Daya Bay Measures the Neutrino Mixing Parameter sin²2θ₁₃

In a letter submitted to the Physical Review (arXiv:1203.1669[hep-ex]), the Daya Bay Collaboration, including members of Virginia Tech's Center for Neutrino Physics, announces the first observation and measurement on the neutrino mixing parameter sin²2θ₁₃. The mixing angle θ₁₃ governs the rate of electron neutrino conversion to muon and tau neutrinos at baselines consistent with atmospheric oscillations. The hunt for theta;₁₃ is currently the focus of several major experiments world wide, who are using either electron antineutrinos from nuclear reactors or beams of muon neutrios/anitneutrinos produced at accelerators.

      In reactor neutrino experiments such as Daya Bay, the oscillation is observed as an electron antineutrino disappearance at a distance of about 2 km from the reactor. The effect can only be measured with confidence in the comparison of detectors placed near the reactor cores – which measure the neutrino flux before significant oscillation has occured – to detectors placed farther away – at a location close to the oscillation maximum. In this way uncertainties associated with antineutrino production in the reactor, the interaction cross section in the detector, and to a lessor degree detector efficiency will cancel in the near/far detector comparison. Daya Bay Collaboration is the first to apply this technique and report results, finding sin²2θ₁₃ = 0.092±0.016(stat.)±0.005(syst.).

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The measured vs. expected signal in each detector, assuming no oscillation. Reactor and survey data are used to compute the weighted average baselines. The oscillation survival probability at the best fit value is given by the smooth curve.

The far detector hall showing three antineurino detectors in the water pool as it is being filled. The far hall is where a deficit of reactor antineutinos was observed relative to the near detector sites. This is the signature of neutrino oscillations and the basis of the discovery and measurement of the oscillation parameter sin²2θ₁₃. (Photo by Joseph Hor © Virginia Tech)

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