[Paper Review] Preparation of the Neutrino-4 experiment on search for sterile neutrino and the obtained results of measurements

A. П. Серебров, R. M. Samoilov+17 more|arXiv (Cornell University)|May 9, 2020

Neutrino Physics Research7 references19 citations

TL;DR

The Neutrino-4 experiment measured reactor antineutrino flux and energy spectrum over 6–12 m from the SM-3 reactor core, observing a sterile neutrino oscillation signal at $\Delta m_{14}^2 = 7.25 \pm 0.13_{\text{st}} \pm 1.08_{\text{sys}} \, \text{eV}^2$ and $\sin^2 2\theta_{14} = 0.26 \pm 0.08_{\text{st}} \pm 0.05_{\text{sys}}$. Combining with gallium and reactor anomaly data yields $\sin^2 2\theta_{14} \approx 0.19 \pm 0.04$ at 4.6σ significance, supporting the 3+1 neutrino model with a sterile neutrino mass of $m_4 = 2.68 \pm 0.13 \, \text{eV}$.

ABSTRACT

The experiment Neutrino-4 had started in 2014 with a detector model and then was continued with a full-scale detector. All steps of preparatory work on this experiment are presented. Were obtained the results of measurements of reactor antineutrino flux and spectrum dependence on the distance in the range 6-12 meters from the center of the reactor core at SM-3 reactor. Using all collected data, we performed the model independent analysis on the oscillation parameters $\Delta m_{14}^2$ and $\sin^22 heta_{14}$. The method of coherent summation of results of measurements allows us to directly observe the effect of oscillations. We observed an oscillation effect in vicinity of $\Delta m_{14}^2=(7.25\pm0.13_{st}\pm1.08_{sys}) ext{eV}^2$ and $\sin^22 heta_{14}= 0.26\pm0.08_{st}\pm0.05_{sys}$. We provide a comparison of our results with results of other experiments on search for sterile neutrino. Combining the result of the Neutrino-4 and the results of measurements of the gallium anomaly and reactor anomaly we obtained value $\sin^22 heta_{14}\approx0.19 \pm 0.04 (4.6\sigma)$. Also was performed comparison of Neutrino-4 results with results of other reactor experiments NEOS, DANSS, STEREO, PROSPECT and MiniBooNE, LSND experiments and the IceCube experiment. Mass of sterile neutrino obtained from data collected in the Neutrino-4 experiment (in assumption $m_4^2\approx \Delta m_{14}^2$) is $m_4=2.68\pm0.13 ext{eV}$. Using the estimations of mixing angles obtained in other experiments and our results we can calculate, within 3+1 neutrino model, masses of electron, muon, and tau neutrinos: $m_{ u_e}^{ ext{eff}}=(0.58\pm0.09) ext{eV}, m_{ u_\mu}^{ ext{eff}}=(0.42\pm0.24) ext{eV}, m_{ u_ au}^{ ext{eff}}\leq 0.65 ext{eV}$. Extended PMNS matrix for 3+1 model with one sterile neutrino is provided, neutrino flavor mixing scheme with sterile neutrino and global fit of reactor experiments.

Motivation & Objective

• To search for sterile neutrinos via reactor antineutrino oscillations at the SM-3 reactor.
• To measure the energy spectrum and flux of reactor antineutrinos as a function of distance (6–12 m) from the core.
• To perform a model-independent analysis of oscillation parameters $\Delta m_{14}^2$ and $\sin^2 2\theta_{14}$ using coherent summation of data.
• To compare results with other experiments (NEOS, DANSS, STEREO, PROSPECT, MiniBooNE, LSND, IceCube) and assess consistency with the 3+1 neutrino model.
• To derive effective masses of active neutrinos ($m_{\nu_e}^{\text{eff}}, m_{\nu_\mu}^{\text{eff}}, m_{\nu_\tau}^{\text{eff}}$) within the 3+1 framework.

Proposed method

• A full-scale detector was deployed at distances 6–12 m from the SM-3 reactor core to measure antineutrino flux and energy spectrum.
• Coherent summation of measurement results enabled direct observation of oscillation effects without model bias.
• Model-independent analysis was applied to extract $\Delta m_{14}^2$ and $\sin^2 2\theta_{14}$ from data, minimizing theoretical assumptions.
• Systematic and statistical uncertainties were evaluated separately to ensure robustness of the oscillation parameter estimates.
• Global fits combined Neutrino-4 data with gallium anomaly and reactor anomaly results to refine $\sin^2 2\theta_{14}$.
• Extended PMNS matrix was constructed for the 3+1 model to describe neutrino flavor mixing including one sterile state.

Experimental results

Research questions

• RQ1 Does the observed reactor antineutrino flux and spectrum at varying distances from the core indicate oscillations into a sterile neutrino state?
• RQ2 What are the model-independent values of $\Delta m_{14}^2$ and $\sin^2 2\theta_{14}$ derived from Neutrino-4 data?
• RQ3 How do Neutrino-4 results compare with those from NEOS, DANSS, STEREO, PROSPECT, MiniBooNE, LSND, and IceCube experiments?
• RQ4 What is the effective mass of the sterile neutrino, assuming $m_4^2 \approx \Delta m_{14}^2$, based on Neutrino-4 measurements?
• RQ5 What are the effective masses of electron, muon, and tau neutrinos when combined with Neutrino-4 results in the 3+1 neutrino model?

Key findings

• Neutrino-4 observed a sterile neutrino oscillation signal at $\Delta m_{14}^2 = 7.25 \pm 0.13_{\text{st}} \pm 1.08_{\text{sys}} \, \text{eV}^2$ and $\sin^2 2\theta_{14} = 0.26 \pm 0.08_{\text{st}} \pm 0.05_{\text{sys}}$.
• Combining Neutrino-4 with gallium and reactor anomaly data yields $\sin^2 2\theta_{14} \approx 0.19 \pm 0.04$ at 4.6σ significance.
• The sterile neutrino mass is estimated as $m_4 = 2.68 \pm 0.13 \, \text{eV}$ under the assumption $m_4^2 \approx \Delta m_{14}^2$.
• Effective electron neutrino mass in the 3+1 model is $m_{\nu_e}^{\text{eff}} = (0.58 \pm 0.09) \, \text{eV}$.
• Effective muon neutrino mass is $m_{\nu_\mu}^{\text{eff}} = (0.42 \pm 0.24) \, \text{eV}$, and tau neutrino mass is bounded by $m_{\nu_\tau}^{\text{eff}} \leq 0.65 \, \text{eV}$.
• An extended PMNS matrix for the 3+1 model was constructed, and a global fit of reactor experiments incorporating Neutrino-4 data supports the existence of a light sterile neutrino.