Session F08: Neutrino Physics: BSM & Reactor Neutrinos

On the Half-Life of 71Ge and the Gallium Anomaly

The Gallium Anomaly is based on the fact that several experiments  have reported (ν_e,e-) reaction rates on ⁷¹Ga  lower than expected.  The vast majority of these reactions are thought to involve transitions between the ground states of ⁷¹Ga and ⁷¹Ge.  The cross section for such reactions can be deduced from the known half-life of ⁷¹Ge of 11.43±0.03 days¹.  This half life was last measured in 1985 and recently it was suggested that a somewhat larger value could reduce or eliminate this anomaly².  As a result, we are remeasuring this important half life.  We produced ⁷¹Ge at the McClellan Nuclear Research Center by irradiating samples of high-purity GeO₂ and detector grade Ge with thermal neutrons.  After allowing approximately two weeks of cooling for short-lived activities to decay, we started three separate measurements using dedicated planar Ge detectors – one using ⁵⁵Fe as a standard, one using ⁵⁷Co as a standard, and one stand-alone measurement, collecting data in one-day time bins. Preliminary results from these three measurements yield half-lives consistent with the known value.  Final results from this experiment will be presented in this talk.

¹ B. Singh and J. Chen, Nucl. Data Sheets 188 (2023) 1

²  C. Giunti et al, arXiv:2212.09722 (2023)   

The Gallium Neutrino Absorption Cross Section and its Uncertainty

The recent Baksan Experiment on Sterile Transitions (BEST) reported a suppressed rate of neutrino absorption on a gallium target, consistent with earlier results from calibration studies of the SAGE and GALLEX/GNO solar neutrino experiments. This discrepancy between expected vs. observed neutrino captures is known as the gallium anomaly, which some have cited as possible evidence for new neutrino physics. To predict the expected capture rate for a given incident neutrino flux, one must know the neutrino capture cross section on ⁷¹Ga, a common systematic of all five gallium experiments. The original BEST analysis used a cross section value computed by Bahcall in 1997. Here, we update the gallium neutrino absorption cross section. To establish both a central value and reasonable uncertainty, we consider a variety of electroweak corrections as well as contributions from transitions to excited states. In the latter case, we carefully extract Gamow-Teller operator strengths from (p,n) and (³He, t) scattering data. These various corrections result in a central value for the neutrino absorption cross section that is reduced by 2% from the value employed in the original BEST analysis. As a result, the significance of the gallium anomaly is slightly reduced, but still stands at roughly 4 standard deviations.   

MAGNETO-v: Search for keV-sterile neutrino using magnetic quantum sensors

Sterile neutrinos with a mass in an order of keV is one of warm dark matter candidates. The mixing between sterile and ordinary neutrinos can lead to measurable distortion in the energy spectra of beta minus decays. The MAGNETO-v experiment aims to detect these keV sterile neutrinos by accurately measuring the beta decay spectrum of Pu-241. By utilizing magnetic quantum sensors with an energy resolution of approximately 100 eV, the experiment captures a significant portion of the decay radiation and precisely measures the energy spectrum consisting of more than one million decays. This talk presents experimental details and preliminary results from the first demonstration experiment performed at the Lawrence Livermore national laboratory.   

Implicit representation modeling neutrino event topologies in water-Cherenkov detectors

Neutrino experiments based on water-Cherenkov detectors have made significant contributions to our understanding of neutrino physics, but they face challenges in accurately modeling detector systematic parameters due to their large size necessary to overcome the smallness of the cross-section for weak interactions. While these experiments have achieved remarkable successes in the past, the future era of precision neutrino physics demands innovative techniques to better comprehend detector systematic uncertainties.

In this talk, I will present modern ideas to represent neutrino event topologies in preparation for the Hyper-Kamiokande experiment, a next-generation underground water-Cherenkov detector, poised to begin construction in the near future and serving as a far detector, positioned 295 km away, for a long baseline neutrino experiment utilizing the upgraded J-PARC beam in Japan. Moreover, it will have the capability to search for proton decay, atmospheric neutrinos, and neutrinos from astronomical sources with unprecedented sensitivity compared to its predecessor, Super-Kamiokande.

