Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

TL;DR

This work presents Borexino's first precise measurement of the solar $^{8}$B neutrino flux via elastic scattering in a liquid scintillator with a 3 MeV electron-recoil threshold. The analysis combines meticulous detector calibration, a stringent muon and cosmogenic background rejection program, and detailed background modeling to extract the $^{8}$B ES signal from 488 live days of data. The measured rate and corresponding equivalent unoscillated flux agree with SNO and Super-K results, yielding a solar electron-neutrino survival probability of $\overline{P}_{ee}=0.29\pm0.10$ at $\langle E_\nu\rangle\approx 8.9$ MeV and supporting the MSW-LMA framework with a detectable transition between low-energy vacuum and high-energy matter-enhanced regimes. The results, alongside Borexino's $^{7}$Be measurements, provide direct evidence for flavor transformation in the solar neutrino sector and demonstrate the viability of liquid scintillator detectors for precision solar neutrino spectroscopy.

Abstract

We report the measurement of electron neutrino elastic scattering from 8B solar neutrinos with 3 MeV energy threshold by the Borexino detector in Gran Sasso (Italy). The rate of solar neutrino-induced electron scattering events above this energy in Borexino is 0.217 +- 0.038 (stat) +- 0.008 (syst) cpd/100 t, which corresponds to the equivalent unoscillated flux of (2.4 +- 0.4 (stat) +- 0.1 (syst))x10^6 cm^-2 s^-1, in good agreement with measurements from SNO and SuperKamiokaNDE. Assuming the 8B neutrino flux predicted by the high metallicity Standard Solar Model, the average 8B neutrino survival probability above 3 MeV is measured to be 0.29+-0.10. The survival probabilities for 7Be and 8B neutrinos as measured by Borexino differ by 1.9 sigma. These results are consistent with the prediction of the MSW-LMA solution of a transition in the solar electron neutrino survival probability between the low energy vacuum-driven and the high-energy matter-enhanced solar neutrino oscillation regimes.

Figures (8)

• Figure 1: Black dots are the measured peak positions of $\gamma$ radiation induced by neutron captures in $^{1}$H (2.223 MeV), $^{12}$C (4.945 MeV), $^{56}$Fe (7.631 MeV) and $^{54}$Fe (9.298 MeV) in the detector center. Red line is the Monte Carlo prediction for $\gamma$ rays generated in the detector center.
• Figure 2: Ratio of the reconstructed radial position of $\gamma$ events from the $^{241}$Am$^9$Be source in Borexino to the source radial position measured by the CCD camera system, as a function of the measured charge.
• Figure 3: Cumulative distribution of events with energy $>$ 3 MeV within a 5 s window after a muon in Borexino. The time distribution has been fit to three decay exponentials. The ensuing exponential lifetimes are $\tau =$ 0.031$\pm$0.002 s, 0.25$\pm$0.21 s, 1.01$\pm$0.36 s and corresponds to the contribution from $^{12}$B, $^{8}$He $+$$^{9}$C $+$$^{9}$Li and $^{8}$B $+$$^{6}$He $+$$^{8}$Li, respectively. The expected and measured rates for these cosmogenic isotopes are summarized in Table \ref{['tab:cosmogenic']}.
• Figure 4: Time profile of events with energy $>$ 3 MeV within 240 s after a muon and within 2 m from its track in the entire Borexino active volume (278 t). The distribution has been fit to the three decay exponentials as in Figure \ref{['fig:TimeAfterMuon']}, plus the $^{11}$Be component, with fixed mean-lives.
• Figure 5: Fit of the radial distribution for events with E$>$3 MeV. The red line represents the uniformly distributed event component in the active mass, the green line the surface contamination, and the blue line is external background.