Naotoshi Okamura

NuTeV anomaly, lepton universality, and nonuniversal neutrino-gauge couplings

The Standard Model (SM) predictions of these parameters based on a global fit to non-NuTeV data, cited as [g L]SM = 0.3042 and [g 2 R]SM = 0.0301 in Ref. [1], differ from the NuTeV result by 3σ in g L. The NuTeV value for g L in Eq. (1) is smaller than its SM prediction, reflecting the fact that the ratios Rν = σ(νμN → νμX)/σ(νμN → μ X) and Rν = σ(νμN → νμX)/σ(νμN → μ X) were smaller than expected by the SM. Thus, possible new physics explanations of the NuTeV anomaly would be those that suppress the neutral current cross sections over the charged current cross sections, or enhance the charged current cross sections over the neutral current cross sections. To this end, two classes of models have been devised. Models of the first class suppress Rν and Rν through new neutrino-quark interactions, mediated by leptoquarks or extra U(1) gauge bosons (Z ’s), which interfere either destructively with the Z-exchange amplitude, or constructively with the W -exchange amplitude [2]. To maintain agreement between the SM and non-NuTeV data, the new interactions must selectively interfere with the νμN (νμN) scattering process, but little else. This severely restricts the types of interactions that may be introduced. Models of the second class suppress the Zνν coupling by mixing the neutrino with heavy gauge singlet states (neutrissimos, i.e. right-handed neutrinos) [3, 4, 5]. For instance, if the SU(2)L active ν is a linear combination of two mass eigenstates with mixing angle θ,

Prospects of very long baseline neutrino oscillation experiments with the KEK / JAERI high intensity proton accelerator

We study physics potential of Very Long Base-Line (VLBL) Neutrino-Oscillation Experiments with the High Intensity Proton Accelerator (HIPA), which will be completed by the year 2007 in Tokai-village, Japan, as a joint project of KEK and JAERI (Japan Atomic Energy Research Institute). The HIPA 50 GeV proton beam will deliver neutrino beams of a few GeV range with the intensity about two orders of magnitude higher than the present KEK beam for K2K experiment. As a sequel to the proposed HIPA-to-Super-Kamiokande experiment, we study impacts of experiments with a 100 kton-level detector and the base-line length of a few-thousand km. The pulsed narrow-band nu_mu beams (NBB) allow us to measure the nu_mu to nu_e transition probability and the nu_mu survival probability through counting experiments at large water-Cerenkov detector. We study sensitivity of such experiments to the neutrino mass hierarchy, the mass-squared differences, the three angles, and one CP phase of the three-generation lepton-flavor-mixing matrix. We find that experiments at a distance between 1,000 and 2,000 km can determine the sign of the larger mass-squared difference (m_3^2-m_1^2) if the mixing between nu_e and nu_3 (the heaviest-or-lightest neutrino) is not too small; 2|U_{e3}|^2(1-|U_{e3}|^2) gsim 0.03. The CP phase can be constrained if the |U_{e3}| element is sufficiently large, 2|U_{e3}|^2(1-|U_{e3}|^2) gsim 0.06, and if the smaller mass-squared difference (m_2^2-m_1^2) and the U_{e2} element are in the prefered range of the large-mixing-angle solution of the solar-neutrino deficit. The magunitude | m_3^2-m_1^2| and the matrix element U_{mu 3} can be precisely measured, but we find little sensitivity to m_2^2-m_1^2 and the matrix element U_{e2}.

Quark lepton unification and lepton flavor nonconservation from a TeV scale seesaw neutrino mass texture

In a recent paper, we pointed out that mixing of the light neutrinos with heavy gauge singlet states could reconcile the Z-pole data from e + e − colliders and the �� (¯�� ) scattering data from the NuTeV experiment at Fermilab. We further noted that the mixing angle required to fit the data is much larger than what would be expected from the conventional seesaw mechanism. In this paper, we show how such mixings can be arranged by a judicious choice of the neutrino mass texture. We also argue that by invoking the unification of the Dirac mass matrix for the up-type quarks and the neutrinos, the mass of the heavy states can naturally be expected to lie in the few TeV range. The model is strongly constrained by the lepton flavor changing process µ → e which requires lepton universality to be violated in the charged channel.

Lifting degeneracies in the oscillation parameters by a neutrino factory

Abstract We study the potential of a very long baseline neutrino oscillation experiment with a neutrino factory and a large segmented water-Cerenkov calorimeter detector in resolving the degeneracies in the neutrino oscillation parameters; the sign of the larger mass-squared difference δ m 13 2 , the sign of | U μ 3 | 2 ( ≡ sin 2 θ ATM ) − 1 / 2 , and a possible two-fold ambiguity in the determination of the CP phase δ MNS . We find that the above problems can be resolved even if the particle charges are not measured. The following results are obtained in our exploratory study for a neutrino factory which delivers 10 21 decaying μ + and μ − at 10 GeV and a 100 kton detector which is placed 2100 km away and is capable of measuring the event energy and distinguishing e ± from μ ± , but not their charges. The sign of δ m 13 2 can be determined for 4 | U e 3 | 2 ( 1 − | U e 3 | 2 ) ≡ sin 2 2 θ RCT ≳ 0.008 . That of sin 2 θ ATM − 1 / 2 can be resolved for sin 2 2 θ ATM = 0.96 when sin 2 2 θ RCT ≳ 0.06 . The CP-violating phase δ MNS can be uniquely constrained for sin 2 2 θ RCT ≳ 0.02 if its true value is around 90° or 270°, while it can be constrained for sin 2 2 θ RCT ≳ 0.03 if its true value is around 0° or 180°.

THE W MASS AND THE U PARAMETER

The Z-pole data from e+e- colliders and results from the NuTeV experiment at Fermilab can be brought into agreement if (1) the neutrino-Z couplings were suppressed relative to the Standard Model (SM), and (2) the Higgs boson were much heavier than suggested by SM global fits. However, increasing the Higgs boson mass will move the theoretical value of the W mass away from its experimental value. A large and positive U parameter becomes necessary to account for the difference. We discuss what type of new physics may lead to such values of U.