Fyzikální ústav Akademie věd ČR

Experiment NOvA at Fermilab, USA, studies neutrinos; their oscillations, mass hierarchy and CP symmetry violation.

Neutrinos are the most abundant massive particle in the universe, but are still poorly understood. They flit through ordinary matter as though it weren’t there. Their unwillingness to interact with matter forced the scientists to build massive particle detector—50 feet tall, 50 feet wide, and 200 feet long — detecting neutrinos fired from 500 miles away.

Researchers have collected data since February 2014, recording neutrino interactions in the 14,000-ton far detector in Ash River, Minnesota, while construction was still underway. This allowed the collaboration to gather data while testing systems before starting operations with the complete detector in November 2014, shortly after the experiment was completed on time and under budget.

The neutrino beam generated at Fermilab passes through an underground near detector, which measures the beam’s neutrino composition before it leaves the Fermilab site. The particles then travel more than 500 miles straight through the earth, no tunnel required, oscillating (or changing types) along the way. About once per second, Fermilab’s accelerator sends trillions of neutrinos to Minnesota, but the elusive neutrinos interact so rarely that only a few will register at the far detector.

When a neutrino bumps into an atom in the NOvA detector, it releases a signature trail of particles and light depending on which type it is: an electron, muon, or tau neutrino. The beam originating at Fermilab is made almost entirely of one type—muon neutrinos—and scientists can measure how many of those muon neutrinos disappear over their journey and reappear as electron neutrinos. If oscillations did not occur, experimenters predicted they would see (in the middle of 2015) 201 muon neutrinos arrive at the NOvA far detector in the data collected; instead, they saw a mere 33, evidence that the muon neutrinos were disappearing as they transformed into the two other flavors. Similarly, if oscillations did not occur, scientists expected to see only one electron neutrino appearance (due to background interactions), but the collaboration saw six such events, evidence that some of the missing muon neutrinos had turned into electron neutrinos.

Similar long-distance experiments such as T2K in Japan and MINOS at Fermilab have seen these muon neutrino to electron neutrino oscillations before.

NOvA will continue to take data for at least six years.

Fermilab’s flagship accelerator set a high-energy neutrino beam world record when it reached 521 kilowatts in 2015, and the laboratory is working on improving the neutrino beam even further for projects such as NOvA and the upcoming Deep Underground Neutrino Experiment (DUNE). Researchers expect to reach 700 kilowatts in 2016 , accumulating neutrino interactions with higher frequency.

While researchers know that neutrinos come in three types, they don’t know which is the heaviest and which is the lightest. Figuring out this ordering—one of the goals of the NOvA experiment—would be a great litmus test for theories about how the neutrino gets its mass. While the famed Higgs boson helps explain how some particles obtain their masses, scientists don’t know yet how it is connected to neutrinos, if at all. The measurement of the neutrino mass hierarchy is also crucial information for neutrino experiments trying to see if the neutrino is its own antiparticle.

Like T2K, NOvA can also run in antineutrino mode, opening a window to see whether neutrinos and antineutrinos are fundamentally different. An asymmetry early in the universe’s history could have tipped the cosmic balance in favor of matter, making the world we see today possible. Soon, scientists will be able to combine the neutrino results obtained by T2K, MINOS and NOvA, yielding more precise answers about scientists’ most pressing neutrino questions.

The NOvA collaboration comprises 210 scientists and engineers from 39 institutions in the United States, Brazil, the Czech Republic, Greece, India, Russia and the United Kingdom. The researchers from the Institute of Physics AS CR contributed to the experiment construction, commissioning, running, maintenance and data analysis. One of the physicists was responsible for uninterrupted detector running and data collection in 2014 - 2015. Such responsible function is appreciation of our responsible attitude to the experiment. Institute of Physics further delivers computing capacities to NOvA via its Regional Centre for Particle Physics, where 1500 data production tasks are simultaneously running.