To address the challenges of modeling systematic uncertainties, I will explore the application of view rendering techniques, including Neural Radiance Field (NeRF), which can implicitly encode detector parameters such as water attenuation length and scattering. Furthermore, these techniques can be employed in event reconstruction, generating new events to enhance the understanding of systematic uncertainties. The ongoing work presented here aims to achieve computationally efficient and comprehensive treatment of systematic uncertainties by accelerating simulations and event reconstruction, enabling the variation of detector parameters for large water-Cherenkov detectors.   

Search for extraterrestrial antineutrinos with the SNO+ detector

All antineutrinos observed to date have originated on Earth, as products of cosmic ray interactions in the atmosphere or nuclear decays, both in nuclear reactors and the planetary crust. We describe a search for extraterrestrial electron antineutrinos in an energy range of 10 MeV < E < 40 MeV using the SNO+ detector, for which the relatively large overburden and distance from operating reactors offer low backgrounds in a scintillator detector. We further present model-independent limits on the astrophysical flux, as well limits on MeV-scale dark matter self-annihilation rates.   

Exploring Current Constraints on Antineutrino Production by 241Pu and Paths Towards the Precision Reactor Flux Era

Nuclear reactors produce thermal energy from the fission of heavy nuclides in the reactor's fuel, such as ²³⁵U, ²³⁸U, ²³⁹Pu,  ²⁴¹Pu, and ²⁴⁰Pu. Each of these nuclides has its own decay chain, which produces beta particles, gamma rays, and antineutrinos. It is well-known the existence of discrepancies between existing reactor antineutrino detector data and theoretical predictions, which creates the need for further investigation. In this context, we have combined data of several experiments based at highly ²³⁵U enriched reactor cores (HEU) with conventional low-enriched cores (LEU) and performed global fits on them. These allowed us to explore new bounds to antineutrino production on the sub-dominant fission isotope of ²⁴¹Pu, which resulted in an IBD yield of σ₂₄₁ = 8.16 ± 3.47 cm2/fission, a value (135 ± 58)% that of current beta conversion models. Furthermore, we considered hypothetical neutrino measurements at HEU, LEU, mixed-oxide, and fast reactor facilities to conjecture on their limits of future IBD yield measurements and what knowledge we hope to obtain with them. We also suggest possible arrangements to maximize our understanding and testing of the current theoretical predictions.   

Final Search for Short-Baseline Neutrino Oscillations with the PROSPECT-I Detector at HFIR

The Precision Reactor Oscillation and SPECTrum (PROSPECT) reactor antineutrino experiment is designed to detect eV-scale sterile neutrino oscillation at short baselines. PROSPECT's segmented detector is positioned approximately 7 meters away from the compact research reactor core at Oak Ridge National Laboratory's High Flux Isotope Reactor. During the data collection period, certain photomultiplier tubes (PMTs) experienced current instabilities, which resulted in the previous search for sterile neutrino oscillation being dominated by statistical uncertainties. However, by using new analysis approaches: multi-period dataset combined with single-ended event reconstruction, we successfully recovered and maximized Inverse Beta Decay (IBD) events while reducing background for each period. The talk will present the final results for sterile neutrino oscillation searches from the PROSPECT experiment using the optimized dataset.   

Final 235U Antineutrino Spectrum Analysis by PROSPECT-I

The PROSPECT experiment, known as the Precision Reactor Oscillation and SPECTrum, aims to examine the spectrum of antineutrinos emitted by the High Flux Isotope Reactor (HFIR) and investigate potential oscillations over short distances. The most recent publication by PROSPECT showcases an improved analysis, enhancing previous findings by incorporating a method called Single Ended Event Reconstruction (SEER) to utilize previously unused segments, as well as employing careful data splitting (DS) of different time periods to maximize the available data.

