It is no secret that neutrinos change flavor or oscillate as they travel from one place to another, and that the amount they change depends on how much time they have to change. This time is directly related to the distance the neutrino traveled and the energy of the neutrino itself. Measuring the distance is easy. The hard part is measuring the neutrino energy.
Read the entire article at the Fermilab web site: http://news.fnal.gov/2018/03/the-secret-to-measuring-an-antineutrinos-energy/