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Results from NOvA

March 14, 2017

20170314_114603Ryan Nichol (right) discussed the NOvA experiment and results. NOvA is the longest baseline accelerator neutrino search, operating on a beam of mainly muon neutrinos created at Fermilab. It measures muon-neutrino disappearance and electron-neutrino appearance, and is now starting to do the same with anti-neutrinos. A sketch of the beam setup is shown below.

novaNeutrinoTelescopes

The decay kinematics implies that a detector at 14.6 mrads from the axis sees a narrowly peaked energy spectrum, 97.5% of it is from muon neuntrinos, and only 0.7% of it is electron neutrinos.

The near and far detectors are both made by extruded plastic cells alternating vertical and horizontal orientation, filled with liquid scintillators. Light is collected by WLS fibres. The far detector has 14 ktons in 896 layers, the near detector is 3.3 ktons.

Ryan showed a couple of graphs that display the readout of the far detector before and after the application of a 10 microsecond spill on-time window based on the beam structure at the source. You can see how the noise is reduced by the timing cut below.

nova1

Above: signals detected in far detector before the 10 microsecond timing cut

nova2

Above: the same event with the timing cut applied, showing the neutrino interaction in two planes of view

The results for muon-neutrino disappearance studies are shown in the slide below.

novaresmu

For electron neutrino appearance studies, the long baseline of NOvA helps, as matter effects in neutrino propagation give higher sensitivity to the experiment for hierarchy studies. A powerful classification algorithm based on a convolutional neural network is used to select electron neutrino events. The signal purity after a selection is of 76%. They observed 33 events with a background of 8.2+-0.8, and obtain a weak preference for a normal hierarchy scenario.

 

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