RENO (experiment)

RENO (experiment)
Name	RENO
Location	Hanbit Nuclear Power Plant
Field	Particle physics
Start	2002 (proposal)
Established	2006 (construction)
Collaborators	Institute for Basic Science, Korea Advanced Institute of Science and Technology, Seoul National University, Sungkyunkwan University, Yonsei University

RENO (experiment) is a South Korean reactor-based neutrino oscillation experiment designed to measure the mixing angle θ13 and to study electron antineutrino disappearance from commercial light-water reactors. Situated near the Hanbit Nuclear Power Plant on the western coast of South Korea, RENO uses identical near and far detectors to compare antineutrino fluxes, enabling precision measurements that complement accelerator-based projects such as T2K and NOvA and reactor efforts like Daya Bay and Double Chooz. The collaboration integrates researchers from major Korean institutions and international partners to address questions about lepton mixing within the PMNS matrix framework and contributions to the broader program of neutrino oscillation physics.

Overview

RENO was proposed to resolve the then-unknown value of the mixing angle θ13 within the Pontecorvo–Maki–Nakagawa–Sakata matrix for three-flavor neutrino oscillations. The experiment exploits the copious electron antineutrino flux produced by the pressurized water reactors at Hanbit Nuclear Power Plant, comparing measurements at two baselines to cancel correlated systematic uncertainties associated with reactor flux models like those tested in studies at Bugey and ILL. RENO’s approach is rooted in disappearance measurements complementary to appearance searches performed by Super-Kamiokande and MINOS, and its results feed into global fits coordinated by groups such as the NuFIT collaboration and analyses connected to KATRIN and cosmological constraints from Planck.

Experimental design

The experimental layout centers on two functionally identical detectors placed at different distances: a near detector to monitor unoscillated spectra and a far detector to observe oscillation-induced deficits. This relative measurement scheme traces methodology akin to CHOOZ follow-ups and leverages time-varying reactor operations at multiple reactor cores similar to arrangements at Daya Bay and Zhangjiang. Baseline lengths, overburden provided by local geology, and shielding choices were optimized to reduce backgrounds from cosmic muons recorded in muon veto systems like those developed for KamLAND and SNO+. The design accounts for oscillation probability dependence on squared-mass differences such as Δm^2_31 and seeks sensitivity over parameter space informed by results from Solar neutrino experiments and accelerator neutrino programs.

Detector components

Each detector incorporates a central volume of gadolinium-doped liquid scintillator contained within an acrylic vessel, surrounded by a non-doped scintillator gamma-catcher and a mineral oil buffer; photomultiplier tubes similar to models used in Super-Kamiokande and Borexino instrument the optical readout. Outer veto layers include water Cherenkov shields and plastic scintillator panels for cosmic-ray muon tagging, design elements influenced by systems at SNO and IceCube prototype modules. Calibration employs radioactive sources such as 137Cs, 60Co, and neutron sources paired with light-emitting diode systems; electronics and data-acquisition architectures draw on technologies developed for Daya Bay and RENO-50 conceptual studies. Mechanical support, shielding, and leak-tight vessels reflect engineering practices from projects at J-PARC and Gran Sasso National Laboratory.

Data collection and analysis

Data acquisition began when detectors achieved stable operation, recording inverse beta decay candidates via prompt positron signals followed by delayed neutron capture on gadolinium, with timing and energy cuts tuned to reject fast neutrons and accidental coincidences as in analyses from Double Chooz and Bugey-3. Background estimation combined reactor-off periods, multiplicity vetoes, and Monte Carlo simulations based on packages like Geant4 and cross-checked against external measurements such as neutron yields measured in CERN test facilities. Statistical analyses used rate-plus-shape fits, covariance matrices, and frequentist or Bayesian inference methods common in Particle Data Group–referenced workflows; systematic uncertainties included reactor power, fuel composition, detector response, and baseline geometry uncertainties informed by reactor engineering records from KEPCO operations at Hanbit.

Results and findings

RENO reported a definitive non-zero value of sin^2(2θ13), contributing a precise measurement of the mixing angle that corroborated and refined earlier indications from T2K and Daya Bay. The measured value of sin^2(2θ13) and constraints on effective squared-mass differences supported the three-flavor oscillation paradigm and informed global fits that impact inferences about the neutrino mass hierarchy investigated by NOvA and planned by JUNO. RENO also provided spectra with distortions such as the so-called 5 MeV excess that challenged reactor flux models developed by groups at Saclay and Bugey and stimulated theoretical work on reactor antineutrino production tied to isotopes like 235U and 239Pu. Secondary analyses explored sterile neutrino limits relevant to anomalies seen in LSND and MiniBooNE and conducted searches for exotic signatures comparable to efforts in PROSPECT.

Collaboration and timeline

The RENO collaboration comprises researchers from institutions including Institute for Basic Science, Korea Advanced Institute of Science and Technology, Seoul National University, Sungkyunkwan University, and Yonsei University, with international interactions involving groups connected to Daya Bay and global neutrino working groups. The project moved from proposal in the early 2000s to construction in the mid-2000s, commissioning in the early 2010s, and successive data releases through the 2010s into the 2020s, aligned with contemporaneous milestones at Daya Bay, Double Chooz, and accelerator programs at J-PARC and Fermilab.

Impact and legacy

RENO’s precise determination of θ13 reshaped priorities in long-baseline programs such as DUNE and Hyper-Kamiokande by establishing an experimentally favorable value that enhances sensitivity to CP violation in the lepton sector. Its observations of spectral anomalies propelled revisions to reactor antineutrino flux models and motivated new short-baseline reactor experiments like PROSPECT and detector concepts such as JUNO-class medium-baseline designs. The experiment strengthened South Korea’s role in international neutrino research, bolstered capacity at participating universities and institutes, and contributed datasets incorporated into global analyses influencing particle physics, astrophysics, and cosmology.

Category:Neutrino experiments