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CERN Land

 
 
 
 
   
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Neutrinos: Now you see them, now you don't

For one of the most abundant known particles in the universe, neutrinos are poorly understood. To fill in the blanks, the NOvA (NuMI Off-Axis νe Appearance) neutrino detector is helping us discover what causes neutrinos to transform from one form to another, which will ultimately bring us closer to understanding the elementary particles of the universe.

「If we want to study neutrinos, we have to build specialized experiments like NOvA to probe directly into the weak nuclear force,」 says Alex Himmel, computing coordinator for the NOvA project. 「And if we want to understand why we're in a universe that's all matter instead of antimatter, we need to understand the weak nuclear force.」

A $278-million international collaboration of nearly 210 scientists and engineers from 39 universities, laboratories, and other institutions, the NOvA project recently had their first big breakthrough when the detector revealed the first evidence of oscillating neutrinos.

NOvA detects neutrinos beamed from Fermi National Accelerator Laboratory』s NuMI (Neutrinos at the Main Injector) near Batavia, Illinois, to the 14,000-ton, multi-story detector located over 500 miles away in a remote area near Ash River, Minnesota. An underground near detector first measures the neutrino composition as it leaves. The neutrinos then pass through matter as though it doesn't exist.

<strong>A 3D look at one of the first neutrino interactions captured in the NOvA far detector.</strong> The dotted red line represents the neutrino beam, generated at Fermilab in Illinois and sent through 500 miles of earth to the far detector. The image on the left is a simplified 3D view of the detector – the top right view shows the interaction from the top of the detector, and the bottom right view shows the interaction from the side of the detector. Courtesy Fermilab.
A 3D look at one of the first neutrino interactions captured in the NOvA far detector. The dotted red line represents the neutrino beam, generated at Fermilab in Illinois and sent through 500 miles of earth to the far detector. The image on the left is a simplified 3D view of the detector – the top right view shows the interaction from the top of the detector, and the bottom right view shows the interaction from the side of the detector. Courtesy Fermilab.

As Himmel explains it, neutrinos come in three forms: muon neutrinos, electron neutrinos, and tau neutrinos. Since very few muon neutrinos become electron neutrinos, and neutrinos interact very rarely, sifting through the haystack of neutrino particles is challenging. How rare are these sightings? In their first run, Himmel's team only saw six.

The team's primary goal is to study the nature of how matter causes neutrinos to change forms. They are looking at differences in how neutrinos and anti-neutrinos travel through the Earth, and trying to understand the differences in mass. 「Two neutrinos are close in mass and the third is very different, but we don't know whether it is heavier or lighter—we just know that they are different,」 says Himmel.

The only way to isolate the electron neutrino signature from background cosmic rays is through computationally intensive techniques. Cosmic rays come through the far detector at about 100,000 every second  — almost 8 billion every day. 「Since we typically have 50 cosmic rays close in time (<0.55 ms) to every neutrino we see, we need serious computing power to search through them for the neutrinos!」 Himmel says.

Relying on the massive computing power of the Open Science Grid (OSG), the NOvA experiment used 10 million CPU hours on the OSG from 28 April to 31 July 2015 to achieve their breakthrough. 「The OSG allows us to look at unbiased samples earlier and understand them earlier. Since this is a new detector, there are many things to understand,」 Himmel says. 「We would not have been able to do the first analysis without the OSG.」