First measurement of pp neutrinos in real time in the Borexino detector

Abstract

The Sun is fueled by a series of nuclear reactions that produce the energy that makes it shine.

Neutrinos (ν) produced by these nuclear reactions exit the Sun

and reach Earth within minutes, providing us with key information

about what goes on at the core of our star. For over twenty years since the first detection of solar neutrinos in the late 1960's, an apparent deficit in their detection rate was known as the <italic>Solar Neutrino Problem</italic>. Today, the Mikheyev-Smirnov-Wolfenstein (MSW) effect is the accepted mechanism by which neutrinos oscillate inside the Sun, arriving at Earth as a mixture of &nu;e, &nu;&mu; and &nu;&tau;, the latter two of which were invisible to early detectors.

Several experiments have now confirmed the observation of neutrino oscillations.

These experiments, when their results are combined together, have demonstrated that neutrino oscillations are well described by the Large Mixing Angle (LMA) solution of the MSW effect.

This thesis presents the first measurement of <italic>pp</italic> neutrinos in the Borexino detector, which is another validation of the LMA-MSW model of neutrino oscillations.

In addition, it is one more step towards the completion of the spectroscopy of <italic>pp</italic> chain neutrinos in Borexino, leaving only the extremely faint <italic>hep</italic> neutrinos undetected. This advance validates the experiment itself and its previous results.

This is, furthermore, the first direct real-time measurement of <italic>pp</italic> neutrinos.

We find a <italic>pp</italic> neutrino detection rate of 143&plusmn;16 (stat)&plusmn;10 (syst) cpd/100 t in the Borexino experiment, which translates, according to the LMA-MSW model, to (6.42&plusmn;0.85)&times;10<super>10</super> cm<super>-2</super> s<super>-1</super>.

We also report on a measurement of neutrons in a dedicated system within the Borexino detector, which resulted in an improved understanding of neutron rates in liquid scintillator detectors at Gran Sasso depths.

This result is crucial to the development of novel direct dark matter detection experiments.