Long distance nu_e -> nu_mu transitions and CP-violation with high intensity beta-beams

Abstract

The recent experimental determinations of a large theta_13 angle have opened the way to a determination of the mass hierarchy and of the CP-violating phase. Experiments based on horn produced (anti-)neutrino conventional beams are presently under development. The event rates are marginal for a definitive search, since they require very intense beams and extremely large detector masses. Zucchelli has proposed a method in which pure (anti-)nu_e beams are generated by the beta-decay of relativistic radio-nuclides stored in a high energy storage ring pointing towards a far away neutrino detector. Since they have a much smaller transverse momentum distribution, the neutrino flux will be much more narrowly concentrated than with a horn. The isomeric doublet Li-8 (anti-nu_e, tau_1/2=0.84s) and B-8 (nu_e, tau_1/2=0.77s) has been studied. Neutrino and antineutrino beams are produced with an average transverse momentum of about 6.5 MeV/c. Radioactive ions may be generated with a dedicated table-top storage ring to supply a suitable ion source to be accelerated at high energies either at FNAL or at CERN. Ions should then extracted from the accelerator and accumulated in a decay storage ring with a long straight section pointing toward the neutrino detector. A massive detector based on liquid Argon technology is probably offering the best opportunities for such future programme. The present ICARUS LAr-TPC experiment has already collected at LNGS events in the relevant neutrino energy region. They should provide a first evidence for a conclusive experimental study of the competing signals and more generally for the actual feasibility of the beta-beam option in a search of the CP violating phase. Additional data may be provided in the near future with the ICARUS and MicroBooNe neutrino experiments located at a short distance neutrino beam and that will collect a much larger number of neutrino events.