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Poster Summary: Supernova Neutrinos with NEWSdm Detector

March 15, 2017

(by Valerio Gentile) The explosion of a Supernova is one of the most energetic phenomena in the Universe. Neutrinos coming from supernovae are largely investigated since they carry almost the total energy emitted by the source and they could directly explain the underlying processes inside a star leading to the explosion.

Although the observation of these events are very uncommon, the new generation of ton-scale detector for dark matter search are sensitive to neutrinos coming from supernova. While they can represent a serious and unremovable background source, a directional detector could provide an unambiguous proof of the SN neutrino interaction.
NEWSdm is a directional detector for Dark Matter search using nuclear emulsions both as target and tracking detector with nanometric resolution.

The detector is surrounded by a shielding for the environmental and cosmogenic background source suppression and it will be placed on an equatorial telescope in order to keep its orientation towards Cygnus constellation fixed. The NEWSdm detector, located in the Gran Sasso Underground Laboratory, is made of a new kind of nuclear emulsion called NIT (Nano Imaging Trackers ) with AgBr crystals of about 40 nm of diameter immersed in an organic gelatine. NIT emulsion make the reconstruction of trajectories with path lengths shorter than 100 nm possible if analyzed by means of microscopes with enough resolution.

In order to detect tracks comparable and shorter than the optical resolution a two steps strategy is adopted: the first one consists of an elliptical fit for all clusters, since the scanning with optical microscope cannot distinguish two grains closer than 200 nm. Indeed, two grains would appear as a single cluster with an elliptical shape, with the major axis along the actual direction of the recoiled nucleus, unlike a single grains from thermal excitation that would appear as spherical.

The first step is followed by a validation of the selected tracks with polarized light analysis using the resonance effect of nanometric metallic grains in a dielectric medium. The resonance effect is sensitive to the shape of nanometric grains: when silver grains are not spherical, the resonant response depends on the polarization of the incident light. This technique provides an accuracy in the grains positions of about 10 nm in both coordinates.

A study was performed to evaluate the capability of the NEWSdm detector to observe supernova neutrinos. In order to discriminate the signal (nuclear recoils induced by SN neutral current neutrino interactions) from the background (radiogenic neutrons and solar neutrinos from 8B) three kinematical variables were used: track length, phi and theta angles. A likelihood ratio test was used to estimate the separation between the signal and the background hypotheses. 16 observed events are required to have a separation larger than 3 sigma CL. The observation of SN neutrinos can be claimed with 3 sigma CL with a 1 ton detector for exposures shorter than five years.

Figure 1: The test statistic (q) for the signal hypothesis tested against the background only hypothesis.The expectation for the background is represented by the blue histogram on the left and for the alternative signal hypothesis by the orange histogram on the right. The red line indicates the median q value for the SN neutrino signal. Assuming 16 observed events, a separation at 3 sigma CL is obtained.



Figure 2: The test statistic (q) as a function of the exposure time for the signal plus background hypothesis (S+B) in blue dotted line. The 3 sigma CL is represented by the red dotted line. The shorter the exposure time the larger the significance of S+B hypothesis, since only the background increase with the time. The observation of SN neutrinos can be claimed with 3 sigma CL with a 1 ton detector for exposures shorter than five years.

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