RENO Experiments Research Articles

Generating CP Violation from a Modified Fridberg-Lee Model

The overall characteristics of the solar and atmospheric neutrino oscillations are approximately consistent with a tribimaximal form of the mixing matrix U of the lepton sector. Exact tribimaximal mixing leads to θ13=0. However, the results from the Daya Bay and RENO experiments have established, such that in comparison to the other neutrino mixing angles, θ13 is small. Moreover, the atmospheric and solar mass splitting differ by two orders of magnitude. These significant differences constitutes the great enthusiasm and main motivation for our research herein reported. Keeping the behavior of U as tribimaximal, we would make a response to the following questions: at some level, whether or not the small parameters such as the solar neutrino mass splitting and Ue3, which vanish in a new framework, can be interpreted as a modified FL neutrino mass model? Subsequently, a minimal single perturbation leads to nonzero values for both of them? Our minimal perturbation matrix is constructed solely from computing the third mass eigenstate, using the rules of perturbation theory. Let us point out that, unlike other investigations, this matrix is not adopted on an ad hoc basis, but is created following a series of steps that we will describe. Also in compared to the original FL neutrino mass model which generalize it by inserting phase factors, our work is more accurate. Subsequently, we produce the following results that add new contributions to the literature: (a) we obtain a realistic neutrino mixing matrix with δ≠0 and θ23=45∘; (b) the solar mass splitting term is dominated by an imaginary term, which could induce the existence of Majorana neutrinos, along with explaining a large CP violation in nature; (c) the ordering of the neutrino masses is normal; however, at the end of the allowed range, it becomes more degenerate (97%); (d) we also obtain the allowed range of the mass parameters, which not only are in accordance with the experimental data but also allow falsifiable predictions for the masses of the neutrinos and the CP violating phases which none of these results has been achieved in the original FL neutrino mass model. Finally, let us emphasize that the results obtained by our framework here are much more efficient compared to those obtained in previous works in terms of currently available experimental data (namely, the best fit column).

Measurement of cosmogenic Li9 and He8 production rates at RENO

We report the measured production rates of unstable isotopes $^9$Li and $^8$He produced by cosmic muon spallation on $^{12}$C using two identical detectors of the RENO experiment. Their beta-decays accompanied by a neutron make a significant contribution to backgrounds of reactor antineutrino events in precise determination of the smallest neutrino mixing angle. The mean muon energy of its near (far) detector with an overburden of 120 (450) m.w.e. is estimated as 33.1 +- 2.3 (73.6 +- 4.4) GeV. Based on roughly 3100 days of data, the cosmogenic production rate of $^9$Li ($^8$He) isotope is measured to be 44.2 +- 3.1 (10.6 +- 7.4) per day at near detector and 10.0 +- 1.1 (2.1 +- 1.5) per day at far detector. This corresponds to yields of $^9$Li ($^8$He), 4.80 +- 0.36 (1.15 +- 0.81) and 9.9 +- 1.1 (2.1 +- 1.5) at near and far detectors, respectively, in a unit of 10$^{-8}$ $\mu^{-1}$ g${^-1}$ cm${^2}$. Combining the measured $^9$Li yields with other available underground measurements, an excellent power-law relationship of the yield with respect to the mean muon energy is found to have an exponent of $\alpha$ = 0.75 +- 0.05.

Measurement of reactor antineutrino flux and spectrum at RENO

The RENO experiment reports measured flux and energy spectrum of reactor electron antineutrinos\,($\overline{\nu}_e$) from the six reactors at Hanbit Nuclear Power Plant. The measurements use 966\,094\,(116\,111)\,$\overline{\nu}_e$ candidate events with a background fraction of 2.39\%\,(5.13\%), acquired in the near\,(far) detector, from August 2011 to March 2020. The inverse beta decay (IBD) yield is measured as (5.852$\,\pm\,$0.124$) \times 10^{-43}$\,cm$^2$/fission, corresponding to 0.941\,$\pm$ 0.019 of the prediction by the Huber and Mueller (HM) model. A reactor $\overline{\nu}_e$ spectrum is obtained by unfolding a measured IBD prompt spectrum. The obtained neutrino spectrum shows a clear excess around 6\,MeV relative to the HM prediction. The obtained reactor $\overline{\nu}_e$ spectrum will be useful for understanding unknown neutrino properties and reactor models. The observed discrepancies suggest the next round of precision measurements and modification of the current reactor $\overline{\nu}_e$ models.

