Sterile Neutrino Search at the NEOS Experiment

Abstract

An experiment to search for light sterile neutrinos is conducted at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search is done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1976 per day with a signal to background ratio of about 22. The shape of the antineutrino energy spectrum obtained from the eight-month data-taking period is compared with a hypothesis of oscillations due to activesterile antineutrino mixing. No strong evidence of 3+1 neutrino oscillation is found. An excess around the 5 MeV prompt energy range is observed as seen in existing longer-baseline experiments. The mixing parameter sin 2 2θ14 is limited up to less than 0.1 for ∆m The mixing among three neutrinos has been well established by experiments performed in the past two decades since the discovery of neutrino oscillations [1][2][3]. Consistent measurements of the two mass differences and the three mixing angles of the standard, three-neutrino mixing model have been reported by oscillation experiments using atmospheric, solar, reactor, and accelerator neutrinos [4]. Nevertheless, the mass hierarchy, the mass of the lightest neutrino, the Dirac or Majorana nature of the neutrino, and the CP phase are yet to be determined [5].Even though the number of active light neutrinos is limited to three by Z boson decay-width measurements [6], it is still possible to have additional neutrinos if they are sterile. Sterile neutrinos can be identified by the occurrence of activesterile neutrino oscillations. A hint for this is the LSND experiment's report of an observation ofν µ →ν e mixing with a frequency corresponding to a mass-squared difference larger than 0.01 eV 2 [7]. Results from the MiniBooNE's test of the LSND signal are, however, inconclusive [8].In addition to the LSND result, there are two other anomalies that could possibly be signs of active-sterile neutrino oscillations. An apparent ν e disappearance over a baseline of a few meters in the GALLEX and SAGE gallium experiments exposed to radioactive sources was reported [9]; the ratio of the numbers of measured and predicted events is 0.88 ± 0.05. A number of short-baseline reactor antineutrino experiments established limits on the presence of neutrino oscillations with eV mass differences by shape analyses of the measured neutrino energy spectra. Among those experiments, the Bugey experimental limits on sterile neutrinos are the most stringent [10]. Mueller et al. [11] found about a 6% deficit in reactor antineutrino event rates compared with the theoretical expectations for the short-baseline reactor experiments, which is the so-called "reactor antineutrino anomaly" (RAA). It can be interpreted as an active-sterile neutrino oscillation with three active neutrinos plus one or more sterile neutrinos, i.e., a 3 + n ν scenario [12,13], compatible with the LSND result. Recent reactor experiments that measured the θ 13 mixing an...

Prospects for improved understanding of isotopic reactor antineutrino fluxes

“…A number of HEU-based experiments have thus far not demonstrated the level of background rejection necessary to achieve the 1∶1 signal-to-background ratio (S:B) assumed in this study [46,55,57]. On the other hand, other short-baseline efforts have either already achieved a S:B ratio far surpassing 1∶1 at LEU reactors [56] or have presented prototype-and Monte Carlo-based evidence to support their achievement of a S:B of 1∶1 or higher at HEU reactors [35,37]. To examine the impact of the S:B on IBD yield precision, we adjust the assumed S:B of all short-baseline measurements in data sets 2-5, while continuing to neglect background contributions for the Daya Bay-like measurements of data sets 1-3.…”

“…A variety of new ton-scale compact reactor antineutrino experiments have either recently been deployed [55][56][57] or will be deployed in the near future [35][36][37] at short baselines from operating LEU [55,56] or HEU [35][36][37] reactors. These experiments' detectors incorporate a wide variety of detection technologies and background rejection techniques to detect an IBD signal despite sizable cosmogenic and ambient backgrounds in their near-surface, nearreactor environments.…”

Neutrinoless double beta decay versus other probes of heavy sterile neutrinos

“…(3+1) and (3+2) phenomenological models. While recent reactor experiments such as DANSS [123] and NEOS [124] have improved the statistical significance of an additional eV-scale sterile state, when combined with the ν e appearance data of MiniBooNE they are in strong tension with the observed ν µ → ν µ accelerator neutrino disappearance of the MINOS, NOνA and IceCube experiments.…”

Prospects of light sterile neutrino oscillation and CP violation searches at the Fermilab Short Baseline Neutrino Facility

“…[26]. We omit the recent MINOS+ [21], IceCube [20] and NEOS [18] constraints from the global fits, although we note that in the future those constraints should be included for more quantitatively accurate results. We expect that the qualitative conclusions drawn in this work stand regardless of inclusion of these more recent constraints in the fit or not.…”

“…[9][10][11]) dictate that the reactor anomaly cannot yet be interpreted decisively as a light sterile neutrino oscillation signature; such interpretations should await either improved reactor antineutrino flux modelling or dedicated searches for light sterile neutrino oscillations at reactor short baselines which are sensitive to distortions in reconstructed event spectra that are L=E dependent. Such searches are now under way with a number of experiments [12][13][14][15][16][17][18].…”