No abstract

In the next decade, a number of experiments will attempt to determine the neutrino mass hierarchy. Feasibility studies for such experiments generally determine the statistic ∆χ 2 . As the hierarchy is a discrete choice, ∆χ 2 does not obey a one degree of freedom χ 2 distribution and so the number of σ's of sensitivity to the hierarchy is not the square root of ∆χ 2 . We present a simple Bayesian formula for the sensitivity to the hierarchy determination that can be expected from the median experiment as a function of ∆χ 2 .

Coherent elastic neutrino-nucleus scattering (CEνNS) can be used to determine the neutron part of nuclear form factors, unlocking intrinsic properties of nuclear structure. In contrast with other such methods, CEνNS is free from both strong interaction effects and Coulomb distortions. We propose precision measurements of CEνNS with an upcoming accelerator facility and determine the corresponding requirements for such a neutrino detector. We find that most significant backgrounds come from fast neutrons, induced by cosmogenic muons or from the pion decays at rest in the target station. With ton-scale liquid noble gas detectors, we will not only achieve percent level precision in the measurement of neutron radii but also clarify contributions of higher-order moments to nuclear form factors.

We report the results of Monte Carlo simulations of a medium baseline reactor neutrino experiment. The difference in baselines resulting from the 1 km separations of Daya Bay and Ling Ao reactors reduces the amplitudes of 1-3 oscillations at low energies, decreasing the sensitivity to the neutrino mass hierarchy. A perpendicular detector location eliminates this effect. We simulate experiments under several mountains perpendicular to the Daya Bay/Ling Ao reactors, considering in particular the background from the TaiShan and YangJiang reactor complexes. In general the hierarchy can be determined most reliably underneath the 1000 meter mountain BaiYunZhang, which is 44.5 km from Daya Bay. If some planned reactors are not built then nearby 700 meter mountains at 47-51 km baselines gain a small advantage. Neglecting their low overhead burdens, hills near BaiMianShi or DongKeng would be the optimal locations. We use a weighted Fourier transform to avoid a spurious dependence on the high energy neutrino spectrum and find that a neural network can extract quantities which determine the hierarchy marginally better than the traditional RL + P V .

We study the advantages of a second identical detector at a medium baseline reactor neutrino experiment. A major obstruction to the determination of the neutrino mass hierarchy is the detector's unknown nonlinear energy response, which even under optimistic assumptions reduces the confidence in a hierarchy determination by about 1σ at a single detector experiment. Various energy response models are considered at one and two detector experiments with the same total target mass. A second detector at a sufficiently different baseline eliminates this 1σ reduction. Considering the unknown energy response, we find the confidence in a hierarchy determination at various candidate detector locations for JUNO and RENO 50. The best site for JUNO's near detector is under ZiLuoShan, 17 km and 66 km from the Yangjiang and Taishan reactor complexes respectively. We briefly describe other advantages, including a more precise determination of θ 12 and the possibility of a DAEδALUS inspired program to measure the CP-violating phase δ using a single pion source about 10 km from one detector and 20 km from the other. Two identical detectors provide a better energy resolution than a single detector, further increasing the confidence in a hierarchy determination.

We propose a novel experimental setup for the determination of the leptonic CP-violating phase δ using the decay at rest (DAR) of µ + from a single source located at distances of 10 and 30 km from two 20 kton organic liquid scintillator detectors. The µ + are created by bombarding a target with a 9 mA beam of 800 MeV protons. With this proposal δ can be determined with a precision of 20 (15) degrees in 6 (12) years. In contrast with the DAEδALUS project, only a single source is required and it runs with a duty factor limited only by maintenance requirements. As a result 9 mA is the maximum instantaneous current, greatly reducing both the technological challenges and the costs.

10 years from now reactor neutrino experiments will attempt to determine which neutrino mass eigenstate is the most massive. In this letter we present the results of more than seven million detailed simulations of such experiments, studying the dependence of the probability of successfully determining the mass hierarchy upon the analysis method, the neutrino mass matrix parameters, reactor flux models, geoneutrinos and, in particular, combinations of baselines. We show that a recently reported spurious dependence of the data analysis upon the high energy tail of the reactor spectrum can be removed by using a weighted Fourier transform. We determine the optimal baselines and corresponding detector locations. For most values of the CP-violating, leptonic Dirac phase δ, a degeneracy prevents NOνA and T2K from determining either δ or the hierarchy. We determine the confidence with which a reactor experiment can determine the hierarchy, breaking the degeneracy.

Gallium and short baseline reactor neutrino experiments indicate a short-distance anomalous disappearance of electron antineutrinos which, if interpreted in terms of neutrino oscillations, would lead to a sterile neutrino mass inconsistent with standard cosmological models. This anomaly is difficult to measure at 1 km baseline experiments because its disappearance effects are degenerate with that of θ 13 . The flux normalization independent measurement of θ 13 at Daya Bay breaks this degeneracy, allowing an unambiguous differentiation of 1-3 neutrino oscillations and the anomalous disappearance at Double Chooz and RENO. The resulting anomaly is consistent with that found at very short baselines and suggests a downward revision of RENO's result for θ 13 . A MCMC global analysis of current cosmological data shows that a quintom cosmology is just compatible at 2σ with a sterile neutrino with the right mass to reproduce the reactor anomaly and to a lesser extent the gallium and LSND/MiniBooNE anomalies. However models in which the sterile neutrino acquires a chameleon mass easily satisfy the cosmological bounds and also reduce the tension between LSND and KARMEN.