What you are looking at
A neutrino is made as one flavour (electron, muon or tau) and can arrive as another. The first tab shows what it arrives as for any distance and energy, and where real experiments sit. The second tab is Daya Bay's final data: 5.55 million reactor antineutrinos, far hall over near halls. Fit the mixing angle θ₁₃ and the mass splitting Δm²₃₂ and compare them with the published values.
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Glossary
- Flavour
- Which charged lepton a neutrino makes when it interacts: an electron, a muon or a tau. A neutrino is born and detected as a flavour, but it travels as a mix of three mass states.
- Why it matters Because the mass states drift out of step, the flavour mix changes on the way: that is oscillation.
- L/E
- Distance travelled over energy, here in km/GeV (the same number as m/MeV). Each term of the oscillation goes as sin²(1.267 Δm² L/E).
- Why it matters Experiments with very different distances and energies see the same physics when their L/E matches.
- Mixing angle θ₁₃
- One of three angles that say how the flavours are made of the mass states. Reactors measure sin²2θ₁₃, the depth of the electron antineutrino's dip at about 1.6 km for MeV energies.
- Why it matters Daya Bay found it non-zero in 2012; it is the smallest of the three angles and the best measured.
- Mass splitting Δm²
- The difference of two mass states' squares, in eV². Δm²₃₂ ≈ 2.5 × 10⁻³ eV² sets the atmospheric oscillation; Δm²₂₁ = 7.53 × 10⁻⁵ eV² the slow solar one.
- Why it matters It sets where along L/E the dip falls; oscillation needs neutrinos to have mass.
- Survival probability
- The chance a neutrino arrives as the flavour it started as. For reactor antineutrinos: 1 − cos⁴θ₁₃ sin²2θ₁₂ sin²Δ₂₁ − sin²2θ₁₃ (cos²θ₁₂ sin²Δ₃₁ + sin²θ₁₂ sin²Δ₃₂), with Δᵢⱼ = 1.267 Δm²ᵢⱼ L/E.
- Two-flavour formula
- 1 − sin²2θ₂₃ sin²Δ₃₂: the survival of muon neutrinos with only two flavours mixing, as atmospheric and IceCube analyses often fit it. The dashed line on the plot; the three-flavour lines differ by a few per cent.
- Inverse beta decay
- An electron antineutrino turns a proton into a neutron and a positron. The positron's flash (the prompt energy) and the neutron's capture on gadolinium a few tens of microseconds later make the pair Daya Bay counts.
- Why it matters The neutrino's energy is the prompt energy plus about 0.78 MeV.
- Far/near ratio
- Events in the far hall over events in the near halls, each scaled by what six reactors would send it without oscillation. Without oscillation it is 1 at every energy.
- Why it matters The reactors' flux, known only to a few per cent, cancels.
- Effective baseline L_eff
- The far hall's distance to the six reactors, averaged by the flux each sends it: about 1.63 km. The plot's horizontal axis is L_eff over the neutrino energy.
- χ²
- The sum over the points of (data − model)² / error². A good fit has χ² near the number of points less the parameters fitted: 24 here.
- Why it matters Fit moves the sliders to where χ² is lowest.
- ± σ
- How far a fitted parameter moves before χ² rises by 1: its one-standard-deviation uncertainty. This fit counts statistics only, so its σ is smaller than the published one, which adds every systematic.
- Daya Bay
- A reactor experiment in southern China, 2011 to 2020: eight identical 20-tonne detectors in three halls, between 360 m and 1.9 km from six reactor cores. Its full data release (Zenodo, 2025) holds 5.55 million IBD candidates over 3158 days.
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