The goal of NO\(\nu\)A is to distinguish electron neutrinos from the muon neutrinos in the beam; therefore, it is important to estimate the amount of background measurements and false positives expected in the data.
Since the Near Detector (ND) is so close to the NuMI beam, it is able to measure the particle before it has had a chance to oscillate or interact with other matter, providing an idea of what results are reasonable in the Far Detector (FD). The data recorded by the ND is separated into 4 different “channels” that are treated seprately. These channels are: muon neutrinos, charged interactions, neutral interactions, and neutral interactions with small \(\nu_{e}\) behavior.
Muon neutrinos travel much farther in the detector than electron, making them considerably easier to detect. See figure \ref{fig:tracks} for more details. From this, its clear that charged interactions do not make up a large amount of the background measurements. The largest source is from neutral currents whose hadronic showers are occasionally misidentified as an electromagnetic shower similar to that of a muon neutrino \cite{Sachdev_2013}.
One method, outlined in \cite{Sachdev_2013}, estimates the amount of hadronic showers produced by a neutral interaction by removing from data those events where the Near Detector identified a muon. The muon is identified using a muon particle identifacation (PID) algorithm that is based on the rate at which a particle looses energy as it passes through an individual plane in the detector. It is important to note that sometimes these particles can be misidentified as muons.
This “Muon-Removed Charged Current” provides a channel that does not contain the main event to be measured by the detectors and provides a data-based way to determine which events would produced charged electromagnetic showers and be identified as an oscillated muon neutrino by the Far Detector even though the Near Detector did not measure it as a muon at the beginning. For an illusration of this technique, see figure \ref{fig:mrcc}.
\label{sect:results}
On February 11, 2014 Fermilab announced that NO\(\nu\)A detected its first signal of a neutrino in the Far Detector, before the detector was finished being built. For the image of the detector track, see figure \ref{fig:results}. While it is too early to draw conclusions from the data, the highly specialized design of the detector arrangement1 coupled with the power of ones ability to manipulate the data for higher resolution will give NO\(\nu\)A a very high level of sensitivity to the neutrino mass hierarchy as well as the CP violating phase 2
Figure \ref{fig:results-cp} shows the probability of \(\nu_{\mu} \rightarrow \nu_{e}\) versus \(\overline{\nu_{\mu}} \rightarrow \overline{\nu_{e}}\) at a fixed neutrino energy of 2 GeV. The solid lines represent the possible values as allowed by the standard model and the dotted lines are the neccessary values to fit current solar neutrino data \cite{Friedland_2012}.
This figure shows a very high level of dependence on the measure probability for \(\nu_{\mu} \rightarrow \nu_{e}\) versus \(\overline{\nu_{\mu}} \rightarrow \overline{\nu_{e}}\) indicating that the measurements taken at NO\(\nu\)A are highly dependent on the CP violating phase.