Search for Sub-eV Sterile Neutrinos at RENO.

We report a search result for a light sterile neutrino oscillation with roughly 2200 live days of data in the RENO experiment. The search is performed by electron antineutrino (ν[over ¯]_{e}) disappearance taking place between six 2.8 GW_{th} reactors and two identical detectors located at 294m (near) and 1383m (far) from the center of the reactor array. A spectral comparison between near and far detectors can explore reactor ν[over ¯]_{e} oscillations to a light sterile neutrino. An observed spectral difference is found to be consistent with that of the three-flavor oscillation model. This yields limits on sin^{2}2θ_{14} in the 10^{-4}≲|Δm_{41}^{2}|≲0.5 eV^{2} region, free from reactor ν[over ¯]_{e} flux and spectrum uncertainties. The RENO result provides the most stringent limits on sterile neutrino mixing at |Δm_{41}^{2}|≲0.002 eV^{2} using the ν[over ¯]_{e} disappearance channel.

Double Chooz θ13 measurement via total neutron capture detection

The establishment of the neutrino oscillations phenomenon as a solution to both solar and atmospheric neutrino anomalies had two consequences: a new oscillation mode, labelled $\mathbf{\theta_{13}}$, and the possibility to observe CP violation, if $\mathbf{\theta_{13}}$ was sizeable. CP violation implies that neutrino oscillations behave differently for neutrinos and anti-neutrinos -- a rare fundamental phenomenon key for our understanding of the Universe. The experimental demonstration of $\mathbf{\theta_{13}}$ has aided the completion of a quest lasting half a century. The best $\mathbf{\theta_{13}}$ knowledge is today inferred from high-precision reactor neutrino disappearance. The Double Chooz (DC) experiment has played a pioneering role in this channel by providing the first positive evidence, in 2011, in combination with the T2K experiment appearance data. The establishment of $\mathbf{\theta_{13}}$ awaited the Daya Bay experiment's observation in 2012; confirmed soon after by the RENO experiment. Today's best knowledge on $\mathbf{\theta_{13}}$ from reactor experiments is a key input to many neutrino experiments. Here DC reports its first multi-detector $\mathbf{\theta_{13}}$ measurement exploiting several unprecedented techniques for a major precision improvement.

Production and optical properties of liquid scintillator for the JSNS2 experiment

The JSNS2 (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source) experiment will search for neutrino oscillations over a 24 m short baseline at J-PARC. The JSNS2 inner detector will be filled with 17 tons of gadolinium-loaded liquid scintillator (LS) with an additional 31 tons of unloaded LS in the intermediate γ-catcher and outer veto volumes. JSNS2 has chosen Linear Alkyl Benzene (LAB) as an organic solvent because of its chemical properties. The unloaded LS was produced at a refurbished facility, originally used for scintillator production by the RENO experiment. JSNS2 plans to use ISO tanks for the storage and transportation of the LS. In this paper, we describe the LS production, and present measurements of its optical properties and long term stability. Our measurements show that storing the LS in ISO tanks does not result in degradation of its optical properties.

Reactor neutrino oscillations as constraints on effective field theory

We study constraints on the Standard Model Effective Field Theory (SMEFT) from neutrino oscillations in short-baseline reactor experiments. We calculate the survival probability of reactor antineutrinos at the leading order in the SMEFT expansion, that is including linear effects of dimension-6 operators. It is shown that, at this order, reactor experiments alone cannot probe charged-current contact interactions between leptons and quarks that are of the (pseudo)vector (V±A) or pseudo-scalar type. We also note that flavor-diagonal (pseudo)vector coefficients do not have observable effects in oscillation experiments. In this we reach novel or different conclusions than prior analyses of non-standard neutrino interactions. On the other hand, reactor experiments offer a unique opportunity to probe tensor and scalar SMEFT operators that are off-diagonal in the lepton-flavor space. We derive constraints on the corresponding SMEFT parameters using the most recent data from the Daya Bay and RENO experiments.

