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Agarwalla, S. K., Huber, P., Tang, J. A., & Winter, W. (2011). Optimization of the Neutrino Factory, revisited. J. High Energy Phys., 01(1), 120–45pp.
Abstract: We perform the baseline and energy optimization of the Neutrino Factory including the latest simulation results on the magnetized iron detector (MIND). We also consider the impact of tau decays, generated by v(mu) -> v(tau) or v(e) -> v(tau) appearance, on the mass hierarchy, CP violation, and theta(13) discovery reaches, which we find to be negligible for the considered detector. For the baseline-energy optimization for small sin(2) 2 theta(13), we qualitatively recover the results with earlier simulations of the MIND detector. We find optimal baselines of about 2 500km to 5 000km for the CP violation measurement, where now values of E-mu as low as about 12 GeV may be possible. However, for large sin(2) 2 theta(13), we demonstrate that the lower threshold and the backgrounds reconstructed at lower energies allow in fact for muon energies as low as 5 GeV at considerably shorter baselines, such as FNAL-Homestake. This implies that with the latest MIND analysis, low-and high-energy versions of the Neutrino Factory are just two different versions of the same experiment optimized for different parts of the parameter space. Apart from a green-field study of the updated detector performance, we discuss specific implementations for the two-baseline Neutrino Factory, where the considered detector sites are taken to be currently discussed underground laboratories. We find that reasonable setups can be found for the Neutrino Factory source in Asia, Europe, and North America, and that a triangular-shaped storage ring is possible in all cases based on geometrical arguments only.
Keywords: Beyond Standard Model; Neutrino Physics
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Coloma, P., Huber, P., & Schwetz, T. (2021). Statistical interpretation of sterile neutrino oscillation searches at reactors. Eur. Phys. J. C, 81(1), 2–13pp.
Abstract: A considerable experimental effort is currently under way to test the persistent hints for oscillations due to an eV-scale sterile neutrino in the data of various reactor neutrino experiments. The assessment of the statistical significance of these hints is usually based on Wilks' theorem, whereby the assumption is made that the log-likelihood is chi 2-distributed. However, it is well known that the preconditions for the validity of Wilks' theorem are not fulfilled for neutrino oscillation experiments. In this work we derive a simple asymptotic form of the actual distribution of the log-likelihood based on reinterpreting the problem as fitting white Gaussian noise. From this formalism we show that, even in the absence of a sterile neutrino, the expectation value for the maximum likelihood estimate of the mixing angle remains non-zero with attendant large values of the log-likelihood. Our analytical results are then confirmed by numerical simulations of a toy reactor experiment. Finally, we apply this framework to the data of the Neutrino-4 experiment and show that the null hypothesis of no-oscillation is rejected at the 2.6 sigma level, compared to 3.2 sigma obtained under the assumption that Wilks' theorem applies.
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