We discuss ways to understand large neutrino mixings using new symmetries of quarks and leptons beyond the standard model for the three allowed patterns of neutrino masses: normal, inverted hierarchy and degenerate masses.

Starting with the unification hypothesis of mixings of quarks and leptons and small quark-like mixings at the see-saw scale, we find that two large mixings for ν_e —νx03BC; andv_μ—v_τ at the weak scale are obtained as a result of renormalization group evolution and radiative magnification if the three neutrinos are quasi degenerate in masses and possess the same CP parity. We also find thatU_e3 remains small and well within the CHOOZ-Palo Verde bound since the correspondingV_ub for CKM mixing is very small. Several testable pedictions are pointed out.

Many experiments are being planned to measure the neutrino mixing angles more precisely. In this note, the theoretical significance of a high precision measurement of these parameters is discussed. It is emphasized that they can provide crucial information about different ways to understand the origin of large atmospheric neutrino mixing and move us closer towards determining the neutrino mass matrix. They may also be able to throw light on the question of lepton-quark unification as well as the existence of any leptonic symmetries. For instance if exact μ↔ τ symmetry in the neutrino mass matrix is assumed to be the reason for maximalv_μ-v_gt mixing, one gets θ₁₃ = 0 and {ie1295-01} can provide information about the way the μ↔ τ symmetry breaking manifests in the case of normal hierarchy.

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 possibilities 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 the 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.

The issue of vacuum stability of standard model (SM) is discussed by embedding it within the TeV scale left–right quark see-saw model. The Higgs potential in this case has only two coupling parameters (𝜆₁, 𝜆₂) and two mass parameters. There are only two physical neutral Higgs bosons (ℎ, 𝐻), the lighter one being identified with the 126 GeV Higgs boson. We explore the range of values for (𝜆₁, 𝜆₂) for which the vacuum is stable for all values of the Higgs fields till 10¹⁶ GeV. Combining with the further requirement that the scalar self-couplings remain perturbative till 10¹⁶ GeV, we find