Abstract
Liquid scintillator detectors play a central role in the detection of neutrinos from various sources. In particular, it is the only technique used so far for the precision spectroscopy of sub-MeV solar neutrinos, as demonstrated by the Borexino experiment at the Gran Sasso National Laboratory in Italy. The benefit of a high light yield, and thus a low energy threshold and a good energy resolution, comes at the cost of the directional information featured by water Cherenkov detectors, measuring B8 solar neutrinos above a few MeV. In this paper we provide the first directionality measurement of sub-MeV solar neutrinos which exploits the correlation between the first few detected photons in each event and the known position of the Sun for each event. This is also the first signature of directionality in neutrinos elastically scattering off electrons in a liquid scintillator target. This measurement exploits the subdominant, fast Cherenkov light emission that precedes the dominant yet slower scintillation light signal. Through this measurement, we have also been able to extract the rate of Be7 solar neutrinos in Borexino. The demonstration of directional sensitivity in a traditional liquid scintillator target paves the way for the possible exploitation of the Cherenkov light signal in future kton-scale experiments using liquid scintillator targets. Directionality is important for background suppression as well as the disentanglement of signals from various sources.8 MoreReceived 10 September 2021Accepted 24 January 2022DOI:https://doi.org/10.1103/PhysRevD.105.052002Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsNeutrinosTechniquesCherenkov detectorsNeutrino detectorsParticle data analysisScintillatorsParticles & Fields
Highlights
Present-day solar neutrino detectors either use scintillation light [1,2] or Cherenkov light [3,4] for neutrino detection
In phase I of the Borexino experiment, which ran from May 16th, 2007, to May 8th, 2010 corresponding to 740.7 d of data acquisition, 19904 events passed the data selection cuts described in Sec
The correlated and integrated directionality (CID) method produces an angular distribution by correlating the direction of the first few photomultiplier tubes (PMTs) hits of each event to the known solar direction and integrating these angles over all the selected events
Summary
Present-day solar neutrino detectors either use scintillation light [1,2] or Cherenkov light [3,4] for neutrino detection. We employ the novel correlated and integrated directionality (CID) method to measure Cherenkov signals of sub-MeV solar neutrinos in a conventional high light yield LS detector, without any specialized hardware or LS mixtures. Though developed for Borexino, other large-volume scintillator detectors like KamLAND [2], JUNO [18], and SNOþ [19] can benefit from this analysis technique This method is different from the event-by-event directional reconstruction using Cherenkov light mentioned previously and instead relies on the well-known position of the neutrino source, which in the case of solar neutrinos is the Sun. This method is different from the event-by-event directional reconstruction using Cherenkov light mentioned previously and instead relies on the well-known position of the neutrino source, which in the case of solar neutrinos is the Sun In this CID technique, we correlate the detected PMT hit pattern to the well-known position of the Sun, and this is integrated over a large number of events.
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