Analysis of bimodality in histograms formed from GALLEX and GNO solar neutrino data

TL;DR

This study investigates the bimodality in GALLEX solar neutrino data, confirming its significance with a new index, and finds that the GNO data do not show bimodality, with implications for solar neutrino variability.

Contribution

Introduces a bimodality index based on probability-transform to assess bimodality significance in solar neutrino data, revealing temporal variations and potential rotational modulation effects.

Findings

GALLEX data is bimodal at 99.98% confidence level

GNO data does not show bimodality

Bimodality correlates with specific time intervals and rotational modulation

Abstract

A histogram display of the solar neutrino capture-rate measurements made by the GALLEX experiment appears to be bimodal, but that of the follow-on GNO experiment does not. To assess the significance of these results, we introduce a "bimodality index" based on the probability-transform procedure. This confirms that the GALLEX measurements are indeed bimodal (at the 99.98 percent confidence level), and that the GNO measurements are not. Tracking the bimodality index as a function of time shows that the strongest contribution to bimodality comes from runs 42 to 62, i.e. from the time interval 1995.1 to 1996.9. The bimodality index for the first half (runs 1 through 33) is 2.56, whereas that for the second half (runs 33 through 65) is 7.05. Power-spectrum analysis shows a similar distinction: the peaks in the power spectrum formed from the second half are stronger than those in the power spectrum formed from the first half, suggesting that bimodality and rotational modulation are related.