Daya Bay Experiment Research Articles

Yifang Wang: high energy physics in China

Abstract On 8 March 2012, Yifang Wang, co-spokesperson of the Daya Bay Experiment and Director of Institute of High Energy Physics, Chinese Academy of Sciences, announced the discovery of a new type of neutrino oscillation with a surprisingly large mixing angle (θ13), signifying ‘a milestone in neutrino research’. Now his team is heading for a new goal—to determine the neutrino mass hierarchy and to precisely measure oscillation parameters using the Jiangmen Underground Neutrino Observatory, which is due for completion in 2020. Neutrinos are fundamental particles that play important roles in both microscopic particle physics and macroscopic universe evolution. The studies on neutrinos, for example, may answer the question why our universe consists of much more matter than antimatter. But this is not an easy task. Though they are one of the most numerous particles in the universe and zip through our planet and bodies all the time, these tiny particles are like ‘ghost’, difficult to be captured. There are three flavors of neutrinos, known as electron neutrino (νe), muon neutrino (νμ), and tau neutrino (ντ), and their flavors could change as they travel through space via quantum interference. This phenomenon is known as neutrino oscillation or neutrino mixing. To determine the absolute mass of each type of neutrino and find out how they mix is very challenging. In a recent interview with NSR in Beijing, Yifang Wang explained how the Daya Bay Experiment on neutrino oscillation not only addressed the frontier problem in particle physics, but also harnessed the talents and existing technology in Chinese physics community. This achievement, Wang reckons, will not be an exception in Chinese high energy physics, when appropriate funding and organization for big science projects could be efficiently realized in the future.

Constraining new physics scenarios in neutrino oscillations

We consider the data of the Daya Bay experiment to constrain the parameter space of models where sterile neutrinos can propagate in a large compactified extra dimension (LED) and models where non-standard interactions affect the neutrino production and detection (NSI). I will show that compactification radius R in LED scenarios can be constrained at the level of 0.57 μm for normal ordering and of 0.19 μm for inverted ordering, at 2σ confidence level. For the NSI model, reactor data put a strong upper bound on the parameter εee at the level of 10-3, whereas the main effect of εeμ and εeτ is a worsening of the determination of θ13.

Measurement of the Reactor Antineutrino Flux and Spectrum at Daya Bay.

This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9 GWth nuclear reactors with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296 721 and 41 589 inverse β decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is (1.55±0.04) ×10(-18) cm(2) GW(-1) day(-1) or (5.92±0.14) ×10(-43) cm(2) fission(-1). This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is 0.946±0.022 (0.991±0.023) relative to the flux predicted with the Huber-Mueller (ILL-Vogel) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than 2σ over the full energy range with a local significance of up to ∼4σ between 4-6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.

The detector system of the Daya Bay reactor neutrino experiment

The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery of ν¯e oscillations over km-baselines. Subsequent data has provided the world׳s most precise measurement of sin22θ13 and the effective mass splitting Δmee2. The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the world׳s most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes, the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors’ baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking.

Reactor Antineutrinos: Present and Future

Neutrinos stand among the least understood particles in the Standard Model. The intense electron antineutrino flux produced by the beta-decays of fission products in nuclear reactors is a very effective way of learning about these elusive particles. A brief review of the current generation reactor antineutrino experiments and their motivation is presented, with a strong emphasis on the Daya Bay experiment. The latter's recent results are described, as well as its prospects for the near future. Likewise, the possibility of measuring the neutrino mass hierarchy with reactor antineutrinos is described in the context of JUNO, a next generation experiment in China that is scheduled to begin operating as early as 2020.

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.

Neutron calibration sources in the Daya Bay experiment

We describe the design and construction of the low rate neutron calibration sources used in the Daya Bay Reactor Anti-neutrino Experiment. Such sources are free of correlated gamma-neutron emission, which is essential in minimizing induced background in the anti-neutrino detector. The design characteristics have been validated in the Daya Bay anti-neutrino detector.

