Rare-event search experiments located on-surface, such as short-baseline reactor neutrino experiments, are often limited by muon-induced background events. Highly efficient muon vetos are essential to reduce the detector background and to reach the sensitivity goals. We demonstrate the feasibility of deploying organic plastic scintillators at sub-Kelvin temperatures. For the NUCLEUS experiment, we developed a cryogenic muon veto equipped with wavelength shifting fibers and a silicon photo multiplier operating inside a dilution refrigerator. The achievable compactness of cryostat-internal integration is a key factor in keeping the muon rate to a minimum while maximizing coverage. The thermal and light output properties of a plastic scintillation detector were examined. We report first data on the thermal conductivity and heat capacity of the polystyrene-based scintillator UPS-923A over a wide range of temperatures extending below one Kelvin. The light output was measured down to 0.8 K and observed to increase by a factor of 1.61 ± 0.05 compared to 300 K. The development of an organic plastic scintillation muon veto operating in sub-Kelvin temperature environments opens new perspectives for rare-event searches with cryogenic detectors at sites lacking substantial overburden.

The precise modeling of the de-excitation of Gd isotopes is of great interest for experimental studies of neutrinos using Gd-loaded organic liquid scintillators. The FIFRELIN code was recently used within the purposes of the STEREO experiment for the modeling of the Gd de-excitation after neutron capture in order to achieve a good control of the detection efficiency. In this work, we report on the recent additions in the FIFRELIN de-excitation model with the purpose of enhancing further the de-excitation description. Experimental transition intensities from the EGAF database are now included in the FIFRELIN cascades, in order to improve the description of the higher energy part of the spectrum. Furthermore, the angular correlations between γ rays are now implemented in FIFRELIN, to account for the relative anisotropies between them. In addition, conversion electrons are now treated more precisely in the whole spectrum range, while the subsequent emission of X rays is also accounted for. The impact of the aforementioned improvements in FIFRELIN is tested by simulating neutron captures in various positions inside the STEREO detector. A repository of up-to-date FIFRELIN simulations of the Gd isotopes is made available for the community, with the possibility of expanding for other isotopes which can be suitable for different applications.

We investigate the possible origins of the reactor antineutrino anomalies in norm and shape within the framework of a summation model where β^{-} transitions are simulated by a phenomenological model of Gamow-Teller decay strength. The general trends of divergence from the Huber-Mueller model on the antineutrino side can be reproduced in both norm and shape. From the exact electron-antineutrino correspondence of the summation model, we predict similar distortions in the electron spectra, suggesting that biases on the reference spectra of fission electrons could be the cause of the anomalies.

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) is a process in which neutrinos scatter on a nucleus which acts as a single particle. Though the total cross section is large by neutrino standards, CE$\nu$NS has long proven difficult to detect, since the deposited energy into the nucleus is $\sim$ keV. In 2017, the COHERENT collaboration announced the detection of CE$\nu$NS using a stopped-pion source with CsI detectors, followed up the detection of CE$\nu$NS using an Ar target. The detection of CE$\nu$NS has spawned a flurry of activities in high-energy physics, inspiring new constraints on beyond the Standard Model (BSM) physics, and new experimental methods. The CE$\nu$NS process has important implications for not only high-energy physics, but also astrophysics, nuclear physics, and beyond. This whitepaper discusses the scientific importance of CE$\nu$NS, highlighting how present experiments such as COHERENT are informing theory, and also how future experiments will provide a wealth of information across the aforementioned fields of physics.

