Narrow Results

We propose a universal mixing hypothesis between quark and lepton sectors at high energy scale (probably GUT scale) where quark–lepton universality holds. Namely in the charged lepton diagonal base, all the other mass matrices for up and down quarks and neutrinos are diagonalized by the same unitary matrix except for the phase elements. Thanks to this hypothesis, the observed values of the Cabibbo–Kobayashi–Maskawa (CKM) quark mixing matrix and the mixing angles θ₁₂ and θ₂₃ in the Maki–Nakagawa–Sakata (MNS) lepton mixing matrix can predict the unknown magnitudes of the mixing angle θ₁₃ and of the CP violating Dirac phase δ in the MNS matrix. Their allowed regions are rather narrow, 0.036 < sin θ₁₃ < 0.048 and 6° < δ < 12°.

The recently observed lepton mixing angle θ₁₃ of the MNS mixing matrix is well incorporated in a universal mixing hypothesis between quark and lepton sectors. This hypothesis asserts that, in the charged lepton diagonal base, all other mass matrices for up- and down-type quarks and light neutrinos are diagonalized by the same unitary matrix except for the phase elements. It is expressed as V_CKM = U_MNS(δ′)^†PU_MNS(δ) for quark mixing matrix V_CKM and lepton mixing matrix U_MNS(δ) in the phenomenological level. Here P is a diagonal phase mass matrix. δ′ is a slightly different phase parameter from the Dirac CP-violating phase δ = 1.1π (best fit) in the MNS lepton mixing matrix.

The universality hypothesis for quark and lepton mixing matrices (CKM and MNS) is further developed. This hypothesis explains why the CKM is almost diagonal whereas the MNS is almost maximally mixed. If this hypothesis is true, the Dirac CP violating phase of the MNS mixing matrix is bounded around π or 0. Quark–lepton mass matrices which realize this hypothesis are constructed, showing simple power law relations among mass matrices for up-type, down-type quarks and neutrinos.

The MINOS experiment will study muon neutrino oscillations in the range of parameter space implied by the measurements with atmospheric neutrinos. The two magnetized toroidal tracking calorimeters, 1 kton at Fermilab and 5.4 kton in the Soudan Mine 735-km away, are constructed out of extruded scintillator strips and use wavelength-shifting and clear fibers read out by multi-anode photomultipliers. The far detector is now more than 3/4 completed and commenced collecting cosmic ray data. The neutrino beamline and the near detector will be finished by the end of 2004. In addition, a calibration detector was also constructed and exposed to the test beams. Over the last two years the entire project has progressed on schedule and budget and with most of the civil construction out of the way the road to this interesting physics program is finally cleared.

I discuss the new physics unveiled by neutrino oscillation experiments. It is fair to say that, while significant theoretical progress has been made, we are yet to construct a unique coherent picture that naturally explains non-zero, yet tiny, neutrino masses and the newly revealed pattern of lepton mixing. I discuss what the challenges are, and point to the fact that more experimental input is required. Finally, I draw attention to the fact that neutrinos may have only just begun to reshape fundamental physics, given the fact that we are still to explain the LSND anomaly and because the neutrino oscillation phenomenon is ultimately sensitive to very small new-physics effects.

The observed hierarchy in the fermion masses, which imply a set of small mass ratios, is not naturally small regarding ’t Hooft’s criteria. In this work, in a model independent approach, we introduce a set of conditions by which fermion mass ratios become natural. Interestingly, these conditions demand that fermion mixing should be described by the four independent mass ratios of each fermion sector. Application of this set of conditions to the standard theory enables one to understand the mains aspects in quark and lepton mixing. This feature can be taken as a strong evidence for the existence in Nature of a flavor symmetry. Also, for this analysis to work in the lepton sector, neutrino masses should have normal ordering with the lightest neutrino mass satisfying the lower bound, $m_{\nu 1}\ge (5.0\pm 0.1)\text{MeV}$, making the approach testable.

The lepton-charge (L_e, L_μ, L_τ) nonconserving interaction leads to the mixing of the electron, muon and tau neutrinos, which manifests itself in spatial oscillations of a neutrino beam, and also to the mixing of the electron, negative muon and tau lepton, which, in particular, may be the cause of the "forbidden" radiative decay of the negative muon into the electron and γ quantum. Under the assumption that the nondiagonal elements of the mass matrices for neutrinos and ordinary leptons, connected with the lepton charge nonconservation, are the same, and by performing the joint analysis of the experimental data on neutrino oscillations and experimental restriction for the probability of the decay μ^- → e^- + γ per unit time, the following estimate for the lower bound of neutrino mass has been obtained: .

The MINOS experiment will study muon neutrino oscillations in the range of parameter space implied by the measurements with atmospheric neutrinos. The two magnetized toroidal tracking calorimeters, 1 kton at Fermilab and 5.4 kton in the Soudan Mine 735-km away, are constructed out of extruded scintillator strips and use wavelength-shifting and clear fibers read out by multi-anode photomultipliers. The far detector is now more than 3/4 completed and commenced collecting cosmic ray data. The neutrino beamline and the near detector will be finished by the end of 2004. In addition, a calibration detector was also constructed and exposed to the test beams. Over the last two years the entire project has progressed on schedule and budget and with most of the civil construction out of the way the road to this interesting physics program is finally cleared.

Some models for the lepton sector, based on seesaw extensions of the Standard Model, are discussed in which the μ–τ interchange symmetry is realized in various ways. The symmetries defining such models and their characteristic predictions for lepton mixing are presented.

The Hyper-Kamiokande is a next generation large water Cherenkov detector proposed to be built in Kamioka, Japan. It aims at precision studies of neutrino oscillation parameters, including the CP violation phase δ_CP , using neutrinos from the J-PARC accelerator as well as atmospheric neutrinos. The detector also serves as a general purpose observatory, expanding the existing sensitivities by an order of magnitude in the searches for nucleon decays, solar and galactic WIMPs, and supernova neutrinos.