Cyclotrons as Drivers for Precision Neutrino Measurements

Abstract

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 DAEδALUS program. Examples of the value of the high precision beams for pursuing Beyond Standard Model interactions are reviewed. New results on a combined DAEδALUS-Hyper-K search for CP -violation that achieve errors on the mixing matrix parameter of 4 • to 12 • are presented.As can be seen, our current knowledge of the mixing matrix values is imprecise. The last entry, U e3 , was found to be non-zero just two years ago [18][19][20][21]. Our current state of measurement of the neutrino mixing matrix is analogous to that of the quark sector in 1995 [22], immediately after the discovery of the top quark. Unlike in the quark sector and its utilization of strong production, neutrino physicists are faced with the difficulty of both weak production and weak decay. Our route to precision therefore drives us to high-intensity sources coupled with ultra-large detectors.Even at this relatively early stage, the νSM has been remarkably predictive. For example, the |U e3 | element was found with the ∆m 2 atm splitting [23] as expected from the model. However, many open questions remain. Figure 1 shows a "hierarchy" of the mass states, after arranging the large and small splittings so that the orientation is consistent with what is seen in the quark sector. It is unclear if the neutrinos are oriented in a "normal hierarchy", as shown, or if the orientation is actually "inverted". Further, although the values of each of the splittings have been measured, the absolute mass of the neutrino is not known. We know there is a 3 × 3 matrix that describes the mixing but we don't know if there is CP -violation present as in the quark sector. There are also more exotic questions surrounding the neutrino and oscillations. For example, are there new forces appearing in neutrino interactions and oscillations? Do exotic non-interacting ("sterile") neutrinos mix with the known active flavors? Hints for all of these possibilities exist [24][25][26][27][28] with evidence extending up to 4σ. The next generation of neutrino experiments must investigate these results and clarify the present picture. If history is any indicator of the future, it is quite likely that these experiments, along with the more conventional ones within the νSM, will raise even more surprises.The next step in neutrino physics requires improved tools, in particular, sources from which the energy distribution and flavor content are very well-defined. The beam energy must be in a range where the neutr...

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“…[108] for a discussion of current constraints), but presenting the parameter space in this fashion shows that DAEδALUS/LENA is sensitive to dark photons that decay predominantly to visible states, and that visible decay experiments already cover some regions in which the A decays invisibly. 17 Note that after including preliminary results from NA48/2 [38], the (g −2) µ window for a visibly decaying A is now fully closed (see also Fig. 3e).…”

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“…DAEδ ALUS (Decay-At-rest Experiment for δ CP studies At the Laboratory for Underground Science) is a phased R&D program leading to a high-sensitivity search for CP-violation [1,2]. This is a unique, cyclotron-driven ν µ → ν e search for a non-zero CP violation parameter, δ CP , in the three-neutrino mixing matrix.…”