Coherent neutrino-nucleus scattering is a promising new tool in the toolbox of electroweak precision measurements at low q-transfer. It will enable precise measurements of standard model (SM) physics like the running of the Weinberg angle but also the search for new physics beyond the SM like sterile neutrinos. The Nucleus experiment aims at the first detection of fully coherent neutrino-nucleus scattering at the Chooz power plant in France, using its two 4GWth reactor cores as high-intensity source for anti-neutrinos. For this endeavour a new experimental site, the Very Near Site (VNS), with a shallow rock overburden of $$\approx {3}$$ m w.e. is under development. To be competitive in this challenging environment, Nucleus developed the novel concept of fiducialised cryogenic bolometers based on CaWO $$_4$$ monocrystals operated at $$\mathscr {O}$$ (10 mK). The signature of a coherent neutrino-nucleus scattering is a nuclear recoil at the 10 eV-scale. Currently, Nucleus is preparing its first phase with 10 g of target mass at the VNS. In this contribution, we will first introduce Nucleus, report its current state and give an outlook to its future.

The NUCLEUS experiment aims at measuring the coherent elastic scattering of nuclear reactor antineutrinos off nuclei using cryogenic calorimeters. Operating at an overburden of 3 m.w.e., muon-induced backgrounds are expected to be dominant. It is therefore essential to develop an efficient muon veto, with a detection efficiency of more than 99 %. This will be realized in NUCLEUS through a compact cube assembly of plastic scintillator panels. In order to prevent a large unshielded area where the cryostat intersects the shielding arrangement without unnecessarily increasing the induced detector dead time, a novel concept has been investigated, featuring a plastic scintillator-based active muon veto operating inside the NUCLEUS cryostat at sub-Kelvin temperatures. The verification of the key physical aspects of this cryogenic muon veto detector led to the first reported measurements of organic plastic scintillators at sub-Kelvin temperatures. The functionality of the principal scintillation process of organic plastic scintillators at these temperatures has been confirmed. On the basis of these findings, a disk-shape plastic scintillator equipped with wavelength shifting fibers and a silicon photomultiplier to guide and detect the scintillation light has been developed. The NUCLEUS cryogenic muon veto will be the first of its kind to be operated at sub-Kelvin temperatures.

NUCLEUS is an experiment conceived for the detection of the coherent elastic neutrino nucleus scattering. The experiment will be carried out in the nuclear power plant of Chooz-B in France that provides an intense flux of anti-neutrinos. The aim of NUCLEUS is to perform a very precise measurement of the coherent elastic scattering below 100 eV by employing extremely sensitive cryogenic detectors based on CaWO$_4$ and Al$_2$O$_3$ target crystals. At present, the NUCLEUS apparatus is in commissioning phase at the Underground Laboratory of the Technical University Munich, where - after its installation and the first measurement phase - the experiment will be fully tested. Thereafter the entire apparatus will be moved to the reactor site at Chooz.

S.P. Wells, T. Averett, D. Barkhuff, D.H. Beck, E.J. Beise, C. Benson, H. Breuer, R. Carr, S. Covrig, J. DelCorso, G. Dodson, C. Eppstein, M. Farkhondeh, B.W. Filippone, T. Forest, P. Frasier, R. Hasty, T.M. Ito, C. Jones, W. Korsch, S. Kowalski, P. Lee, E. Maneva, K. McCarty, R.D. McKeown, J. Mikell, B. Mueller, P. Naik, M.L. Pitt, J. Ritter, V. Savu, D. T. Spayde, M. Sullivan, R. Tieulent, E. Tsentalovich, B. Yang, and T. Zwart Center for Applied Physics Studies, Louisiana Tech University, Ruston, LA 71272, USA Department of Physics, College of William and Mary, Williamsburg, V A 23187, USA Bates Linear Accelerator Center, Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA University of Illinois at Urbana− Champaign, Urbana, IL 61801, USA University of Maryland, College Park, MD 20742, USA Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, USA Department of Physics, V irginia Polytechnic Institute and State University, Blacksburg, V A 24061, USA Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506, USA Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA (SAMPLE Collaboration)

Different extensions of the standard model of particle physics, such as braneworld or mirror matter models, predict the existence of a neutron sterile state, possibly as a dark matter candidate. This Letter reports a new experimental constraint on the probability p for neutron conversion into a hidden neutron, set by the STEREO experiment at the high flux reactor of the Institut Laue-Langevin. The limit is p<3.1×10^{-11} at 95% C.L. improving the previous limit by a factor of 13. This result demonstrates that short-baseline neutrino experiments can be used as competitive passing-through-walls neutron experiments to search for hidden neutrons.

