Mix and mass: teasing out neutrino transformations

In its recent update NOvA recorded 33 electron-neutrino event candidates at its far detector (grey band). That measurement is compared to expectations for normal (blue) and inverted (red) mass hierarchies as a function of the CP-violating phase δCP. CP violation is largest when δCP takes on values of π/2 and 3π/2 and absent for values of 0, π and 2π.

In its recent update NOvA recorded 33 electron-neutrino event candidates at its far detector (grey band). That measurement is compared to expectations for normal (blue) and inverted (red) mass hierarchies as a function of the CP-violating phase δCP. CP violation is largest when δCP takes on values of π/2 and 3π/2 and absent for values of 0, π and 2π.

It is an exciting time to be a neutrino physicist. At the start of the century, the neutrino experiments Super-K, based in Japan, and SNO, based in Canada, revealed the phenomenon of oscillations, lifting the veil on many neutrino mysteries. Rather than closing the case on neutrinos, this game-changing, Nobel Prize-winning discovery opened a realm of possibilities for scientists to further investigate their nature.

“Oscillation” is how scientists describe the way neutrinos transform from one of their three types into another. The three types are called electron, muon and tau.

When the NOvA experiment was first conceived in the early 2000s, muon neutrinos were known to transform mostly into tau neutrinos. NOvA measures the small fraction of the muon neutrinos produced at Fermilab that also oscillate into electron neutrinos on their 810-kilometer trip to a detector at Ash River, Minnesota. This fraction gives us insight into some important questions:

  • Are there two light and one heavy neutrino, or vice versa? The first scenario is called the normal hierarchy, and the second the inverted hierarchy. This difference shows up during the voyage through the matter of the Earth’s crust and means that muon neutrinos and antineutrinos oscillate into electron neutrinos at different rates.
  • Are the oscillations of neutrinos and antineutrinos intrinsically different? Differences between the behavior of matter and antimatter particles are said to violate CP symmetry. CP violation in neutrinos could help explain why the universe is made out of matter and not antimatter.
These graduate students contributed to this analysis. Top row, from left: Erika Catano-Mur, Iowa State University; Marco Colo, William and Mary; Deepika Grover, Banaras Hindu University; Liudmila Kolupaeva, Joint Institue for Nuclear Research, Dubna; Bottom row, from left: Kuldeep Kaur Mann, Panjab University; Dan Pershey, Caltech; Fernanda Psihas, Indiana University

These graduate students contributed to this analysis. Top row, from left: Erika Catano-Mur, Iowa State University; Marco Colo, William and Mary; Deepika Grover, Banaras Hindu University; Liudmila Kolupaeva, Joint Institue for Nuclear Research, Dubna; Bottom row, from left: Kuldeep Kaur Mann, Panjab University; Dan Pershey, Caltech; Fernanda Psihas, Indiana University

Fermilab is producing record numbers of neutrinos for NOvA. But, of course, there is a catch: Of the roughly 10 billion trillion neutrinos produced, only a handful of them interact in the detector at Minnesota. And with 150,000 cosmic-ray particles crossing the detector every second, finding the neutrinos that come from the Fermilab accelerator complex is “winning the lottery” meets “needle in a haystack.” To find its neutrinos, NOvA uses a novel technique based on advances in computer vision technology, which automatically categorizes cosmic-ray and neutrino event pictures in a way inspired by the human eye.

NOvA released new results on the rate of electron neutrino appearance at the XXVII International Conference on Neutrino Physics and Astrophysics, or Neutrino 2016, which ended last week. NOvA observed 33 electron neutrinos appearing in its muon neutrino beam; without oscillations, only eight counts from backgrounds would have been expected. This rate is close to the maximum expected in standard three-flavor oscillation scenarios and, in combination with constraints from the muon neutrino disappearance result also reported last week, prefers the normal hierarchy and large CP violation. Although this preference is not yet statistically significant, the new results rule out a range of possibilities in the inverted hierarchy scenario helping narrow in on the correct solution.

NOvA will continue to take data using neutrinos until spring 2017, when it will switch to using antineutrinos. New clues about the differences between neutrino and antineutrino oscillations may be just around the corner.

Chris Backhouse of Caltech will present NOvA results at Fermilab in a special Joint Experimental-Theoretical Seminar on Wednesday, July 20.

Fernanda Psihas is a graduate student at Indiana University.