[Paper Review] Study of the wave packet treatment of neutrino oscillation at Daya Bay

Fengpeng An, A. B. Balantekin+218 more|arXiv (Cornell University)|Aug 4, 2016

Neutrino Physics Research3 citations

TL;DR

This study investigates neutrino oscillations at the Daya Bay reactor neutrino experiment using a wave packet model to test deviations from the standard plane wave approximation. By analyzing high-statistics antineutrino data across nine baselines, the authors derive the first experimental limits on the relative intrinsic momentum dispersion, σ_rel, finding 2.38×10⁻¹⁷ < σ_rel < 0.23 at 95% C.L., with improved upper bound σ_rel < 0.20 when accounting for detector and reactor dimensions.

ABSTRACT

The disappearance of reactor $\barν_e$ observed by the Daya Bay experiment is examined in the framework of a model in which the neutrino is described by a wave packet with a relative intrinsic momentum dispersion $σ_ ext{rel}$. Three pairs of nuclear reactors and eight antineutrino detectors, each with good energy resolution, distributed among three experimental halls, supply a high-statistics sample of $\barν_e$ acquired at nine different baselines. This provides a unique platform to test the effects which arise from the wave packet treatment of neutrino oscillation. The modified survival probability formula was used to fit Daya Bay data, providing the first experimental limits: $2.38 \cdot 10^{-17} &lt; σ_{ m rel} &lt; 0.23$. Treating the dimensions of the reactor cores and detectors as constraints, the limits are improved: $10^{-14} \lesssim σ_{ m rel} &lt; 0.23$, and an upper limit of $σ_{ m rel} &lt;0.20$ is obtained. All limits correspond to a 95\% C.L. Furthermore, the effect due to the wave packet nature of neutrino oscillation is found to be insignificant for reactor antineutrinos detected by the Daya Bay experiment thus ensuring an unbiased measurement of the oscillation parameters $\sin^22θ_{13}$ and $Δm^2_{32}$ within the plane wave model.

Motivation & Objective

• To test the validity of the plane wave approximation in neutrino oscillation by introducing a wave packet treatment with intrinsic momentum dispersion.
• To constrain the relative intrinsic momentum dispersion σ_rel using high-precision reactor antineutrino data from the Daya Bay experiment.
• To assess whether wave packet effects induce measurable decoherence or bias in the measurement of oscillation parameters sin²2θ₁₃ and Δm²₃₂.
• To improve experimental limits on σ_rel by incorporating physical constraints from reactor core and detector dimensions.
• To determine whether wave packet effects are significant enough to affect the unbiased determination of neutrino oscillation parameters in the plane wave model.

Proposed method

• Utilized a modified neutrino survival probability formula incorporating wave packet effects, including the localization term D²kj and coherence length Lcoh.
• Fitted the wave packet model to Daya Bay's high-statistics antineutrino data collected at nine different baselines using three experimental halls and eight detectors.
• Applied a χ²/ndf minimization technique to compare the wave packet model with the standard plane wave model, using the p-value and confidence level (95%) for statistical inference.
• Incorporated physical constraints on the spatial width of the neutrino wave packet (σx) derived from reactor core and detector dimensions to refine the σ_rel bounds.
• Computed the coherence length Lcoh and decoherence length Ld to interpret the bounds on σ_rel in terms of physical scales.
• Used the average momentum p = 4 MeV of detected reactor antineutrinos to convert σ_rel bounds into spatial width σx limits via σx ≈ 1/(2σp).

Experimental results

Research questions

• RQ1 Does the wave packet treatment of neutrino oscillation produce measurable deviations from the plane wave model in the Daya Bay experiment?
• RQ2 What are the experimental limits on the relative intrinsic momentum dispersion σ_rel of reactor antineutrinos?
• RQ3 How do physical constraints from reactor core and detector dimensions affect the bounds on σ_rel?
• RQ4 To what extent do wave packet effects influence the measurement of sin²2θ₁₃ and Δm²₃₂ in the Daya Bay experiment?
• RQ5 Is the decoherence effect from wave packet spreading significant enough to bias the oscillation parameter determination?

Key findings

• The first experimental limits on the relative intrinsic momentum dispersion are established as 2.38×10⁻¹⁷ < σ_rel < 0.23 at 95% confidence level.
• When physical constraints from reactor core and detector dimensions are applied, the lower bound improves to σ_rel > 10⁻¹⁴, corresponding to a regime where the localization term D²kj is negligible.
• An improved upper limit of σ_rel < 0.20 is obtained at 95% C.L. under these physical constraints, indicating that wave packet effects are negligible for the measured oscillation parameters.
• The spatial width of the neutrino wave packet is constrained to 10⁻¹¹ cm ≲ σx ≲ 2 m, with the lower bound ruling out σx ≳ 1 km.
• The best-fit wave packet model yields χ²/ndf = 245.9/(256−4), with a p-value of 0.596, slightly lower than the plane wave model’s p-value of 0.614, indicating no significant preference for the wave packet model.
• The decoherence effect due to the wave packet nature of neutrino oscillation is found to be insignificant for reactor antineutrinos in the Daya Bay experiment, ensuring an unbiased measurement of sin²2θ₁₃ and Δm²₃₂ within the plane wave model.