Feasibility and physics potential of detecting ⁸B solar neutrinos at JUNO *

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) features a 20 kt multi-purpose underground liquid scintillator sphere as its main detector. Some of JUNO's features make it an excellent location for B solar neutrino measurements, such as its low-energy threshold, high energy resolution compared with water Cherenkov detectors, and much larger target mass compared with previous liquid scintillator detectors. In this paper, we present a comprehensive assessment of JUNO's potential for detecting B solar neutrinos via the neutrino-electron elastic scattering process. A reduced 2 MeV threshold for the recoil electron energy is found to be achievable, assuming that the intrinsic radioactive background U and Th in the liquid scintillator can be controlled to 10 g/g. With ten years of data acquisition, approximately 60,000 signal and 30,000 background events are expected. This large sample will enable an examination of the distortion of the recoil electron spectrum that is dominated by the neutrino flavor transformation in the dense solar matter, which will shed new light on the inconsistency between the measured electron spectra and the predictions of the standard three-flavor neutrino oscillation framework. If eV , JUNO can provide evidence of neutrino oscillation in the Earth at approximately the 3 (2 ) level by measuring the non-zero signal rate variation with respect to the solar zenith angle. Moreover, JUNO can simultaneously measure using B solar neutrinos to a precision of 20% or better, depending on the central value, and to sub-percent precision using reactor antineutrinos. A comparison of these two measurements from the same detector will help understand the current mild inconsistency between the value of reported by solar neutrino experiments and the KamLAND experiment.

Radioactivity control strategy for the JUNO detector

“…The stricter requirements on the radiopurity of the LS (LS-solar) will make it possible to study solar neutrinos with the JUNO experiment, with the external background made totally negligible by the application of more stringent fiducial volume cuts [11].…”

“…The lack of a coincidence signature, in this case, poses even more stringent demands on the background level in the energy region below about 20 MeV and was already studied in ref. [11].…”

“…The expected IBD rate in the JUNO CD induced by reactor neutrinos is about 60 counts per day (cpd), while the singles rate from natural radioactivity should be controlled to less than 10 counts per second, leading to ∼1 cpd accidental coincidence (see section 5.3) with default antineutrino selection cuts, similar to 8 He/ 9 Li and geoneutrinos background sources. However, for solar neutrino detection whose signal is a single event, this limit will be several orders of magnitude lower and achieved by applying more stringent fiducial volume (FV) and timing cuts to remove natural radioactivity and cosmogenic backgrounds, as described in a dedicated paper [11]. We should point out that 14 C and 85 Kr are not considered here JHEP11(2021)102 due to their decay energy (Q β =156 keV and Q β =687 keV, respectively) below the default energy cut for IBD reactions (0.7 MeV -introduced in section 3.1).…”

Borexino Results on Neutrinos from the Sun and Earth

“…JUNO [140], the first multi-kton liquid scintillator detector plans to complete its construction in south China in 2022. With its 20 kton LS target, it has a potential to measure 8 B neutrinos down to an unprecedented low threshold of 2.5 MeV [48], discover the small expected flux of hep solar neutrinos, and to collect the world's largest statistics of geoneutrinos [141]. The future Jinping experiment [49] aims to perform precision spectroscopy of CNO solar neutrinos and geoneutrinos, taking ad-vantage of its shielding against cosmic muons in the world's deepest laboratory located at Jinping in China.…”

“…Even today, they are at the base of the most precise determination of the θ 12 mixing angle [46]. More recently, they are being used in searches for physics beyond the Standard Model [17,47] and are among the goals of future experiments [48][49][50][51]. The vast majority (about 99%) of the energy produced in the Sun comes from a series of reactions fusing Hydrogen to Helium, called pp chain.…”