Borexino’s search for low-energy neutrino and antineutrino signals correlated with gamma-ray bursts

On January 2017 the Borexino collaboration published [Astroparticle Physics 86 (2017) 11-17] the result of the search for neutrino and antineutrino events correlated with 2350 gamma-ray bursts (GRBs).  The search was performed over Borexino data collected between December 2007 and November 2015, and no statistically significant excess over background was observed.

Our analysis combined for the first time two semi-independent data acquisition systems: the primary Borexino readout optimized for solar neutrinos up to a few MeV, and a fast waveform digitizer system tuned for events above 1 MeV.

 

The above plot (as given in Fig.2 of our paper) shows the fluence upper limits for electron antineutrinos from GRBs versus antineutrino energy, considering the detection through Inverse Beta Decay interaction.

The Borexino results are shown in comparison with results from other experiments such as SuperKamiokande, SNO, and KamLAND. Here you can find our result, as depicted in the plot, for both the Borexino primary data acquisition and Borexino FADC analyses.

This detection channel was possibile only thanks to the particularly radio-pure scintillator of Borexino.This plot (the same as given in Fig.4 of our publication)  shows instead the Borexino 90% C.L. fluence upper limits obtained through neutrino-electron elastic scattering for \nu_e (black line), \overline{\nu}_e (red dotted line), \nu_{\mu,\tau} (blue line), and \overline{\nu}_{\mu,\tau} (green dotted line). Limits for \nu_e given by SNO and SuperKamiokande are depicted as well. Related Borexino data can be found here.

Seasonal Modulation of the 7Be Solar Neutrino Rate in Borexino

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:

  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.

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 \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.