Abstract
In this paper we focus on the Earth matter effects for the solar parameter determination by a medium baseline reactor experiment such as JUNO. We derive perturbative expansions for the mixing angles θ12 and θ13 as well as the Δm212 and Δm312 in terms of the matter potential relevant for JUNO. These expansions, up to second order in the matter potential, while simple, allow one to calculate the electron antineutrino survival probability to a precision much better than needed for the JUNO experiment. We use these perturbative expansions to semi-analytically explain and confirm the shift caused by the matter effects on the solar neutrino mixing parameters θ12 and Δm212 which were previously obtained by a purely numerical χ2 analysis. Since these shifts do not satisfy the naive expectations and are significant given the precision that can be achieved by the JUNO experiment, a totally independent cross check using a completely different method is of particular importance. We find that these matter effect shifts do not depend on any of the details of the detector characteristics apart from the baseline and earth mass density between reactor(s) and detector, but do depend on the normalized product of reactor neutrino spectrum times the inverse-beta decay cross-section. The results of this manuscript suggests an alternative analysis method for measuring sin2θ12 and Δm212 in JUNO which would be a useful cross check of the standard analysis and for the understanding of the Wolfenstein matter effect. The explanation of these shifts together with a quantitative understanding, using a semi-analytical method, is the principal purpose of this paper.
Highlights
Jiangmen Underground Neutrino Observatory (JUNO) [1] is a medium baseline reactor antineutrino experiment with ∼ 50 km kilometer baseline that is currently under construction
Despite that we are studying the tiny effect of matter potential, since we are interested to estimate only the matter induced shift of the solar parameters, we believe that taking into account the variation of baselines have essentially no impact on our results
We have shown that the maximum difference between the vacuum and matter oscillation probability occurs at the solar oscillation minimum, around 3 MeV for the JUNO experiment and has a magnitude of 3.5% (4.0%) including energy resolution smearing
Summary
Jiangmen Underground Neutrino Observatory (JUNO) [1] is a medium baseline reactor antineutrino experiment with ∼ 50 km kilometer baseline that is currently under construction. One of the secondary goals of this experiment is to measure the solar neutrino mixing parameters (sin θ12 and ∆m221) and the atmospheric ∆m2’s with sub percent precision. We first derive a simple perturbative expansion for the mixing angles and ∆m2’s in matter Using these matter mixing angles and matter ∆m2’s one can calculate the electron antineutrino survival probability with a precision that is beyond what is needed for the JUNO experiment. Comparing these results it is clear that the naive expectation can explain either the shift for sin θ12 or ∆m221 but not both at the same time using a fixed neutrino energy.
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