Session EM: Mini-Symposium on Sterile Neutrinos

An overview of the physics possibilities and the planned searches for sterile neutrinos

There are many planned experiments on sterile neutrino searches in the world using radiation sources, reactors, atmospheric neutrinos and accelerators. Physics motivation of the searches and the overview of the experiments including the recent trend are shown in this talk.   

Cosmological neutrino counting, light WIMPs, and nuclear physics

Constraints from big-bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) limit the allowed number of neutrinolike particle species (of which only three can participate in the standard-model weak interaction) through their influence on the expansion rate of the universe. However, thermally-populated ``light WIMPs'' with mass $< 20$ MeV that couple to neutrinos or to the electromagnetic plasma would alter these limits. We have examined the observational consequences of light WIMPs for BBN and the CMB, assuming alternately that the WIMPs couple strongly either to the electromagnetic plasma or to the neutrinos. Light WIMPs that couple to neutrinos are disfavored compared with the standard model, while WIMPs that couple to the plasma are slightly favored over the standard model and could make a fourth thermally-populated neutrino species consistent with current data. In either case, current data imply a lower limit on the WIMP mass of 0.5 MeV to about 5 MeV, depending on the WIMP properties. We present the derived constraints and comment on their coupling to the underlying nuclear rates, particularly that of $d(p,\gamma)^3\mathrm{He}$.   

Sterile neutrino signatures in core-collapse supernova simulations

We have explored the impact of a fourth right handed sterile neutrino on core-collapse supernovae. We utilize a relativistic hydrodynamic spherical supernova model. We show that it is possible that oscillations between a sterile neutrino and electron neutrino (or their antiparticles) will enhance the supernova explosion energy by efficiently transporting neutrino energy from the core to just behind the shock. We have considered a range of masses and mixing angles, including those consistent with sterile neutrino dark matter. We find that the supernova explosion energy can be significantly increased due to the rapid transport of electron antineutrinos as sterile neutrinos from the core to behind the shock where they convert back to active neutrinos. This mechanism enhances the neutrino heating in the region behind the shock and leads to increased luminosities of all three neutrino flavors in addition to an enhanced explosion kinetic energy. We also show that the inclusion of sterile neutrinos leads to a unique oscillatory behavior in the emergent neutrino luminosities from the cyclic depletion of the neutrino density due to oscillations to a sterile neutrino.   

Status of fits for sterile neutrino models on global data

Anomalous signals in various short baseline neutrino experiments suggest the addition of one or more sterile neutrinos to the existing oscillation framework. Fits have been made to 3+n (n=1,2,3) models with sterile mass splittings in the 1 eV$^2$ range. This talk will provide an update to the current state of the art analyses of global data. The parameter goodness-of-fit is a statistical test used to address the dilution of the standard $\chi^2$ test due to insensitive bins. This talk will also present the results of an ongoing study into the effects of nuisance (pull) parameters on this test.   

Investigating the Reactor Antineutrino Anomaly with Beta Spectroscopy

The Reactor Antineutrino Anomaly is a discrepancy between the expected flux of antineutrinos from nuclear reactors and the detected flux. This anomaly is often explained by either the existence of a fourth, sterile neutrino or by incorrect calculations of the predicted number of reactor antineutrinos. Calculations of the expected flux assume that all the fission product $\beta$ decays have spectral shapes that are nearly identical to the allowed shape. However, many of the highest energy transitions are first forbidden and may have a different spectral shape, which could alter the predicted antineutrino flux and explain the anomaly. We will perform measurements of the shapes of $\beta$ decay spectra on the isotopes that have the biggest impact on the Reactor Antineutrino Anomaly. Those nuclei, starting with $^{92}$Rb, will be produced at the CARIBU facility at Argonne National Laboratory and the $\beta$ spectra will be measured in plastic scintillators. The energy response of the plastic scintillators will be calibrated by studying the allowed $\beta$ decay of $^{8}$Li.   

Beta spectral measurements for improved reactor antineutrino spectra

Analysis of reactor-produced antineutrino oscillation data requires knowledge of the underlying flux of antineutrinos emanating from the reactor. Results from these experiments are in tension with models that have mixing only among the three active neutrino flavors of the Standard Model. Knowledge of reactor antineutrino flux is based on inversion of total reactor beta spectra measured at the Institut Laue Langevin in the 1980s. Recent reanalysis of that data has resulted in a significant 3\% upward shift in the antineutrino flux with implications for the possible existence of sterile neutrinos. The potential to provide a secondary beta spectral measurement is presented with attention given to (1) activation of actinide foils using a neutron source with a energy spectrum that is tailored to ILL, reactor core, and ``pure fast'' neutron spectra and (2) absolute normalization of the beta emission rate per fission through ancillary measurements of gamma-ray emissions from the activated foils and post-irritation destructive radiochemical assay. These efforts are focused on checking the systematic uncertainties associated with the underlying beta spectra that are used to infer the reactor antineutrino spectra for operational nuclear power reactors.   

