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Poster excerpt 16: Determination of Supernova Direction in a Liquid Scintillator Detector

March 4, 2015

(The following text has been produced by Y.P. Cheng)

cheng1The supernovae is assumed as the explosion in the final stage of the stellar evolution, one of the most violent and spectacular events in the Universe. A core collapse supernovae (SN) may be not optically visible at all due to dust obscuration or because some collapsing stars may never blow up into supernovae. However, neutrino signal emerging from the core provides a way to determine supernovae direction. Jiangmen Underground Neutrino Observatory (JUNO) is a Liquid scintillator (LS) reactor antineutrino experiment aiming to determine the neutrino mass hierarchy. It can detect about 5000 neutrino events from a 10 kpc supernovae. On the right is a picture of JUNO detector.

Cerenkov light in liquid scintillator can be used to reconstruct particle track because it’s highly directional. Most cerenkov light is absorbed and reemitted as scintillation light, not directional anymore. The fact that cerenkov light is produced instantaneously, while scintillation light has different time constants (several nanoseconds), and cerenkov light moves faster in LS due to longer wavelength, makes it possible to separate these two lights[1]. Below is a picture showing separation of these two lights by their hittime difference. PMT Transit Time Spread(tts) makes their separation more challenging.

cheng2

Then hits generated from Cerenkov light are used for positron direction reconstruction in liquid scintillator. The direction of the positron track is determined by taking the centroid of all vectors pointing from the reconstructed vertex (from other algorithms, such as likelihood algorithm) to the Cerenkov hits on the detector. For mean energy positron from supernovae anti-electron neutrino, the 1 sigma uncertainty is 23°, which means 68.3% reconstructed positron direction are in a 23° angle cone centered the true positron direction.

Since positron direction information can be estimated from above procedures (1ns PMT tts is taken into consideration), neutrino direction can be calculated on a per event basis using conservation of momentum and energy. This set of equation can be solved by looking up in a two dimensional map. Two difference methods are applied to derive SN direction. One is the same as Ref[2], only neutron direction is used. SN direction uncertainty is 11.21°. The other take the statistically averaged anti-electron neutrino direction as SN direction with same statistics as method one, then the SN direction uncertainty is 8.17°. Thus, it helps supernovae direction determination if additive positron direction information is used.

References:

[1] Arxiv 1307.5813

[2] Phys.Rev.D 61 (1999) 012001

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