New Initiatives on Lepton Flavor Violation and Neutrino Oscillation with High Intense Muon and Neutrino Sources

The following sections are included:

• PREFACE

• Contents

Potential applications of muons to various fields of scientific research have been recognized since the discovery of the muon among cosmic-rays. Recent progress is reviewed here with special emphasis on (1) muon-catalyzed fusion for fusion energy development, (2) an electron labelling method via muon spin-relaxation measurements for life science applications and (3) the application of high-energy muons and neutrinos to disaster prevention, such as the construction of a data-base for predictions of volcanic eruption and earthquakes.

In the coming years, experiments underway will increase the sensitivity to charged lepton flavor violation by four orders of magnitude. These experiments will stringently probe weak scale physics. I review the status of global symmetries in the standard model and present several well-motivated models that predict observable lepton flavor violation. Finally, I describe what we might learn from future experimental results, whether positive or null.

In this article we review lepton flavor violation in supersymmetric models with right-handed neutrinos which naturally explain the tiny neutrino masses.

Supersymmetric (SUSY) models with R-parity violation (RPV) can be an alternative scenario for non-zero neutrino masses. Within this framework, we discuss the lepton-flavor violating (LFV) processes µ → eγ, µ → 3e and the µ → e conversion in nuclei. We find the interesting features of LFV in RPV models, which are very different from other neutrino models such as SUSY models with heavy right-handed neutrinos. We show that a search for all the LFV processes is important to distinguish between the different models, and the measurement of P-odd asymmetries in polarized µ → 3e is also useful to reveal the nature of LFV.

The observation of atmospheric neutrinos in Super-Kamiokande experiment indicates a large lepton flavor violation (LFV) among fundamental interactions in addition to neutrino masses. The minimal supersymmetric standard model augmented by right-handed neutrinos is an attractive candidate of neutrino masses and LFV. It possibly leads to a sizable mixing between the second and the third generation left-handed sleptons. In this talk, we examine probable signals of LFV in the production and decay of such left-handed sleptons at an e⁺e^- linear collider and a µ⁺µ^- collider.

The present experimental situation regarding neutrino oscillations is first summarized, followed by an overview of selected grand unified models which have been proposed to explain the various scenarios with three active neutrinos and their right-handed counterparts. Special attention is given to the general features of the models and their ability to favor some scenarios over others.

The transition strengths of the µ^- → e^- conversion channels, calculated with several nuclear methods, are exploited to put constraints on the parameters of a model dependent Lagrangian which involves interactions mediated by various particles (W-, Z-bosons, virtual photons, etc.) as well as SUSY R-parity violating interactions. We focus on the particularly interesting nuclei ⁴⁸Ti and ²⁷Al, i.e. the stopping targets of the µ^- → e^- experiments SINDRUM2 and MECO.

Decay modes with a violation of lepton flavor conservation (LFV) test fundamental standard model extension ¹. The most stringent upper limit on LFV-decay modes comes from the search for muon electron conversion in muonic atoms with the SINDRUM II spectrometer at PSI²: Γ(µ^-Ti → e^-Ti^g.s.)/Γ(µ^-Ti capture) < 6.1 × 10^-13 (90% CL). This report summarizes the main SINDRUM II results, searches for µe conversion on different nuclear targets (Ti, Au, Pb) and for charge-changing muon conversion to a positron. Preliminary results of this years µe measurement on gold are reported.

An experiment is described to search for muon to electron conversion in the field of a nucleus with one event sensitivity, normalized to the muon capture rate on the same nucleus, of 2 × 10^-17. The muon to electron transition does not conserve lepton flavor. For it to occur at an observable rate new physics is required, beyond the usual Standard Model and minimal extensions that include the possibility of light massive neutrinos.

The results of simulations based on GEANT 3.21 library and technical details of the proposed MECO experiment are discussed in this paper: time structure of the proton beam, experimental setup and fluxes of secondary particles at entrance of the detector solenoid.

A Japanese PRISM project which is aiming to construct a highly-intense slow muon source with narrow energy spread and high purity is described.

A facility PRISM (Phase Rotated Intense Slow Muon) is currently under an intense design work as one of physics projects for the forthcoming high intensity proton synchrotron in Tokai, Japan¹. With this high intensity muon beam (10¹² muons per second), we would be able to push forward physics of lepton flavor violation in near future. The branching ratio of µ^- - e^- conversion could be studied at a level of 10^-18, and muonium-antimuonium conversion could be also studied below a level of .

The Search for Lepton Flavor Violation(LFV), the new physics search beyond the Standard Model, has a long history since muon was discovered in 1937. A new search for µ⁺ → e⁺γ decay is planed with sensitivity of 10^-14 at Paul Scherrer Institute(PSI),¹ and detectors have been developed. The concept and status of detector development are presented in this talk.

