Testing Matter Effects in Very Long Baseline Neutrino Oscillation Experiments∗

Assuming three-neutrino mixing, we study the capabilities of very long baseline neutrino oscillation experiments to verify and test the MSW effect and to measure the lepton mixing angle θ13. We suppose that intense neutrino and antineutrino beams will become available in so–called neutrino factories. We find that the most promising and statistically significant results can be obtained by studying νe → νμ and ν̄e → ν̄μ oscillations which lead to matter enhancements and suppressions of wrong sign muon rates. We show the θ13 ranges where matter effects could be observed as a function of the baseline. We discuss the scaling laws of rates, significances and sensitivities with the relevant mixing angles and experimental parameters. Our analysis includes fluxes, event rates and statistical aspects so that the conclusions should be useful for the planning of experimental setups. We discuss the subleading ∆m21 effects in the case of the LMA MSW solution of the solar problem, showing that they are small for L & 7000 km. For shorter baselines, ∆m21 effects can be relevant and their dependence on L offers a further handle for the determination of the CP-violation phase δ. Finally we comment on the possibility to measure the specific distortion of the energy spectrum due to the MSW effect. ∗Work supported in part by ”Sonderforschungsbereich 375 für Astro-Teilchenphysik” der Deutschen Forschungsgemeinschaft, by the TMR Network under the EEC Contract No. ERBFMRX–CT960090 and by the Italian MURST under the program “Fisica Teorica delle Interazioni Fundamentali”. Email: [email protected] Email: [email protected]–muenchen.de Email: [email protected] Also at: INRNE, Bulgarian Academy of Sciences, 1789 Sofia, Bulgaria. Email: [email protected]