Measurement of Reactor Antineutrino Oscillation Amplitude and Frequency at RENO.

The RENO experiment reports more precisely measured values of θ_{13} and |Δm_{ee}^{2}| using ∼2200 live days of data. The amplitude and frequency of reactor electron antineutrino (ν[over ¯]_{e}) oscillation are measured by comparing the prompt signal spectra obtained from two identical near and far detectors. In the period between August 2011 and February 2018, the far (near) detector observed 103 212 (850 666) ν[over ¯]_{e} candidate events with a background fraction of 4.8% (2.0%). A clear energy and baseline dependent disappearance of reactor ν[over ¯]_{e} is observed in the deficit of the measured number of ν[over ¯]_{e}. Based on the measured far-to-near ratio of prompt spectra, we obtain sin^{2}2θ_{13}=0.0896±0.0048(stat)±0.0047(syst) and |Δm_{ee}^{2}|=[2.68±0.12(stat)±0.07(syst)]×10^{-3} eV^{2}.

Search for sterile neutrinos at RENO

The RENO experiment was designed to measure a neutrino mixing angle, θ13, by detecting electron antineutrinos emitted from the Hanbit nuclear reactors in Korea, and succeeded to measure θ13 from the disappearance mode in three neutrino frame. We investigate the possibility of sterile neutrinos existence at RENO experiment and compare data with Monte Carlo generated in four neutrino frame. In this talk, we present some recent results using chi-square analysis method. The probability deficit curve as a function of an effective baseline and the excluded contour plot in sin2(2 θ14) – Δ(m41)2 space will be shown.

Experimental study of decoherence effect at Daya Bay

The last unknown neutrino mixing angle has been successfully measured by the Daya Bay, RENO and Double Chooz experiments. The used oscillation probability formula is based on the plane-wave model. A wave-packet model is necessary for a self-consistent description of neutrino oscillations. The oscillation probability formula in the wave-packet model describes effects missing in the plane-wave approximation, such as delocalization, decoherence and dispersion which all depend on a single unknown parameter σp – momentum dispersion of the neutrino wave-packet. The survival probability formula in the wave-packet model was used to fit the Daya Bay data. The high-statistics data set of reactor interactions in the Daya Bay experiment allows us to study the neutrino oscillation in the wave-packet model and provide the first experimental constraint of σp. Some results of our study are reported in this paper.

New results from RENO & prospects with RENO-50

This paper briefly describes recent progress of RENO and next generation future prospect of the reactor neutrino oscillation experiment, RENO-50. Recently the RENO experiment has updated its latest value on sin22θ13 and provided new results on 5 MeV excess, Δm2ee, θ13 with n-H analysis, absolute antineutrino flux measurement, and sterile neutrino search. It gives rich programs of neutrino properties, detector development, nuclear monitoring and application. Using reactor neutrinos, the future RENO-50 experiment will search for more precise measurement of θ12, Δm212 and mass hierarchy.

Reactor antineutrino shoulder explained by energy scale nonlinearities?

The Daya Bay, Double Chooz and RENO experiments recently observed a significant distortion in their detected reactor antineutrino spectra, being at odds with the current predictions. Although such a result suggests to revisit the current reactor antineutrino spectra modeling, an alternative scenario, which could potentially explain this anomaly, is explored in this letter. Using an appropriate statistical method, a study of the Daya Bay experiment energy scale is performed. While still being in agreement with the γ calibration data and B12 measured spectrum, it is shown that a O(1%) deviation of the energy scale reproduces the distortion observed in the Daya Bay spectrum, remaining within the quoted calibration uncertainties. Potential origins of such a deviation, which challenge the energy calibration of these detectors, are finally discussed.