Probing non-standard interactions at Daya Bay

AbstractIn this article we consider the presence of neutrino non-standard interactions (NSI) in the production and detection processes of reactor antineutrinos at the Daya Bay experiment. We report for the first time, the new constraints on the flavor non-universal and flavor universal charged-current NSI parameters, estimated using the currently released 621 days of Daya Bay data. New limits are placed assuming that the new physics effects are just inverse of each other in the production and detection processes. With this special choice of the NSI parameters, we observe a shift in the oscillation amplitude without distorting the L/E pattern of the oscillation probability. This shift in the depth of the oscillation dip can be caused by the NSI parameters as well as by θ13, making it quite difficult to disentangle the NSI effects from the standard oscillations. We explore the correlations between the NSI parameters and θ13 that may lead to significant deviations in the reported value of the reactor mixing angle with the help of iso-probability surface plots. Finally, we present the limits on electron, muon/tau, and flavor universal (FU) NSI couplings with and without considering the uncertainty in the normalization of the total event rates. Assuming a perfect knowledge of the event rates normalization, we find strong upper bounds ∼ 0.1% for the electron and FU cases improving the present limits by one order of magnitude. However, for a conservative error of 5% in the total normalization, these constraints are relaxed by almost one order of magnitude.

Probing new physics scenarios in accelerator and reactor neutrino experiments

We perform a detailed combined fit to the disappearence data of the Daya Bay experiment and the appearance and disappearance data of the Tokai to Kamioka (T2K) one in the presence of two models of new physics affecting neutrino oscillations, namely a model where sterile neutrinos can propagate in a large compactified extra dimension and a model where non-standard interactions (NSI) affect the neutrino production and detection. We find that the Daya Bay ⨁ T2K data combination constrains the largest radius of the compactified extra dimensions to be μm at 2σ C.L. (for the inverted ordering of the neutrino mass spectrum) and the relevant NSI parameters in the range , for particular choices of the charge parity violating phases.

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.

Recent results from Daya Bay experiment

This manuscript is a short summary of my talk given at ICNFP2014 Conference. Here we report on new results of $\sin^22\theta_{13}$ and $\Delta m^2_\text{ee}$ measurements, search for the sterile neutrino within $10^{-3} \text{ eV}^2 <\Delta m^2_{41}<0.1\text{ eV}^2$ domain and precise measurement of the reactor absolute antineutrino flux.

Daya Bay Reactor Antineutrino Experiment

With 217 days of data-taking, using six antineutrino detectors deployed in three experimental halls near the Daya Bay Nuclear Power Plant complex, the Daya Bay experiment has obtainted sin22θ13 = 0.090−0.009+0.008 from the rate and energy spectra analysis under the three-neutrino framework. In addition, the value of effective mass-squared difference |Δmee2| = (2.59+0.19-0.20) × 10−3 eV2 is directly measured through νe disappearance channel for the first time, which is ee−0.20 consistent with the |Δm2μμ| measured by muon neutrino disappearance channel.

Recent results from Daya Bay experiment

This manuscript is a short summary of my talk given at ICNFP2014 Conference. Here we report on new results of sin2 2θ13 and Δm2ee measurements, search for the sterile neutrino within 10−3 eV2 < Δm241 < 0.1 eV2 domain and precise measurement of the reactor absolute antineutrino flux.

The Antineutrino Energy Structure in Reactor Experiments

The recent observation of an energy structure in the reactor antineutrino spectrum is reviewed. The reactor experiments Daya Bay, Double Chooz, and RENO have reported a consistent excess of antineutrinos deviating from the flux predictions, with a local significance of about 4σbetween 4 and 6 MeV of the positron energy spectrum. The possible causes of the structure are analyzed in this work, along with the different experimental approaches developed to identify its origin. Considering the available data and results from the three experiments, the most likely explanation concerns the reactor flux predictions and the associated uncertainties. Therefore, the different current models are described and compared. The possible sources of incompleteness or inaccuracy of such models are discussed, as well as the experimental data required to improve their precision.

Application of an acrylic vessel supported by a stainless-steel truss for the JUNO central detector

After the success of the Daya Bay experiment, the Jiangmen Underground Neutrino Observatory (JUNO) was launched to measure neutrino-mass hierarchy and oscillation parameters and to study other neutrino physics. Its central detector is set for antineutrinos from reactors, the Earth, the atmosphere, and the Sun. The main requirements of the central detector are containment of 20 kt of liquid scintillator, as the target mass, and 3% energy resolution. It is about a ball-shape detector of 38.5 m with ∼75% coverage of PMT on its inner surface. The design of such a huge detector is a big challenge because it must meet the requirements for several different types of physics measurement and possess the feasibility and reliability in its structure and engineering, all at reasonable time and cost. One option for the JUNO central detector is a hyper-scale acrylic ball submerged in the water to shield the background. This paper proposes a structural scheme for such an acrylic ball that is supported by a stainless-steel truss, inspired by point-supported glass-curtain walls in civil engineering. The preliminary design of the scheme is completed and verified by finite element (FE) method using ABAQUS. FE analysis shows that the scheme can control the stress level of the acrylic ball within the limit of 5 to 10 MPa, in accordance with the demand of the design objective of the central detector. The scheme is of outstanding global stability and allows various choices on local connections. We prove that the scheme is of good feasibility and should be a reasonable option for the central detector.

