Testing matter effects in very long baseline neutrino oscillation experiments

Abstract

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 θ 13 . 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 ν e → ν µ andν e →ν µ oscillations which lead to matter enhancements and suppressions of wrong sign muon rates. We show the θ 13 ranges where matter effects could be observed as a function of the baseline. We discuss the scaling 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 ∆m 2 21 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, ∆m 2 21 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.

Systematic exploration of the neutrino factory parameter space including errors and correlations

“…A simple approximative result which is valid for small values of θ 13 was derived in [11], where also expressions for larger values of θ 13 are given. Similar formulas were also derived in [12,2].…”

“…The expansion allows, however, to make use of two experimentally justified facts: First, the hierarchy of the neutrino mass splittings, i.e. α = ∆m 2 12 /∆m 2 31 ≪ 1, which allows to identify hierarchy suppressed "small ∆m 2 effects". Second, the mixing angle θ 13 is small with sin 2 2θ 13 < 0.1 [13], leading to further suppression factors as outlined in section 2.1.…”

“…We will also see in section 4.3 that θ 12 and ∆m 2 21 can be measured only very poorly in neutrino factory experiments. To a good approximation only the product ∆m 2 21 sin 2θ 21 can be determined, which can be expected already from the the appearance probability in eq. (3).…”

“…Results obtained from neutrino factories cannot compete with results expected from the long baseline reactor experiment KamLAND [22]. It is thus very important to study how external input on ∆m 2 21 and θ 12 (e.g. from KamLAND) helps to improve the extraction of CP-violating effects at a neutrino factory.…”

“…[1,2,3]). Among the main points discussed are the achievable precision for the oscillation parameters, the optimal baseline and the best muon energy, the search for matter effects, the search for CP violation and specifically, whether measurements of the CP phase are possible.…”