Leptogenesis, i.e. the creation of a lepton asymmetry in the early Universe, may occur through the decay of heavy singlet (right-handed) neutrinos. If we require it not to be erased by physics beyond the Standard Model at the TeV energy scale, then only 2 candidates are possible if they are subgroups of E₆. These 2 solutions happen to be also the only ones within 1σ of the atomic parity violation data and the invisible Z width. Lepton flavor violation is predicted in one model, as well as in another unrelated model of neutrino masses where the observable decay of a doubly charged scalar would determine the relative magnitude of each element of the neutrino mass matrix.

We show that if the presently observed L/E-fratness of the electron-like event ratio in the Super-Kamiokande atmospheric neutrino data is confirmed then the indicated ratio must be unity. Further, it is found that once CP is violated the exact L/E flatness implies: (a) The CP-violating phase, in the standard parameterization, is narrowed down to two possibilities ±π/2, and (b) The mixing between the second and the third generations must be maximal. With these results at hand, we argue that a dedicated study of the L/E-flatness of the electron-like event ratio by Super-Kamiokande can serve as an initial investigatory probe of CP violation in the neutrino sector.

The physics agenda for future long-baseline neutrino oscillation experiments is outlined and the prospects for accomplishing those goals at future neutrino facilities are considered. Neutrino factories can deliver better reach in the mixing and mass-squared parameters but conventional super-beams with large water or liquid argon detectors can probe regions of the parameter space that could prove to be interesting.

Recent status of neutrino oscillation phenomenology with four neutrinos is reviewed. It is emphasized that the so-called (2+2)-scheme as well as the (3+1)-scheme are still consistent with the recent solar and atmospheric neutrino data.

We observe that neutrino oscillation experiments are sensitive to flavor change from direct flavor-violating interactions in addition to neutrino oscillations, and we explore the consequences of this "confusion" effect. We show that near detectors at neutrino factories and MINOS could improve present constraints on new physics by more than an order of magnitude. In addition, neutrino factories could observe CP violation from direct interactions.

We study the optimal setup for observation of the CP asymmetry in neutrino factory experiments — the baseline length, the muon energy and the analysis method. First, we point out that the statistical quantity which has been used in previous works doesn't represent the CP asymmetry. Then we propose the more suitable quantity, which is sensitive to the CP asymmetry. We investigate the behavior of with ambiguities of the theoretical parameters. The fake CP asymmetry due to the matter effect increases with the baseline length and hence the error in the estimation of the fake CP asymmetry grows with the baseline length due to the ambiguities of the theoretical parameters. Namely, we lose the sensitivity to the genuine CP-violation effect in longer baseline.

The considerable interest raised by the discovery of neutrino oscillations and recent progress in studies of muon colliders has triggered interest in considering a neutrino factory at CERN. This paper explains the reference scenario, indicates the other possible choices and mentions the R&D that are foreseen.

The Japanese scheme of a neutrino factory based on fixed-field alternating(FFAG) gradient accelerators is presented in this paper. The FFAG based neutrino factory has several advantages compared with the linear accelerator based designs in U.S. and Europe.

It is shown that explanations of atmospheric neutrino anomaly other than ν_µ - ν_τ oscillations (e.g. decay, decoherence and ν_µ - ν_τ - ν_KK mixing) can be tested at future facilities. Stringent tests of CPT invariance in neutrino oscillations can also be performed.

A long baseline experiment is under preparation to start from the year 2006 with a conventional neutrino beam from JHF and Super-Kamiokande as the far detector. The precision for the ν_µ disappearance channel are δ(Δm²) ~ 2 × 10^-4eV² and δ(sin² 2θ₂₃) ~ 0.01 by five year exposure. The sensitivity for the ν_e appearance channel is sin² 2θ₁₃ ~ 6 × 10^-3 at the atmospheric neutrino best fit point.

MOON(Molybdenum Observatory Of Neutrinos) is a high sensitive detector used for spectroscopic studies of neutrinos(ν) in ¹⁰⁰Mo nuclear micro-laboratories. MOON can be used for spectroscopic studies of Majorana ν masses by double beta decays(ββ) of ¹⁰⁰Mo and real-time studies of low energy solar ν's and supernova ν's by inverse beta decays. Nuclear responses are crucial for ν studies in nuclei. Nuclear weak responses for ββ - ν, solar-ν, supernova-ν, and for ν's of astrophysics interest are studied well by means of intense ν beams combined with large ν detectors.

Hyper-Kamiokande, a very large water Cherenkov detector with mass of ~ 1 Mton, is discussed for a next generation nucleon decay experiment at Kamioka. Other physics possibilities with this detector are also mentioned. The sensitivities of such a large water Cherenkov detector are studied for the p → e⁺π⁰ decay mode, assuming 40%, 10%, and 4.4% photocathode coverages, with the same or similar selection criteria as the Super-Kamiokande analysis and with a tighter total momentum cut.

We propose here a large homogeneous calorimeter as the next generation neutrino detector for ν factories and/or conventional ν beams. The active media is chosen to be water for obvious economical reasons. The Čerenkov light produced in water is sufficient to have good energy resolution, and the pattern recognition is realized by a modular water tank structure. Monte Carlo simulations demonstrate that the detector performance is excellent for identifying neutrino CC events while rejecting background events.

The following sections are included:

• Scientific Program