---
title: "Short reactor baselines: look for a change with distance | Sandbox Physics"
description: "Comparing spectra at several distances removes much of the flux-model dependence. STEREO, PROSPECT-I and DANSS ruled out parts of the simplest sterile model."
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language: "en"
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  zh-CN: "https://sandboxphysics.com/zh/stories/neutrinos/short-baseline-reactor-sterile-tests/"
last_modified: "2026-10-11"
---

[2026 Nobel Prize in Physics](https://sandboxphysics.com/nobel/2026/physics/)

# Short reactor baselines: look for a change with distance

A low total count can have several causes. Detectors split into segments, or moved between positions, test a sharper prediction: a new oscillation would reshape the spectrum over just a few meters.

Sandbox Physics · 2026-10-10 · Updated 2026-10-11 · Interpreting evidence

## A sterile neutrino would predict a change with distance

A sterile neutrino would not feel the ordinary weak interactions. If electron antineutrinos mixed with an additional mass state, some could lose their detectable electron flavor in flight. This is a hypothesis to test; the reactor rate discrepancy does not establish such a particle.

The simplest common model adds one state to the three known active flavors. At short enough baselines, the new, larger mass-squared gap dominates the oscillation phase and the effects of the known smaller gaps are negligible. Its amplitude and frequency fix a pattern in both distance and energy.

The expression below, in natural units and without matter effects, shows the opportunity: compare different travel distances at the same energy. An error in the source spectrum shared by all positions can survive in absolute counts but largely cancels in that relative comparison.

$$
\begin{aligned}P_{ee}&\simeq1-\sin^2(2\theta_{14})\sin^2\phi_{41}\\\phi_{41}&=\frac{\Delta m^2_{41}L}{4E}\end{aligned}
$$

Short-baseline approximation for three active states plus one extra state, neglecting the smaller known splittings, in natural units. A real prediction must also average over core size, interaction positions and energy response.

Sources: [1] · [2] · [3] · [4]

## STEREO and PROSPECT compare segments of one detector

STEREO ran beside the compact research reactor at the Institut Laue–Langevin in Grenoble, France. Its six target cells sat at different distances around ten meters from the core. Uranium-235 fission dominates that reactor, so the source mixture was simple, and the cells gave simultaneous spectra to compare.

PROSPECT-I used a segmented, lithium-loaded liquid-scintillator detector at the High Flux Isotope Reactor in Oak Ridge, United States. Its final oscillation analysis grouped events into six distance ranges between roughly seven and nine meters. Segmentation and the neutron signature help control backgrounds in a detector close to the surface.

Recording all cells at once reduces the need to match reactor histories from separate runs, but each cell still has its own response and efficiency. Calibration must keep an energy-scale difference from posing as a distance effect. Segments are a useful control only when each part is understood well enough to compare.

Sources: [1] · [2] · [5]

1. 01 **Several distances**

   Use a segmented detector or move it

2. 02 **Relative spectra**

   Control detector response and reactor history

3. 03 **Specified model**

   Average over geometry, then test the parameter region

How the test is designed, not simulated or published exclusion data. Relying less on the flux model still leaves the assumptions of the signal model.

## DANSS moves the same detector beneath a power reactor

DANSS used a highly segmented plastic-scintillator detector beneath a core at the Kalinin nuclear power plant in Russia. The 2018 analysis compared positions about 10.7 to 12.7 meters from the core. Moving one detector changes the baseline while keeping most of its detection hardware the same.

A movable detector gains one advantage and takes on another challenge. Positions are sampled at different times, so changes in fuel and power must be tracked or averaged appropriately. A commercial core is also larger than a compact research core; antineutrinos come from a spread of production points, not one point at its center.

Those geometric spreads wash out rapid oscillations, and finite energy resolution does the same. An ideal model curve can look striking while the variation left after detector smearing is much weaker. Comparing experiments requires their actual core dimensions, baseline distributions and responses, not just the shortest quoted distance.

Sources: [3] · [1] · [2]

## What these null results rule out

STEREO’s final result, published in 2023, found its cell spectra consistent with no extra oscillation and excluded the region favored by the reactor anomaly over its sensitive range. It also measured a source spectrum that disagreed with the reference. A source anomaly can persist even after an oscillation explanation is rejected.

The final PROSPECT-I analysis, submitted in 2024 and published in 2025, used the full 2018 sample with an improved selection across several periods. Its spectra at different baselines agreed with one another. It extended the excluded regions and rejected the best-fit point reported by Neutrino-4; that tests one point, not every possible new state.

DANSS’s 2018 result likewise drew exclusion regions from spectral ratios at different positions. These are dated analyses, and this article does not combine every collaboration’s current limit. Each confidence region belongs to a specific model, dataset and statistical method, and should be quoted with them.

Sources: [1] · [2] · [3]

## Excluding the favored region does not exclude every sterile neutrino

An exclusion contour shows where a real experiment can tell the difference; its sensitivity is not the same everywhere. If an oscillation is very slow over the sampled distances, or very fast compared with the geometric and energy spreads, its relative signature is weak. Small mixing is hard to test too.

In relative-spectrum discussions, “model-independent” usually means less dependence on an absolute source spectrum. The analysis still depends on propagation assumptions, geometry, response and statistical choices, and an extra-state fit still specifies the signal it tests.

The shared lesson is about method: change the distance while controlling the source and the detector, then look for a correlated pattern, not just a missing total. These reactor experiments test one concrete explanation of the anomalies; solar calibration, accelerator and cosmological questions need their own data and assumptions.

Sources: [3] · [2] · [1] · [6]

## What we learned

Comparing spectra at several distances removes much of the dependence on the predicted source spectrum and tests a specific oscillation pattern. STEREO, PROSPECT-I and DANSS rule out parts of the simplest model with one extra state. Those exclusions do not rule out every sterile-neutrino theory.

