First Directional Measurement of Sub-MeV Solar Neutrinos with Borexino

Area of Science:

• Particle Physics
• Astrophysics
• Nuclear Physics

Background:

• Solar neutrinos provide insights into nuclear processes within stars.
• Previous measurements of low-energy solar neutrinos have faced challenges in directional reconstruction.

Purpose of the Study:

• To measure sub-MeV solar neutrinos using Cherenkov radiation.
• To demonstrate a novel technique correlating photon hits with solar position.
• To validate the standard solar model predictions for neutrino interactions.

Main Methods:

• Utilized the Borexino detector at Laboratori Nazionali del Gran Sasso.
• Employed a novel method correlating individual photon hits with the Sun's known position.
• Selected events in an energy window of 0.54–0.74 MeV using scintillation light.

Main Results:

• Measured 10,887 solar neutrinos within the specified energy window.
• Determined a 7Be neutrino interaction rate of 51.6 counts/(day·100 ton), consistent with predictions.
• Excluded the no-neutrino hypothesis with >5σ confidence.

Conclusions:

• Successfully demonstrated the use of directional Cherenkov information for sub-MeV solar neutrinos.
• Provided experimental proof of principle for hybrid event reconstruction using Cherenkov and scintillation signatures.
• Confirmed agreement with standard solar model predictions and previous Borexino results.