[Paper Review] Short-baseline Reactor Neutrino Oscillation

H. Seo|arXiv (Cornell University)|Jan 24, 2017

Neutrino Physics Research3 citations

TL;DR

This paper summarizes the latest results from short-baseline reactor neutrino experiments—Daya Bay, RENO, and Double Chooz—on the precise measurement of the neutrino mixing angle $\theta_{13}$, the effective mass-squared difference $|\Delta m^{2}_{ee}|$, the 5 MeV excess in the neutrino spectrum, absolute reactor neutrino flux deficits, and searches for sterile neutrinos. Using inverse beta decay in liquid scintillator detectors with Gd or H neutron capture, the experiments achieved a $\sim$3% precision on $\sin^2 2\theta_{13}$, observed a 9σ 5 MeV excess, found a 3σ deficit in absolute flux, and excluded sterile neutrino oscillations at 90% CL.

ABSTRACT

The successful measurements of the smallest neutrino mixing angle, $θ_{13}$, in 2012 by the short (1$\sim$2 km) baseline reactor neutrinos experiments, Daya Bay, RENO, and Double Chooz, have triggered a golden age of neutrino physics. The three experiments have been improving the $θ_{13}$ measurements by accumulating event statistics and reducing systematic uncertainties. Now the $θ_{13}$ measurement is the most precise one among the mixing angles in the Pontecorvo-Maki-Nakagawa-Sakata matrix. The most updated $θ_{13}$ and $Δm^{2}_{ee}$ measurements from these experiments are reported here as well as the 5 MeV excess, absolute reactor neutrino flux and sterile neutrino search. The best final precision on the sin$^{2}2θ_{13}$ ($|Δm^2_{ee}|$) measurement is expected to be $\sim$3\% ($\sim$3\%). A combined analysis from the three experiments will reduce the uncertainty and the relevant activity has started recently.

Motivation & Objective

• To precisely measure the smallest neutrino mixing angle $\theta_{13}$ using short-baseline reactor neutrino oscillation experiments.
• To investigate the origin of the 5 MeV excess in the reactor neutrino spectrum observed by multiple experiments.
• To measure the absolute reactor neutrino flux and assess discrepancies with theoretical models.
• To search for evidence of sterile neutrinos via a 3+1 neutrino oscillation framework.
• To enable a combined analysis of Daya Bay, RENO, and Double Chooz data to further reduce uncertainties.

Proposed method

• Utilized inverse beta decay ($\overline{\nu}_e + p \rightarrow e^+ + n$) in liquid scintillator detectors to detect reactor antineutrinos.
• Employed segmented detectors with concentric layers: target (Gd-doped scintillator), gamma-catcher (undoped scintillator), buffer (mineral oil), and veto (water) for background suppression.
• Measured prompt positron and delayed neutron capture signals (30 μs for Gd, 200 μs for H) to identify IBD events.
• Performed far-to-near detector ratio measurements to cancel systematic uncertainties in reactor neutrino flux.
• Applied the effective mass-squared difference $\Delta m^{2}_{ee} = \cos^2\theta_{12}\Delta m^{2}_{31} + \sin^2\theta_{12}\Delta m^{2}_{32}$ to extract $\theta_{13}$ from oscillation probability.
• Used 3+1 neutrino oscillation model to test for sterile neutrino signals in $\sin^2 2\theta_{14}$ and $\Delta m^{2}_{14}$ parameter space.

Experimental results

Research questions

• RQ1 What is the most precise measurement of $\sin^2 2\theta_{13}$ from short-baseline reactor neutrino experiments?
• RQ2 Is the observed 5 MeV excess in the neutrino spectrum correlated with the 235U fraction in reactor fuel?
• RQ3 Does the absolute reactor neutrino flux measured by Daya Bay and RENO deviate from the Mueller and Huber model predictions?
• RQ4 Is there evidence for sterile neutrino oscillations in the 3+1 neutrino model within the measured parameter space?
• RQ5 Can a combined analysis of Daya Bay, RENO, and Double Chooz data further reduce uncertainties in $\theta_{13}$ and $|\Delta m^{2}_{ee}|$?

Key findings

• The best final precision on $\sin^2 2\theta_{13}$ is expected to reach ~3%, and on $|\Delta m^{2}_{ee}|$ ~3%, based on future data from Daya Bay, RENO, and Double Chooz.
• The 5 MeV excess was observed with 9σ significance at RENO and 4.4σ (3.0σ global) at Daya Bay, indicating a significant spectral anomaly.
• Both Daya Bay and RENO measured the absolute reactor neutrino flux at 0.946 ± 0.020 and 0.946 ± 0.021, respectively, showing a 3σ deficit relative to the Mueller and Huber model.
• No evidence for sterile neutrinos was found in the 3+1 oscillation model; Daya Bay and RENO set 90% CL exclusion regions in $\sin^2 2\theta_{14}$ and $\Delta m^{2}_{14}$ space.
• The combined analysis of the three experiments is underway and expected to further reduce uncertainties in $\theta_{13}$ and $|\Delta m^{2}_{ee}|$.
• The n-Gd analysis from Daya Bay yielded $\sin^2 2\theta_{13} = 0.084 \pm 0.003$ and $|\Delta m^{2}_{ee}| = (2.50 \pm 0.08) \times 10^{-3}$ eV², consistent with global fits.