## Try it in the Lab

[Reactor coincidence](https://sandboxphysics.com/labs/reactor-coincidence/) [Neutrino flavor propagation](https://sandboxphysics.com/labs/neutrino-flavor-propagation/)

## Primary sources & revision

1. [STEREO Collaboration · Uranium-235 spectrum and sterile-neutrino test (Nature, 2023; arXiv v3, 2024)](https://arxiv.org/abs/2210.07664v3)
2. [PROSPECT Collaboration · Final PROSPECT-I Short-Baseline Oscillation Search (2024 submission; 2025 publication)](https://arxiv.org/abs/2406.10408v2)
3. [DANSS Collaboration · Search for sterile neutrinos at the DANSS experiment (2018)](https://arxiv.org/abs/1804.04046)
4. [Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)](https://pdg.lbl.gov/2024/reviews/rpp2024-rev-neutrino-mixing.pdf)
5. [PROSPECT Collaboration · Experimental science and detector design](https://prospect.yale.edu/science)
6. [Particle Data Group · Statistics (2025)](https://pdg.lbl.gov/2025/reviews/rpp2025-rev-statistics.pdf)

First published 2026-10-10; last revised 2026-10-11. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.

[← Previous: The reactor anomaly and the spectrum bump ask different questions](https://sandboxphysics.com/stories/neutrinos/reactor-anomaly-and-spectrum-bump/) [Next: Geoneutrinos: what can particles tell us about the Earth’s heat? →](https://sandboxphysics.com/stories/neutrinos/geoneutrinos-earth-radiogenic-heat/)

## Keep reading

Neutrinos · Reading path

### Read reactors and the Earth

1. [Article **Where does a reactor’s antineutrino spectrum come from?**](https://sandboxphysics.com/stories/neutrinos/reactor-antineutrino-spectrum-origins/)
2. [Article **KamLAND: testing a solar mystery with reactors on Earth**](https://sandboxphysics.com/stories/neutrinos/kamland-reactor-oscillations/)
3. [Article **Daya Bay: why compare a near detector with a far one?**](https://sandboxphysics.com/stories/neutrinos/daya-bay-near-far/)
4. [Article **RENO and Double Chooz: why repeat the mixing-angle measurement?**](https://sandboxphysics.com/stories/neutrinos/reno-double-chooz-independent-mixing-tests/)
5. [Article **JUNO: why must a huge detector resolve tiny spectral features?**](https://sandboxphysics.com/stories/neutrinos/juno-fine-spectrum-and-mass-ordering/)
6. [Article **The reactor anomaly and the spectrum bump ask different questions**](https://sandboxphysics.com/stories/neutrinos/reactor-anomaly-and-spectrum-bump/)
7. [You are here **Short reactor baselines: look for a change with distance**](https://sandboxphysics.com/stories/neutrinos/short-baseline-reactor-sterile-tests/)
8. [Article **Geoneutrinos: what can particles tell us about the Earth’s heat?**](https://sandboxphysics.com/stories/neutrinos/geoneutrinos-earth-radiogenic-heat/)

Neutrinos · Topic

### Unexpected results and sterile neutrinos

Gallium, reactor and timing experiments have all reported surprises. Which ones held up, and could a fourth kind of neutrino explain the rest?

1. [Article **How does a faint signal become a neutrino discovery?**](https://sandboxphysics.com/stories/neutrinos/neutrino-discovery-evidence-chain/)
2. [Article **Gallium experiments: opening the Sun’s low-energy window**](https://sandboxphysics.com/stories/neutrinos/gallium-low-energy-solar-neutrinos/)
3. [Article **Why can a neutrino change flavor?**](https://sandboxphysics.com/stories/neutrinos/neutrino-flavor-oscillations/)
4. [Article **The reactor anomaly and the spectrum bump ask different questions**](https://sandboxphysics.com/stories/neutrinos/reactor-anomaly-and-spectrum-bump/)
5. [You are here **Short reactor baselines: look for a change with distance**](https://sandboxphysics.com/stories/neutrinos/short-baseline-reactor-sterile-tests/)
6. [Article **OPERA: tau appearance and a corrected clock anomaly**](https://sandboxphysics.com/stories/neutrinos/opera-tau-appearance-and-timing-correction/)
7. [Article **Sterile neutrinos: an explanation must pass several tests**](https://sandboxphysics.com/stories/neutrinos/sterile-neutrinos-anomalies-and-tests/)

Neutrinos · Topic

### Neutrinos from reactors and accelerators

Reactors and accelerators are neutrino sources that people control. What have detectors placed near them and far from them shown?

1. [Article **Cowan and Reines: finding the neutrino in two flashes**](https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/)
2. [Article **KamLAND: testing a solar mystery with reactors on Earth**](https://sandboxphysics.com/stories/neutrinos/kamland-reactor-oscillations/)
3. [Article **Daya Bay: why compare a near detector with a far one?**](https://sandboxphysics.com/stories/neutrinos/daya-bay-near-far/)
4. [Article **RENO and Double Chooz: why repeat the mixing-angle measurement?**](https://sandboxphysics.com/stories/neutrinos/reno-double-chooz-independent-mixing-tests/)
5. [You are here **Short reactor baselines: look for a change with distance**](https://sandboxphysics.com/stories/neutrinos/short-baseline-reactor-sterile-tests/)
6. [Article **K2K and MINOS: test flavor change with a distance you choose**](https://sandboxphysics.com/stories/neutrinos/k2k-minos-controlled-baselines/)
7. [Article **T2K: why look slightly away from the beam axis?**](https://sandboxphysics.com/stories/neutrinos/t2k-off-axis-beam-and-appearance/)