Narrow Results

A brief review is given of some recent works where baryogenesis and dark matter have a common origin within the U(1) extensions of the Standard Model (SM) and of the minimal supersymmetric Standard Model (MSSM). The models considered generate the desired baryon asymmetry and the dark matter to baryon ratio. In one model, all of the fundamental interactions do not violate lepton number, and the total $B-L$ in the Universe vanishes. In addition, one may also generate a normal hierarchy of neutrino masses and mixings in conformity with the current data. Specifically, one can accommodate ${𝜃}_{13}\sim 9^{\circ }$ consistent with the data from Daya Bay reactor neutrino experiment.

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 and the first direct measurement of the mass-squared difference 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.

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 non-zero value in 2012. An improved measurement of the oscillation amplitude and the first direct measurement of the mass-squared difference 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.

Neutrons induced by cosmic-ray muons are a significant background for underground experiments studying neutrino oscillations, neutrino-less double beta decay, dark matter and other rare-event signals. The Daya Bay Reactor Antineutrino experiment consists of 8 antineutrino detectors (AD) placed in three experimental halls at different baselines from six nuclear reactors. Each AD contains 20 tons of Gd-doped liquid scintillator, serving as the main target for antineutrinos interacting via the inverse beta-decay (IBD) reaction. The data from Daya Bay allows to make a competitive measurement of neutron production by cosmogenic muons at depths of 250, 265 and 860 meters-water-equivalent.

The conventional formula for neutrino oscillation probability is based on the neutrino plane-wave assumption, which is a good approximation but not theoretically self-consistent. Wave-packet treatment is necessary for a self-consistent description of neutrino oscillation phenomenon. The oscillation of reactor neutrino observed by the Daya Bay experiment is examined in the framework of a model in which the neutrino is described by a wave packet with a relative intrinsic momentum dispersion σ_rel. The modified survival probability formula based on neutrino wave-packet (WP) treatment was used to fit the Daya Bay data, providing the first quantitative limits on 95% confidence level: 2.38 · 10⁻¹⁷ < σ_rel < 0.232. Furthermore, the effect due to the wave packet nature of neutrino oscillation is found to be insignificant in the Daya Bay experiment, which ensures an unbiased measurement of the oscillation parameters sin² 2θ₁₃ and $\Delta m_{32}^2$ within the plane wave model.

A sterile neutrino at the eV or sub-eV scale was suggested as a possible explanation of the electron antineutrino excess observed in the LSND and MiniBooNE experiments. Searches for a light sterile neutrino have been independently performed by the MINOS and the Daya Bay experiments using the muon (anti)neutrino and electron antineutrino disappearance channels, respectively. In a recent analysis, results from both experiments are combined with those from Bugey-3 reactor neutrino experiment to constrain oscillations into light sterile neutrinos, setting stringent limits on sin² 2θ_μe over six orders of magnitude in the sterile mass-squared splitting $\Delta m_{41}^2$. The parameter space allowed by the LSND and MiniBooNE experiments is excluded for $\Delta m_{41}^2<0.8{\text{eV}}^2$ at 95% CL_s.

The Daya Bay experiment uses reactor antineutrino disappearance to measure the neutrino mixing parameter θ₁₃. A variety of deep neural networks are tested with a well-understood uncorrelated accidental background to the inverse beta decay signal to assess the utility of deep learning approaches for characterizing and discriminating backgrounds. Crucially, the training procedures are data-driven and do not rely on simulated events to train the neural networks. The eventual goal of this technique is to reduce the correlated β-n background, which results from the decay of ⁹Li produced by cosmic-ray muons. This background contributes the largest systematic uncertainty in the determination of θ₁₃.

With eight functionally identical electron anti-neutrino detectors deployed at three experimental halls near three high-power nuclear reactor complexes, the Daya Bay Reactor Neutrino Experiment has achieved unprecedented precision in measuring the neutrino mixing angle θ₁₃ via neutron captured on gadolinium (nGd). Combining 217 days of data collected using six antineutrinos detectors with 1013 days of data using eight detectors, a relative comparison of the rates and positron energy spectra of the detectors located far (~1500-1950m) relative to those near the reactors (~350-600m) gave sin² 2θ₁₃ = 0.0841 ± 0.0027(stat) ± 0.0019(syst). In addition, an independent analysis using samples based on neutron captured on hydrogen (nH) is validated, which has yielded sin² 2θ₁₃ = 0.071 ± 0.011. With the nGd result and the nH result combined, an improvement in precision can be reached. The technical details and the combination of nGd result and nH result will be presented in this proceeding.

The discovery of neutrino oscillations is a direct indication of physics beyond the Standard Model. The so-called atmospheric and solar sectors have been explored by several experiments, meanwhile the mixing angle θ₁₃ connecting both sectors remains unknown. In contrast to accelerator experiments, reactor neutrinos arise as a clean probe to search for a non-vanishing value of this angle. A new generation of multi-detector reactor experiments, starting operation by 2010-2011, aims at achieving sensitivities to sin²(2θ₁₃) down to 0.01. This will allow for the exploration of the first hints pointing to a non-zero value of θ₁₃, provided by global fits of available neutrino data.