Category Archives: Solar neutrinos

Seasonal Modulation of the 7Be Solar Neutrino Rate in Borexino

On June 2017, the Borexino collaboration published [Astroparticle Physics 92 (2017) 21-29] an improved measurement of time periodicities of the 7Be solar neutrino interaction rate.

This yearly modulation was observed on 4 years (December 2011 – December 2015) of Borexino Phase II data.

The period, amplitude, and phase of the observed time evolution of the signal are consistent with its solar origin, and the absence of an annual modulation is rejected at 99.99% C.L.

The data were analyzed using three methods:

  1. the sinusoidal fit;
  2. the Lomb-Scargle method;
  3. the Empirical Mode Decomposition techniques.

All the analysis methods clearly confirm the presence of an annual modulation of the 7Be solar neutrino interaction rate and show no signs of other periodic time variations.

This plot (as given in Fig.4 of our paper) shows the \beta-like event rate [cpd/100 ton] along with the best fit in 30.4-days long bins, starting from Dec 11, 2011 . The red line describes the analytical fit equation:

    \[\mathrm{R(t)=R_0 + \overline{R}} \left[ 1 +  \epsilon \cos \frac {2 \pi} {\mathrm{T}}  \left( \mathrm{t} - \phi \right) \right]^2.\]

Related data (Nbin, rate and errors) are available here.

The robustness of the fit was studied by varying the bin size and the fitting methods. The following plot (as given in Fig. 5 of the publication) shows the rate of \beta-like events passing selection cuts in 7-days long bins. This time, the red line is the best fit resulting from the Lomb-Scargle analysis. Associated data (Nbin, rate and errors) can be found here.

Neutrinos from the primary proton-proton fusion process in the Sun

In 2014, the Borexino collaboration published (Nature 512, pp 383 2014) the first real-time measurement of neutrinos from the proton-proton fusion process.

In the core of the Sun, energy is released through sequences of nuclear reactions that convert hydrogen into helium. The primary reaction is thought to be the fusion of two protons with the emission of a low-energy neutrino. These so-called pp neutrinos constitute nearly the entirety of the solar neutrino flux, vastly outnumbering those emitted in the reactions that follow. Although solar neutrinos from secondary processes have been observed, proving the nuclear origin of the Sun’s energy and contributing to the discovery of neutrino oscillations, those from proton–proton fusion have hitherto eluded direct detection. Here we report spectral observations of pp neutrinos, demonstrating that about 99% of the power of the Sun, 3.84 × 1033 ergs per second, is generated by the proton–proton fusion process.

Fig3This plot (as given in Fig. 3 of the Borexino Nature paper) reports the fit of the Borexino energy spectrum between 165 and 590 keV and the relative residuals. The properties of the data bins and residuals are available here.

FigExt2This plot, as shown in “Extended Data Figure 2″ of the Borexino Nature paper, reports the ± 1\sigma prediction of the MSW-LMA solution for the electron survival probability as well as the survival probability points associated to the Borexino experimental measurements of pp, 7Be, pep and 8B solar neutrino flux.

Numerical values for the electron survival probability band are available here  while the coordinates of the experimental Borexino measurements can be found here.