By Hamish Johnston
Deep in the North Woods in Minnesota the snow is starting to melt, and the giant NOvA Far Detector is coming to life. Designed to register the arrival of neutrinos that will be created 810 km away at Fermilab near Chicago, the detector has recorded its first 3D images. These are not of neutrinos, but of the trajectories of fast-moving particles that are created in a process that begins with a cosmic ray colliding with Earth’s atmosphere.
The Far Detector is a series of liquid-scintillator sectors, and particles are tracked as they move from sector to sector. By studying these tracks, physicists should be able to discern between the extremely rare detection of a neutrino and the much more common background noise from cosmic rays.
The above image shows a large shower of energetic particles created by a muon, which itself was created by a cosmic ray. While this is exactly what NOvA scientists want to discriminate against when they are looking for neutrinos, such events are very useful when it comes to calibrating the detector.
“Everybody loves cosmic rays for this reason,” explains Fermilab’s Mat Muether. “They are simple and abundant and a perfect tool for tuning up a new detector.”
When the experiment is up and running, neutrinos will be created at Fermilab by smashing protons into a target to produce pions and kaons, which then decay into neutrinos. These will first be sent through the NOvA Near Detector at Fermilab – which is a much smaller version of the Far Detector. This will allow physicists to characterize the neutrino beam before it embarks on the 810 km journey through solid rock.
On the way to the North Woods, some muon neutrinos will change (or oscillate) into electron neutrinos. By measuring the rate at which this change occurs – and also the rate of the corresponding antineutrino oscillation – physicists hope to gain insight into several important unanswered questions in physics. These include what are the precise masses of the three different neutrino types, and why is there much more matter than antimatter in the universe?
The NOvA experiment is expected to begin neutrino measurements later this year.
There’s more about NOvA and other Fermilab projects here – including audio interviews with Fermilab physicists.