On January 2017, the Borexino collaboration presented (currently, the paper has been submitted to the Astroparticle Physics Journal and is available on Arxiv only) 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:
- the sinusoidal fit;
- the Lomb-Scargle method;
- 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.
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 Arxiv pubblication) shows the rate of -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.