![]() Only 78 such interactions were observed, in line with the well-established result originally observed by Super-Kamiokande that muon neutrinos are indeed oscillating into other flavours as they travel. For the latest data, the NOvA team predicts 470 selected ν μ charged current interactions in the far detector if neutrino oscillations do not occur. Additionally, each event must be far enough from the detector edges to ensure that the entire final state has been recorded and that the event is not due to an incoming cosmic ray. To keep this background at bay in the ν μ→ ν μ measurement, each recorded track is assigned a muon likelihood based on key track features such as overall length and the rate of energy deposition. ![]() ![]() Neutral pions in these recoil systems can mimic electrons, while charged pions can mimic muons. ![]() An important background to both the ν μ and ν e charged-current channels comes from neutral-current interactions, whereby the neutrino exits the detector and leaves behind only a hadronic recoil system. Electrons, in contrast, create more compact electromagnetic showers with well-characterised longitudinal and transverse profiles. Muons produced in charged-current ν μ interactions in NOvA leave long straight tracks of detector activity that can span hundreds of cells (see the image of a muon-neutrino interaction in the NOvA detector). To identify the flavour of an interacting neutrino, researchers look for tell-tale signs of a muon or an electron in the recorded event. A muon-neutrino interaction in the NOvA detector, as viewed by the vertical cells (the neutrino entered from the left). The results released at Neutrino 2016 are based on this data set, and highlighted here are the measurements of ν μ → ν μ (corresponding to muon-neutrino survival) and ν μ → ν e (electron-neutrino appearance). As of May 2016, the experiment has accumulated 16% of its planned total. NOvA has been collecting data with the NuMI beam since February 2014, and full operations began the following October upon completion of the far detector ( CERN Courier July/August 2014 p30). The detectors are identical in their structure, consisting of 4 × 6 cm liquid scintillator-filled PVC cells in alternating planes in order to provide two orthogonal 2D views of particle trajectories. NOvA’s 300 tonne near detector, which is located 1 km downstream of the neutrino source, measures the rate, energy spectrum and flavour composition of the neutrino beam prior to significant flavour oscillations, while the 14,000 tonne far detector is located 810 km downstream in northern Minnesota. NOvA was conceived to address these unknowns using two detectors together with the intense beam of muon neutrinos provided by Fermilab’s NuMI neutrino source. But is there a new symmetry that underlies this apparent ν μ/ν τ equality? And if the equality breaks down as measurements improve, which flavour will dominate? A third major unknown is whether neutrinos violate CP symmetry, as allowed by the complex phase δ of the leptonic mixing matrix. Past experimental data are consistent with ν 3 being equal parts ν μ and ν τ, in addition to a small amount of ν e. A second set of questions relate to the flavour admixture of the ν 3 state. The basic features of the three-flavour neutrino oscillation framework have been fleshed out, and the NOvA experiment in the US – which presented its latest results at the Neutrino 2016 conference in London earlier this month – is poised to address many of the remaining unknowns related to neutrino masses and their mixing.Īmong these is whether neutrinos obey a “normal” or “inverted” mass hierarchy: that is, whether the mass eigenstate with the least ν e content (called ν 3) is the heaviest or lightest of the three (see “A portal to new physics”). The breakthrough results from the Super-Kamiokande and SNO experiments, which showed that neutrinos oscillate between their three flavours, marked the start of nearly two decades of tremendous progress in neutrino physics. The NuMI horn at Fermilab, which focuses mesons produced in the target into a 675 m-long volume where they can decay to produce neutrinos. Results shed light on the ordering of neutrino masses.
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