[Paper Review] Impact of Wave Packet Separation in Low-Energy Sterile Neutrino Searches

C. Argüelles, Toni Bertólez-Martínez+1 more|arXiv (Cornell University)|Jan 13, 2022

Neutrino Physics Research4 citations

TL;DR

This paper demonstrates that wave packet (WP) effects—specifically, neutrino wave packet separation due to finite size—resolve the tension between reactor and radioactive-source experiments in low-energy sterile neutrino searches. By modeling decoherence from WP spreading, the authors show that plane-wave approximations overestimate oscillation visibility, and that WP effects naturally reconcile the 2–5σ signals from radioactive sources with the null results from reactors, without requiring new physics beyond standard oscillation theory with finite wave packets.

ABSTRACT

Light sterile neutrinos have been motivated by anomalies observed in short-baseline neutrino experiments.Among them, radioactive-source and reactor experiments have provided evidence and constraints, respectively, for electron neutrino disappearance compatible with an eV-scale neutrino. The results from these observations are seemingly in conflict. This letter brings into focus the assumption that the neutrino wave packet can be approximated as a plane wave, which is adopted in all analyses of such experiments. We demonstrate that the damping of oscillations, e.g., due to a finite wave packet size, solve the tension between these electron-flavor observations and constraints.

Motivation & Objective

• To address the long-standing tension between short-baseline sterile neutrino signals (e.g., LSND, BEST) and null results from reactor experiments.
• To investigate whether the plane-wave approximation, commonly used in neutrino oscillation analyses, breaks down for eV-scale sterile neutrinos due to finite wave packet size.
• To demonstrate that wave packet decoherence effects can explain why radioactive-source experiments observe electron neutrino disappearance while reactor experiments do not.
• To provide a consistent theoretical framework using wave packet formalism that reconciles conflicting experimental results in low-energy sterile neutrino searches.

Proposed method

• Modeling neutrino states as wave packets instead of plane waves to account for spatial extent and decoherence effects in oscillation probability calculations.
• Using the wave packet formalism to compute the effective oscillation probability, including the suppression due to wave packet separation over baseline distances.
• Applying the wave packet model to two key experiments: the PROSPECT reactor experiment (10 baselines, 160 energy bins) and the BEST gallium experiment (51Cr source, two detector regions).
• Constructing a χ² test statistic that minimizes normalization uncertainties by comparing relative event rates across baselines and energy bins, using the Huber-Mueller flux for 235U.
• Incorporating detector efficiency, response matrices, and geometric corrections (ξ_in/out) to match experimental data in the BEST analysis.
• Performing a global fit using both PROSPECT and BEST data, with wave packet effects included in the oscillation probability P_ee(L,E).

Experimental results

Research questions

• RQ1 Can wave packet effects resolve the discrepancy between electron neutrino disappearance signals in radioactive-source experiments and null results in reactor experiments?
• RQ2 How does finite wave packet size affect the visibility of sterile neutrino oscillations in low-energy experiments?
• RQ3 To what extent does the plane-wave approximation fail for eV-scale sterile neutrino oscillations, particularly in experiments with short baselines and low energies?
• RQ4 Can the inclusion of wave packet decoherence explain both the 2–5σ signals in BEST and the null results in PROSPECT within a single theoretical framework?

Key findings

• Wave packet separation suppresses oscillation visibility, especially at short baselines, due to decoherence from finite neutrino source and detector sizes.
• The inclusion of wave packet effects reconciles the 5σ signal from the BEST experiment with the null results from the PROSPECT reactor experiment, resolving the long-standing tension.
• The plane-wave approximation overestimates oscillation amplitude, leading to false exclusion of sterile neutrino parameters that are consistent with radioactive-source data.
• The model shows that for eV-scale sterile neutrinos, the coherence length is comparable to or shorter than typical experimental baselines, making wave packet effects non-negligible.
• The global fit including wave packet effects yields a consistent description of both BEST and PROSPECT data, with no need to invoke new physics beyond standard oscillation theory with finite wave packets.
• The study demonstrates that the apparent contradiction between experiments arises not from physics but from the use of an invalid approximation (plane wave) in data analysis.