GSDC: A Unique Data Center in Korea for HEP research

Global Science experimental Data hub Center (GSDC) at Korea Institute of Science and Technology Information (KISTI) is a unique data center in South Korea established for promoting the fundamental research fields by supporting them with the expertise on Information and Communication Technology (ICT) and the infrastructure for High Performance Computing (HPC), High Throughput Computing (HTC) and Networking. GSDC has supported various research fields in South Korea dealing with the large scale of data, e.g. RENO experiment for neutrino research, LIGO experiment for gravitational wave detection, Genome sequencing project for bio-medical, and HEP experiments such as CDF at FNAL, Belle at KEK, and STAR at BNL. In particular, GSDC has run a Tier-1 center for ALICE experiment using the LHC at CERN since 2013. In this talk, we present the overview on computing infrastructure that GSDC runs for the research fields and we discuss on the data center infrastructure management system deployed at GSDC.

Observation of Energy and Baseline Dependent Reactor Antineutrino Disappearance in the RENO Experiment.

The RENO experiment has analyzed about 500 live days of data to observe an energy dependent disappearance of reactor ν[over ¯]_{e} by comparing their prompt signal spectra measured in two identical near and far detectors. In the period between August of 2011 and January of 2013, the far (near) detector observed 31 541 (290 775) electron antineutrino candidate events with a background fraction of 4.9% (2.8%). The measured prompt spectra show an excess of reactor ν[over ¯]_{e} around 5MeV relative to the prediction from a most commonly used model. A clear energy and baseline dependent disappearance of reactor ν[over ¯]_{e} is observed in the deficit of the observed number of ν[over ¯]_{e}. Based on the measured far-to-near ratio of prompt spectra, we obtain sin^{2}2θ_{13}=0.082±0.009(stat)±0.006(syst) and |Δm_{ee}^{2}|=[2.62_{-0.23}^{+0.21}(stat)_{-0.13}^{+0.12}(syst)]×10^{-3} eV^{2}.

Slow control systems of the Reactor Experiment for Neutrino Oscillation

The RENO experiment has been in operation since August 2011 to measure reactor antineutrino disappearance using identical near and far detectors. For accurate measurements of neutrino mixing parameters and efficient data taking, it is crucial to monitor and control the detector in real time. Environmental conditions also need to be monitored for stable operation of detectors as well as for safety reasons. In this paper, we report the design, hardware, operation, and performance of the slow control system.

Predicting leptonic CP phase by considering deviations in charged lepton and neutrino sectors

Recently, the reactor mixing angle has been measured precisely by Daya Bay, RENO, and T2K experiments with a moderately large value. However, the standard form of neutrino mixing patterns such as bimaximal, tri-bimaximal, golden ratio of types A and B, hexagonal, etc., which are based on certain flavor symmetries, predict vanishing . Using the fact that the neutrino mixing matrix can be represented as , where Ul and result from the diagonalization of the charged lepton and neutrino mass matrices and is a diagonal matrix containing Majorana phases, we explore the possibility of accounting for the large reactor mixing angle by considering deviations both in the charged lepton and neutrino sector. In the charged lepton sector we consider the deviation as an additional rotation in the (12) and (13) planes, whereas in the neutrino sector we consider deviations to various neutrino mixing patterns through (13) and (23) rotations. We find that with the inclusion of these deviations it is possible to accommodate the observed large reactor mixing angle , and one can also obtain limits on the charge-conjugation parity-violating Dirac phase and Jarlskog invariant JCP for most of the cases. We then explore whether our findings can be tested in the currently running NuMI Off-axis ve Appearance experiment with three years of data taking in neutrino mode followed by three years with the anti-neutrino mode.