Latest developments on reactor antineutrino spectra and test of the reactor antineutrino anomaly with Nucifer

The detection of electron antineutrinos emitted in the decay chains of the fission products in nuclear reactors associated with accurate simulations provides an efficient tool to investigate the neutrino oscillation phenomenon. I will review the latest developments in the computation of reactor antineutrino spectra of the four main fissile isotopes, U235, U238, Pu239, and Pu241. I will discuss the recent observation of a spectral distortion at 4-6 MeV, not expected by the models. Then I will focus on experiments at short baselines, less than 100 meters, providing new results on the reactor antineutrino anomaly that could be interpreted as a hint for a new (sterile) neutrino state. I will then discuss the search for light sterile neutrino with the Nucifer detector being currently taking data at 7 m away from the Osiris research reactor at CEA-Saclay. Follow-up experiments in Europe will be discussed.   

PROSPECT, A US based short-baseline reactor neutrino experiment

Several recent calculations of the reactor antineutrino flux have revealed a possible deficit when compared to measurements at baselines between 10-100 m. Similar anomalous results are also seen in other electron antineutrino disappearance experiments. In addition, recent measurements of the shape of the reactor antineutrino spectrum observe statically significant deviations from the calculated spectrum. These discrepancies could be a sign of new physics. Precision measurements of the reactor antineutrino spectrum at very short baselines (order 1-10 m) can be used to probe these anomalies, for example, searching for possible oscillations into sterile neutrino species, as well as provide valuable data for safeguards purposes and reactor flux predictions. Given proper site optimization, detector design, and background reduction, an experiment mounted at a typical US research reactor can provide 5 sigma discovery potential for the favored oscillation parameter space with 3 years of detector live time. We will discuss the technical challenges and recent progress in mounting the PROSPECT experiment.   

Detector Design and Background Simulations for the PROSPECT Short-Baseline Reactor Experiment

PROSPECT is a U.S.-based, multi-phase, 2-detector reactor antineutrino experiment whose primary goals are to probe short-baseline oscillations and perform a precise measurement of the reactor antineutrino spectrum at a research reactor. There are several challenges in developing a detector for such a measurement, including little-to-no cosmic ray attenuating overburden and relatively compact spaces in which the detectors can be deployed. The baseline detector design being considered for the experiment is a segmented organic scintillator array since this arrangement can provide the position resolution required for oscillation studies in a compact geometry as well a topology based event identification capability. To study this design concept the PROSPECT Collaboration has developed a versatile simulation framework in which the geometric parameters characterizing an array can be easily varied. This simulation includes optical photon propagation and realistic background generation to best mimic experimental conditions. In this presentation we will describe this simulation framework and detector design and background response studies performed with it.   

Quest for the fourth neutrino with SOX

Both accelerator and reactor based neutrino experiments show indications of neutrino changing oscillations at a very short baseline, different from the standard three neutrino flavor mixing picture. Placement of the PBq antineutrino generator 144Ce-144Pr (followed by monoenergetic 51Cr neutrino generator) in the close vicinity of the Borexino liquid scintillator antineutrino detector, provides a unique opportunity to test the short baseline hypothesis for the actual L/E oscillation signature. The project is called SOX (Source in Borexino detector). We will present the physics potential of the experiment, current status of the source production and plans for the deployment.   

A New Cubic Meter Scale Neutrino Detector for Seeking Sterile Neutrino Signatures at a Reactor

We describe a new type of detector under construction to study electron anti-neutrinos a few meters from a nuclear reactor to look for oscillations, potentially due to sterile neutrinos, and addressing the ``Reactor Neutrino Anomaly.'' This detector is made possible by a natural synergy between the miniTimeCube and mini-LENS programs. It features a ``Raghavan Optical Lattice'' (ROL) consisting of cubical cells filled with liquid scintillator (doped to improve neutron detection). Cell walls are thin acrylic planes with a low-index film, resulting in total internal reflection guiding most of the light down the 3 cardinal directions. The six orthogonal photomultiplier tubes efficiently collect light from each cell, allowing event topology determination on a cellular level, and vertex resolution to about one cm using timing. The resulting excellent spatial and energy resolution, coupled with event topology, allows discerning the inverse beta decay signal, and the putative oscillation pattern, even in the presence of other backgrounds. We will discuss venues, efficiency, sensitivity and status of the project.   

Light Sterile Neutrino Search at Daya Bay

It has been well established through neutrino oscillation that neutrinos have masses and their flavors mix. In 2012, The Daya Bay Reactor Neutrino Experiment discovered that the last unknown neutrino mixing angle $\theta_{13}$ is non-zero. With more than one million reactor antineutrino interactions recorded since December 2011, besides the most precise measurement of $\sin^{2}2\theta_{13}$, the Daya Bay experiment also has high sensitivity in sterile neutrino search due to its multiple baselines. In this talk, I will discuss the spectral analysis of the light sterile neutrino search in the largely unexplored region of $10^{-3}~{\rm eV^{2}} < |\Delta m^2| < 0.3 {\rm eV^{2}}$ at Daya Bay through the electron antineutrino disappearance channel.