The utilization of SEER and DS has resulted in a significant increase in data quantity and a higher signal to background ratio, enabling PROSPECT to achieve one of the most accurate measurements of the antineutrino spectrum emitted from a purely 235U-fueled reactor. By comparing these measurements with the Huber-Mueller conversion model, a localized excess is observed within the energy range of 5 MeV to 7 MeV. These findings are consistent with observations made by other reactor experiments. The magnitude of the excess observed by PROSPECT, relative to that reported in commercial reactor experiments, provides new insights into the origin of the discrepancy between data and model.

In this presentation, we will delve into PROSPECT’s updated dataset and discuss the technique used to map the reconstructed energy spectrum to the antineutrino energy through a process called unfolding. Furthermore, we will highlight the main outcomes presented in the latest publication, emphasizing their implications for enhancing our understanding of current antineutrino spectrum models originating from nuclear reactors.   

PROSPECT-I Determination of Efficiency Uncertainty for Antineutrino Flux

The Precision Reactor Oscillation and SPECTrum (PROSPECT) experiment is a short-baseline reactor experiment designed to measure the spectrum of antineutrinos and search for evidence of short baseline sterile neutrino oscillations. From 2018 to 2019, the first-generation detector, PROSPECT-I, took data while located roughly 7 m from the High Flux Isotope Reactor (HFIR), an 85 MW, compact core, highly enriched research reactor at Oak Ridge National Laboratory (ORNL). PROSPECT-I was a segmented ⁶Li-loaded liquid scintillator detector which detected neutrinos via inverse beta decay (IBD) events, by detecting the scintillation light of the IBD positron and other particles produced when the IBD neutron captured on the ⁶Li. PROSPECT-I has demonstrated the highest signal-to-background ratio of any surface antineutrino detector with minimal overburden, placing stringent limits on eV scale sterile neutrino oscillations, setting new direct limits on boosted dark matter models, and providing one of the most precise measurements to date of the ²³⁵U antineutrino spectrum. This talk will present ongoing work by the PROSPECT collaboration towards making an absolute flux measurement of the reactor antineutrinos, and detail ongoing work on measuring the uncertainties in the detection efficiency.   

Trade-off between statistics and systematics for a PROSPECT-I absolute flux measurement

The Precision Reactor Oscillation and Spectrum Experiment (PROSPECT) is a short-baseline reactor antineutrino experiment designed to perform a sterile neutrino oscillation search and make a precise measurement of the neutrino energy spectrum from a compact reactor core, located at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. In its first phase, PROSPECT demonstrated a superior signal-to-background while operating with minimal overburden, motivating an absolute reactor antineutrino flux measurement at the few percent level that has potential impacts on understanding the reactor antineutrino anomaly and the global flux picture, as well as reactor monitoring with neutrino detectors. This talk will present experimental progress in selecting the optimal fiducialization and run time of PROSPECT data for the flux analysis in order to maximize available statistics from HFIR while minimizing systematic error to achieve a statistical uncertainty below 1%. It will illustrate the statistical trade-offs associated with neutron mobility and leakage into dead detector material. Experimental and analytical procedures used to calculate and minimize the uncertainty on HFIR’s thermal power will also be addressed.   

Tracing Antineutrino Direction with PROSPECT

PROSPECT, the Precision Reactor Oscillation and SPECTrum Experiment, is a short-baseline reactor-based neutrino experiment, examining the spectrum of antineutrinos emitted by the High Flux Isotope Reactor. Antineutrinos are detected through Inverse Beta Decay, a nuclear reaction which occurs when an electron antineutrino scatters off of a proton to produce a positron and neutron. The lithium doped liquid scintillator and optically isolated segments allow for precise event localization. As the neutron retains the direction of the antineutrino on average, PROSPECT is in a unique position to reconstruct the direction of incoming reactor antineutrinos.Directionality results from an analysis of PROSPECT data are compared to Monte Carlo simulations. This talk will present a comparison of the measured average antineutrino direction to the known reactor position, and methods used to cross-check the results using background radioactivity.