Antineutrinos from nuclear reactors: recent oscillation measurements

Nuclear reactors are the most intense man-made source of antineutrinos, providing a useful tool for the study of these particles. Oscillation due to the neutrino mixing angle is revealed by the disappearance of reactor over ∼km distances. Use of additional identical detectors located near nuclear reactors reduce systematic uncertainties related to reactor emission and detector efficiency, significantly improving the sensitivity of oscillation measurements. The Double Chooz, RENO, and Daya Bay experiments set out in search of using these techniques. All three experiments have recently observed reactor disappearance, and have estimated values for of 9.3◦ ± 2.1°, 9.2◦ ± 0.9°, and 8.7◦ ± 0.4° respectively. The energy-dependence of disappearance has also allowed measurement of the effective neutrino mass difference, ≈ . Comparison with ≈ from accelerator measurements supports the three-flavor model of neutrino oscillation. The current generation of reactor experiments are expected to reach ∼3% precision in both and . Precise knowledge of these parameters aids interpretation of planned measurements, and allows future experiments to probe the neutrino mass hierarchy and possible CP-violation in neutrino oscillation. Absolute measurements of the energy spectra of deviate from existing models of reactor emission, particularly in the range of 5–7 MeV.

Neutrino mixing matrix and masses from a generalized Friedberg-Lee model

The overall characteristics of the solar and atmospheric neutrino oscillation are approximately consistent with a tribimaximal form of the mixing matrix $U$ of the lepton sector. Exact tribimaximal mixing leads to $\theta_{13}=0$. However, recent results from Daya Bay and RENO experiments have established a nonzero value for $\theta_{13}$. Keeping the leading behavior of $U$ as tribimaximal, we use a generalized Fridberg-Lee neutrino mass model along with a complementary ansatz to incorporate a nonzero $\theta_{13}$ along with CP violation. We generalize this model in two stages: In the first stage we assume $\mu-\tau$ symmetry and add imaginary components which leads to nonzero phases. In the second stage we add a perturbation with real components which breaks the $\mu-\tau$ symmetry and this leads to a nonzero value for $\theta_{13}$. The combination of these two generalizations leads to CP violation. Using only two of the experimental data, we can fix all of the parameters of our model and predict not only values for the other experimental data, which agree well with the available data, but also the masses of neutrinos and the CP violating phases and parameters. These predictions include the following: $\langle m_{\nu_e} \rangle\approx(0.033-0.037)~eV$, $\langle m_{\nu_\mu} \rangle\approx(0.043-0.048)~ eV$, $\langle m_{\nu_\tau} \rangle\approx(0.046-0.051)~ eV$, and $59.21^{\circ}\lesssim \delta\lesssim 59.34^{\circ}$

Trimaximal TM1 neutrino mixing in [formula omitted] with spontaneous CP violation

The measurement of the reactor angle by the Daya Bay and RENO experiments in 2012 has ruled out the tri-bimaximal paradigm. Adopting an S4 family symmetry, we propose direct models of the trimaximal type TM1 in which the tri-bimaximal Klein symmetry of the neutrino sector is broken to a residual Z2 symmetry. In such a scenario, the solar mixing angle is decreased compared to its tri-bimaximal value by about 1°, thus bringing it in excellent agreement with experimental observation. The atmospheric mixing angle, on the other hand, depends on the CP violating Dirac phase δ. Imposing CP conservation in the family symmetry limit, we show how to break the CP symmetry via flavon VEVs with well-defined complex phases, so that sizable deviations of the atmospheric angle from maximal mixing, consistent with the latest global fits, are produced.

Detection methods at reactor neutrino experiments

In 2011 and 2012, the smallest neutrino mixing angle θ13 was determined to be unexpected large by reactor neutrino experiments Daya Bay, Double Chooz, and RENO, and accelerator experiments T2K and MINOS. The most precise measurement is sin22θ13=0.089±0.010(stat.)±0.005(syst.), provided by Daya Bay. The measurement of θ13 opened the gateway to the mass hierarchy and CP phase measurements. It also marked the beginning of precision measurements in neutrino studies. With near-far relative measurement and improvements in detector design, the relative precision of neutrino detectors reached 0.2%. Detection methods for reactor neutrinos are reviewed. The highlighted techniques include gadolinium-doped liquid scintillator, three-layer detectors, functionally identical detectors, reflective panel, background shielding, etc. The next generation reactor neutrino experiment Daya Bay II and its technical challenges are briefly described.