$ exchange and $S_3$ permutation with the current data. We show that the $μ-τ$ exchange symmetry for $M_ν$ can be satisfied for any value of the absolute neutrino mass, but for $M_ν$ to satisfy the $S_3$ symmetry, neutrino masses have to be quasi-degenerate. On the other hand, both these symmetries are currently allowed for $M_M$ for all values of absolute neutrino mass and both mass orderings.

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arXiv2008

Renormalization group evolution of neutrino mixing parameters near $θ_{13} = 0$ and models with vanishing $θ_{13}$ at the high scale

Renormalization group (RG) evolution of the neutrino mass matrix may take the value of the mixing angle $θ_{13}$ very close to zero, or make it vanish. On the other hand, starting from $θ_{13}=0$ at the high scale it may be possible to generate a non-zero $θ_{13}$ radiatively. In the most general scenario with non-vanishing CP violating Dirac and Majorana phases, we explore the evolution in the vicinity of $θ_{13}=0$, in terms of its structure in the complex ${\cal U}_{e3}$ plane. This allows us to explain the apparent singularity in the evolution of the Dirac CP phase $δ$ at $θ_{13}=0$. We also introduce a formalism for calculating the RG evolution of neutrino parameters that uses the Jarlskog invariant and naturally avoids this singular behaviour. We find that the parameters need to be extremely fine-tuned in order to get exactly vanishing $θ_{13}$ during evolution. For the class of neutrino mass models with $θ_{13}=0$ at the high scale, we calculate the extent to which RG evolution can generate a nonzero $θ_{13}$, when the low energy effective theory is the standard model or its minimal supersymmetric extension. We find correlated constraints on $θ_{13}$, the lightest neutrino mass $m_0$, the effective Majorana mass $m_{ee}$ measured in the neutrinoless double beta decay, and the supersymmetric parameter $\tanβ$.

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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.

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arXiv2008

Identifying neutrino mass hierarchy at extremely small theta(13) through Earth matter effects in a supernova signal

Collective neutrino flavor transformations deep inside a supernova are sensitive to the neutrino mass hierarchy even at extremely small values of theta(13). Exploiting this effect, we show that comparison of the antineutrino signals from a galactic supernova in two megaton class water Cherenkov detectors, one of which is shadowed by the Earth, will enable us to distinguish between the hierarchies if sin^2 theta(13) < 10^{-5}. On the other hand, the observation of Earth effects in the inverted hierarchy for sin^2 theta(13) > 10^{-3} will provide a robust observable signature of collective oscillations occurring deep inside the supernova.

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arXiv2008

CPT violation in long baseline neutrino experiments: a three flavor analysis

We explore possible signals of CPT violation in neutrinos in the complete three-flavor framework. Employing a systematic expansion in small parameters, we analytically estimate the CPT violating contributions to the survival probabilities of $ν_μ, \barν_μ, ν_e$ and $\barν_e$. The results indicate that, in spite of the large number of CPT violating parameters, only a small number of combinations are relevant for oscillation experiments. We identify the combinations that can be constrained at the long baseline experiments, and show that their contribution to the neutrino Hamiltonian can be bounded to $< 10^{-23}$ GeV, by considering the NOvA experiment for the muon sector, and neutrino factories for the electron sector.This formalism also allows us to translate the bounds on the parameters describing non-standard interactions of neutrinos into the bounds on CPT violating quantities.

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arXiv2008

Collective neutrino oscillations in non-spherical geometry

The rich phenomenology of collective neutrino oscillations has been studied only in one-dimensional or spherically symmetric systems. Motivated by the non-spherical example of coalescing neutron stars, presumably the central engines of short gamma-ray bursts, we use the Liouville equation to formulate the problem for general source geometries. Assuming the neutrino ensemble displays self-maintained coherence, the problem once more becomes effectively one-dimensional along the streamlines of the overall neutrino flux. This approach for the first time provides a formal definition of the ``single-angle approximation'' frequently used for supernova neutrinos and allows for a natural generalization to non-spherical geometries. We study the explicit example of a disk-shaped source as a proxy for coalescing neutron stars.

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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.

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arXiv2007

Neutrinos from a core collapse supernova

The neutrino burst from a galactic supernova can help determine the neutrino mass hierarchy and $θ_{13}$, and provide crucial information about supernova astrophysics. Here we review our current understanding of the neutrino burst, flavor conversions of these neutrinos, and model independent signatures of various neutrino mixing scenarios.

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arXiv2007

Tree FCNC and non-unitarity of CKM matrix

We discuss possible signatures of the tree level FCNC, which results from the non-unitarity of CKM matrix. We first define the unitaity step-by-step, and possible test of the non-unitaity through the 4-value-KM parametrization. We, then, show how the phase angle of the unitary triangle would change in case of the vector-like down quark model. As another example of tree FCNC, we investigate the leptophobic $Z'$ model and its application to the recent $B_s$ mixing measurements.

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arXiv2007

Radiatively broken symmetries of nonhierarchical neutrinos

Symmetry-based ideas, such as the quark-lepton complementarity (QLC) principle and the tri-bimaximal mixing (TBM) scheme, have been proposed to explain the observed mixing pattern of neutrinos. We argue that such symmetry relations need to be imposed at a high scale $Λ\sim 10^{12}$ GeV characterizing the large masses of right-handed neutrinos required to implement the seesaw mechanism. For nonhierarchical neutrinos, renormalisation group evolution down to a laboratory energy scale $λ\sim 10^3$ GeV tends to radiatively break these symmetries at a significant level and spoil the mixing pattern predicted by them. However, for Majorana neutrinos, suitable constraints on the extra phases $α_{2,3}$ enable the retention of those high scale mixing patterns at laboratory energies. We examine this issue within the Minimal Supersymmetric Standard Model (MSSM) and demonstrate the fact posited above for two versions of QLC and two versions of TBM. The appropriate constraints are worked out for all these four cases. Specifically, a preference for $α_2 \approx π$ (i.e. $m_1 \approx -m_2$) emerges in each case. We also show how a future accurate measurement of $θ_{13}$ may enable some discrimination among these four cases in spite of renormalization group evolution.

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