Precision measurement of the 7Be solar neutrino interaction rate in Borexino

TL;DR

These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.

Abstract

A direct measurement of the 0.862 MeV 7Be solar neutrino interaction rate performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso yields 46.0$\pm1.5_{\rm stat}$$^{+1.6}_{-1.5\,\rm syst}$ counts/day/(100 tons). Our result is the first direct measurement of a sub-MeV solar neutrino rate with an accuracy better than 5%. The hypothesis of no oscillation for 7Be solar neutrinos is rejected at 4.9$σ$ C.L. Using the latest Standard Solar Model (SSM) flux predictions, the result leads directly to a precise determination of the survival probability for solar $ν_e$'s in vacuum, and permits us to probe with unprecedented sensitivity the transition between the matter-enhanced and vacuum-dominated neutrino oscillation regimes characteristic of the MSW-LMA solution to the solar neutrino problem.

Figures (2)

• Figure 1: Two example fitted spectra; the fit results in the legends have units [counts/(day$\cdot$100 ton)]. Top: A Monte Carlo based fit over the energy region 270--1600~keV to a spectrum from which some, but not all, of the $\alpha$ events have been removed using a PSA cut, and in which the event energies were estimated using the number of photons detected by the PMT array. Bottom: An analytic fit over the 290--1270 keV energy region to a spectrum obtained with statistical $\alpha$ subtraction and in which the event energies were estimated using the total charge collected by the PMT array. In all cases the fitted event rates refer to the total rate of each species, independent of the fit energy window.
• Figure 2: The global experimental constraints on the low energy solar $P_{ee}$. For the $^{7}$Be point, which shows the current result, the inner (red) error bars show the experimental uncertainty, while the outer (blue) error bars show the total (experimental + SSM) uncertainty. The remaining points were obtained following the procedure in bib:barger, wherein the survival probabilities of the low energy ($pp$), medium energy, and high energy ($^8$B) solar neutrinos are obtained, with minimal model dependence, from a combined analysis of the results of all solar neutrino experiments. To illustrate Borexino's effect on the low energy $P_{ee}$ measurements, the green (dashed) points are calculated without using the Borexino data. The MSW-LMA prediction is also shown for comparison; the band defines the 1-$\sigma$ range of the mixing parameter estimate in bib:pdg2010, which does not include the current result.