The HLMA project: determination of high delta-m^2 LMA mixing parameters and constraint on |U_e3| with a new reactor neutrino experiment

TL;DR

This paper proposes a new reactor neutrino experiment at Heilbronn to precisely measure solar neutrino mixing parameters and constrain |U_e3|, especially if the HLMA region is realized, complementing KamLAND's capabilities.

Contribution

It introduces a novel medium baseline reactor experiment at Heilbronn designed to measure solar neutrino parameters and |U_e3| with improved sensitivity in the HLMA region.

Findings

Calculated expected neutrino interaction rates and spectra.

Assessed background signals and detection strategies.

Estimated sensitivity to |U_e3| in different mass hierarchy scenarios.

Abstract

In the forthcoming months, the KamLAND experiment will probe the parameter space of the solar large mixing angle (LMA) MSW solution as the origin of the solar neutrino deficit with $\nuebar$'s from distant nuclear reactors. If however the solution realized in nature is such that $\Dm2_{sol} \gsim 2 \cdot 10^{-4}$ eV$^2$ (thereafter named the HLMA region), KamLAND will only observe a rate suppression but no spectral distortion and hence it will not have the optimal sensitivity to measure the mixing parameters. In this case, we propose a new medium baseline reactor experiment located at Heilbronn (Germany) to pin down the precise value of the solar mixing parameters. In this paper, we present the Heilbronn detector site, we calculate the $\nuebar$ interaction rate and the positron spectrum expected from the surrounding nuclear power plants. We also discuss the sensitivity of such an experiment to |U_e3| in both normal and inverted neutrino mass hierarchy scenarios. We then outline the detector design, estimate background signals induced by natural radioactivity as well as by in-situ cosmic ray muon interaction, and discuss a strategy to detect the anti-neutrino signal 'free of background'.