As we enter the age of precision measurement in neutrino physics, improved flux sources are required. These must have a well defined flavor content with energies in ranges where backgrounds are low and cross-section knowledge is high. Very few sources of neutrinos can meet these requirements. However, pion/muon and isotope decay-at-rest sources qualify. The ideal drivers for decay-at-rest sources are cyclotron accelerators, which are compact and relatively inexpensive. This paper describes a scheme to produce decay-at-rest sources driven by such cyclotrons, developed within the DAE ALUS program. Examples of the value of the high precision beams for pursuing Beyond Standard Model interactions are reviewed. New results on a combined DAE ALUS—Hyper-K search for CP violation that achieve errors on the mixing matrix parameter of 4° to 12° are presented. 1. Introduction As we reach the 100th anniversary of the birth of Bruno Pontecorvo, neutrino physics is facing a transition. Neutrino oscillations are well established, albeit in a different form from what Pontecorvo expected [1, 2]. We have a data-driven “Neutrino Standard Model,” ( SM) which, despite questions about its underlying theoretical description, is remarkably predictive. Now, the neutrino community must pivot from “searches” to “precision measurements,” in which we can test the SM. The transition requires new and better tools for these measurements and further calls for original approaches to experiments. The SM is simply described in Figure 1. The three known neutrino flavors mix within three mass states. The separations between the states, or “mass splittings,” are defined as , for The historical name for the smaller splitting ( ) is and the larger mass splitting ( is referred to as , in honor of the solar and atmospheric experiments that established the existence of each. The early solar [3–6] and atmospheric [7–9] experiments have been joined by new results [10–15] to establish this phenomenology [16]. Figure 1: Illustration of the “ SM” showing mass states and mixings. Note that this drawing depicts only one possible mass ordering. There remain many open questions that surround this data-driven picture of neutrinos and oscillations. The mixings are described with a matrix, commonly called the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, that connects the mass eigenstates ( , , and ) to the flavor eigenstates ( , , and ): where the ranges indicate our knowledge of each of the entries [27]. Together with the mass splittings, the mixing matrix is pictorially represented in Figure 1, in which
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