An important scientific result

Fermilab Director Pier Oddone

I must begin on a somber note, as this week we are completing the previous reduction in force program that had not been applied to the computing sector last year due to that sector’s ongoing restructuring. Involuntary separations due to budget issues are very painful and we very much regret the loss of each employee. We have no choice, however, when the budgets do not maintain our desired staffing level.

Now, to news that is much more positive for our future neutrino program. Last week, the Daya Bay Reactor Neutrino Experiment reported the first definitive results on the rate at which muon neutrinos convert to electron neutrinos. The parameter that describes that conversion is sin2 2 θ13, and I will simply refer to it as θ13. For many years, and so long as we did not see this process occur, we worried that the rate described by θ13 could be zero or very low. Perhaps a new symmetry was at play to make the number zero or very small. Instead, θ13 is just about as large as it could be given the previous upper limit determined by the CHOOZ Reactor Experiment more than ten years ago.

This number is critically important because it is a fundamental parameter that determines our ability to study matter-antimatter symmetry in neutrino interactions. The violation of matter-antimatter symmetry in neutrinos is one of the most promising paths to understand why we live in a universe made of matter. Had θ13 been zero, then the physics program of the Long-Baseline Neutrino Experiment would have been more limited because it would not be able to study matter-antimatter symmetry. This large value of θ13 is the last missing piece to fall into place to make sure that we can achieve the full promise of LBNE. It is also great news for NOvA, confirming that the experiment has a very good shot at determining how the spectrum of neutrino masses is arranged – a critical question for grand unified theories and for the interpretation of other critical experiments such as neutrino-less double beta decay experiments that explore the nature of neutrinos.

The discovery that muon neutrinos convert to electron neutrinos comes after a set of exciting measurements that gave us very strong hints of a non-zero θ13 over the last year: results from the T2K experiment in Japan, MINOS at Fermilab and Double CHOOZ in France. While none of the experiments were definitive, their combination made a very strong suggestion that θ13 would be relatively large. The Daya Bay Neutrino Experiment was designed to measure the rate even if it was ten times smaller than the value recently reported. Because this goal was very ambitious and required a high-rate experiment, data was collected and analyzed quickly, only after a few weeks of running: a truly impressive feat!