Constraining new physics scenarios in neutrino oscillations from Daya Bay data

We perform for the first time a detailed fit to the $\bar \nu_e \to \bar \nu_e$ disappearance data of the Daya Bay experiment to constrain the parameter space of models where sterile neutrinos can propagate in a large compactified extra dimension (LED) and models where non-standard interactions affect the neutrino production and detection (NSI). We find that the compactification radius $R$ in LED scenarios can be constrained at the level of $0.57 \, \mu m$ for normal ordering and of $0.19\, \mu m$ for inverted ordering, at 2$\sigma$ confidence level. For the NSI model, reactor data put a strong upper bound on the parameter $\varepsilon_{ee}$ at the level of $\sim 10^{-3}$, whereas the main effect of $\varepsilon_{e\mu}$ and $\varepsilon_{e\tau}$ is a worsening of the determination of $\theta_{13}$.

Reactor antineutrino experiments

Neutrinos are elementary particles in the standard model of particle physics. There are three flavors of neutrinos that oscillate among themselves. Their oscillation can be described by a 3×3 unitary matrix, containing three mixing angles θ12, θ23, θ13, and one CP phase. Both θ12 and θ23 are known from previous experiments. θ13 was unknown just two years ago. The Daya Bay experiment gave the first definitive nonzero value in 2012. An improved measurement of the oscillation amplitude [Formula: see text] and the first direct measurement of the [Formula: see text] mass-squared difference [Formula: see text] were obtained recently. The large value of θ13 boosts the next generation of reactor antineutrino experiments designed to determine the neutrino mass hierarchy, such as JUNO and RENO-50.

The muon system of the Daya Bay Reactor antineutrino experiment

The Daya Bay experiment consists of functionally identical antineutrino detectors immersed in pools of ultrapure water in three well-separated underground experimental halls near two nuclear reactor complexes. These pools serve both as shields against natural, low-energy radiation, and as water Cherenkov detectors that efficiently detect cosmic muons using arrays of photomultiplier tubes. Each pool is covered by a plane of resistive plate chambers as an additional means of detecting muons. Design, construction, operation, and performance of these muon detectors are described.

Independent measurement of the neutrino mixing angleθ13via neutron capture on hydrogen at Daya Bay

A new measurement of the $\theta_{13}$ mixing angle has been obtained at the Daya Bay Reactor Neutrino Experiment via the detection of inverse beta decays tagged by neutron capture on hydrogen. The antineutrino events for hydrogen capture are distinct from those for gadolinium capture with largely different systematic uncertainties, allowing a determination independent of the gadolinium-capture result and an improvement on the precision of $\theta_{13}$ measurement. With a 217-day antineutrino data set obtained with six antineutrino detectors and from six 2.9 GW$_{th}$ reactors, the rate deficit observed at the far hall is interpreted as $\sin^22\theta_{13}=0.083\pm0.018$ in the three-flavor oscillation model. When combined with the gadolinium-capture result from Daya Bay, we obtain $\sin^22\theta_{13}=0.089\pm0.008$ as the final result for the six-antineutrino-detector configuration of the Daya Bay experiment.

Onsite data processing and monitoring for the Daya Bay experiment

The Daya Bay Reactor Neutrino Experiment started running on September 23, 2011. The offline computing environment, consisting of 11 servers at Daya Bay, was built to process onsite data. With the current computing ability, onsite data processing is running smoothly. The Performance Quality Monitoring system (PQM) has been developed to monitor the detector performance and data quality. Its main feature is the ability to efficiently process multiple data streams from the three experimental halls. The PQM processes raw data files from the Daya Bay data acquisition system, generates and publishes histograms via a graphical web interface by executing the user-defined algorithm modules, and saves the histograms for permanent storage. The fact that the whole process takes only around 40 minutes makes it valuable for the shift crew to monitor the running status of all the sub-detectors and the data quality.