Ja n 20 09 Toward CP - even Neutrino Beam

The best method of measuring CP violating effect in neutrino oscillation experiments is to construct and use a neutrino beam made of an ideal mixture of ν̄e and νe of monochromatic lines. The conceptual design of such a beam is described, together with how to measure the CP-odd quantity. We propose to exploit an accelerated unstable hydrogen-like heavy ion in a storage ring, whose decay has both electron capture and bound beta decay with a comparable fraction. It is of great current interest, both to microphysics and to cosmology, to clarify how the matterantimatter asymmetry is generated. One key idea is the leptogenesis [1] in which the lepton asymmetry of the universe is first generated by CP violating lepton number violating processes, and is later converted to the baryon asymmetry via high temperature Band L-violating electroweak processes. To advance this idea further, it is important to deepen our understanding on the origin of CP violation in the leptonic sector. Measurement of CP violation in the neutrino sector is thus one of the most fundamental problems facing physics beyond the standard particle physics. In this note we develop a concept of CP-even neutrino beam, which serves this purpose. Moreover, it is one of the most important experimental issues in neutrino physics to determine the relevant CP parameter in neutrino oscillation experiments. The ideal neutrino beam for CP measurement would be a mixture of monochromatic νe and ν̄e beam for which the detector response is symmetrical, producing after the oscillation equal numbers of μ if CP is conserved. Such a beam is referred to as a CP-even neutrino beam in this paper. Our proposal is to simultaneously use the monochromatic neutrino νe of electron capture (EC) and anti-neutrino ν̄e of bound beta (bβ) decay from hydrogen-like heavy ions. Such ions do exist, for example [2] 114 49 In 48+ , 110 47 Ag 46+ , 108 47 Ag 46+ , 104 45 Rh 44+ . (1) The concept of CP-even neutrino beam is realized, to a large extent, by this kind of beam. It is an extension of the ideas of monochromatic neutrino beam using EC proposed in [3], [4], and beta beam proposed in [5], [6]. Work supported in part by the Grant-in-Aid for Science Research from the Ministry of Education, Science and Culture of Japan No.18654046, No.18684008, and No. 13135207