A recently completed neutrino detector called NOvA has an online webcam where you can watch cosmic rays collisions in near real-time. Since most of us aren’t lucky enough to have a cosmic ray detector at home, this webcam is a nice reminder of just how ubiquitous these energetic particles from the cosmos really are.
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| Created from screenshots of the the NOνA Far Detector Event Display |
The zoo of cosmic ray particles is a whole field of study unto itself with many projects trying to determine the exact composition of particles and from where they originate. But for the physicists trying to study neutrinos these cosmic rays are all just noise.
What NOνA physicists are really interested in are tracks like this:
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| The footprint of a neutrino? A neutrino moving from right to left likely collided with an atom in the NOvA Near Detector and sparked the fan-like shower of particle tracks seen in the second panel from the left. Screenshot from the NOνA Near Detector Event Display. |
The fan-like shower of particles in the second panel is likely caused by an invisible neutrino speeding from right to left and colliding with an atom.
Neutrinos are extremely light, extremely fast particles that rarely interact with other kinds of matter—it’s estimated that 65 billion neutrinos produced in the Sun stream through a square centimeter on the Earth’s surface every second but very few of them ever interact. Nevertheless, the vast number of neutrinos created in stars, in supernovas, and other high-energy astrophysical objects could make up a significant portion of the mass in the universe and are thought to help drive cosmic processes.
The NOνA experiment hopes to closely observe neutrino interactions in order to fully understand their role in the universe. The underground experiment generates pulses of 100 thousand billion (1014) neutrinos every second at Fermilab near Chicago and sends them 500 miles away to Ash River, Minnesota.
The vast majority of these neutrinos fly straight through the 60 meter long detector as if it were invisible, but a couple per week will leave a tell-tale trace in the form of a shower of other particles. Scientists know the exact speed and direction of the neutrinos and use this to pick out the rare collisions.
I was able to capture the above screenshot of a likely neutrino event yesterday afternoon after only a few minutes, but the reason I was so lucky is that this image is from the Near Detector webcam. The Near Detector is located at Fermilab, right next to where the six foot wide beam of neutrinos is generated. As a result the Near Detector gets the full blast of neutrinos before they spread out and speed away and neutrino collisions are relatively common.
By the time the neutrinos reach the Far Detector 500 miles away, the width of the beam is several miles across and the density of neutrinos is much lower. But physicists really want to find traces of neutrinos in the Far Detector as this can reveal what happened to the neutrinos along the way.
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| The “horn” focuses and steers beams of pions that eventually decay into neutrinos. Credit: NOνA/Fermilab |
Neutrinos vary between three different types or “flavors” of neutrino: electron, muon, and tau neutrinos. All neutrinos in the NOνA experiment are originally muon neutrinos created from the decay of pion particles into muons and muon neutrinos. If electron neutrinos are detected at the Far Detector, then physicists know that some of the muon neutrinos changed into electron neutrinos along the way and they can estimate the rate at which this happens.
The switch from muon to electron neutrinos has never been observed, and this is one of the key goals of the NOvA experiment. NOνA also hopes to discover which of the three types of neutrinos is the heaviest and whether neutrinos behave in the same way as their antimatter counterpart, the antineutrino. The last goal could help answer the big question of why the universe today has more matter than antimatter.
But neutrinos are devilishly hard to detect. This video from Fermilab shows what it takes to pinpoint the neutrino, akin to “searching all of North and South America for one person”.
NOνA is still in the early stages of its six year first run, with the potential to unlock many cosmic mysteries. As particle physicist Jon Butterworth described last week in the Guardian, “as [NOνA] collects more and more data it will tell us more about the mixing and the nature of the neutrino, a particle which is one of the most abundant in the universe and which may hold clues to some of the big puzzles which remain open in physics”.
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By Tamela Maciel, also known as “pendulum”