Many low-threshold experiments observe sharply rising event rates of yet unknown origins below a few hundred eV, and larger than expected from known backgrounds. Due to the significant impact of this excess on the dark matter or neutrino sensitivity of these experiments, a collective effort has been started to share the knowledge about the individual observations. For this, the EXCESS Workshop was initiated. In its first iteration in June 2021, ten rare event search collaborations contributed to this initiative via talks and discussions. The contributing collaborations were CONNIE, CRESST, DAMIC, EDELWEISS, MINER, NEWS-G, NUCLEUS, RICOCHET, SENSEI and SuperCDMS. They presented data about their observed energy spectra and known backgrounds together with details about the respective measurements. In this paper, we summarize the presented information and give a comprehensive overview of the similarities and differences between the distinct measurements. The provided data is furthermore publicly available on the workshop's data repository together with a plotting tool for visualization.

The PROSPECT and STEREO collaborations present a combined measurement of the pure ^{235}U antineutrino spectrum, without site specific corrections or detector-dependent effects. The spectral measurements of the two highest precision experiments at research reactors are found to be compatible with χ^{2}/ndf=24.1/21, allowing a joint unfolding of the prompt energy measurements into antineutrino energy. This ν[over ¯]_{e} energy spectrum is provided to the community, and an excess of events relative to the Huber model is found in the 5-6 MeV region. When a Gaussian bump is fitted to the excess, the data-model χ^{2} value is improved, corresponding to a 2.4σ significance.

The Nucleus experiment aims to measure coherent elastic neutrino nucleus scattering of reactor anti-neutrinos using cryogenic calorimeters. Operating at an overburden of 3 meters of water equivalent, muon-induced backgrounds are expected to be one of the dominant background contributions. Besides a high efficiency to identify muon events passing the experimental setup, the Nucleus muon veto has to fulfill tight spatial requirements to fit the constraints given by the experimental site and to minimize the induced detector dead-time. We developed highly efficient and compact muon veto modules based on plastic scintillators equipped with wavelength shifting fibers and silicon photo multipliers to collect and detect the scintillation light. In this paper, we present the full characterization of a prototype module with different light read-out configurations. We conclude that an efficient and compact muon veto system can be built for the Nucleus experiment from a cube assembly of the developed modules. Simulations show that an efficiency for muon identification of >99 % and an associated rate of 325 Hz is achievable, matching the requirements of the Nucleus experiment.

Coherent elastic neutrino-nucleus scattering (CEvNS) offers a unique way to study neutrino properties and to search for new physics beyond the Standard Model. The NUCLEUS experiment aims at measuring the CEvNS signal from reactor antineutrinos using newly developed cryogenic detectors with ultra-low recoil energy threshold. The experiment is currently under construction for a blank assembly and later planned to be installed in between the two 4.25 GW(th) nuclear reactors of the Chooz B power plant in the French Ardennes. This proceeding presents an overview of the first phase of the NUCLEUS experiment. A general description of the experimental apparatus and its expected sensitivity to the CEvNS is provided.

This article reports the measurement of the $^{235}$U-induced antineutrino spectrum shape by the STEREO experiment. 43'000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure $^{235}$U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around $E_\nu \simeq 5.3$ MeV. A gaussian fit of this local excess leads to an amplitude of $A = 12.1 \pm 3.4\%$ (3.5$\sigma$).

Coherent elastic neutrino-nucleus scattering (CEνNS) offers a unique way to study neutrino properties and to search for new physics beyond the Standard Model. The NUCLEUS experiment aims to measure CEνNS of reactor anti-neutrinos down to unprecedented low nuclear recoil energies. The novel gram-scale cryogenic detectors feature an ultra-low energy threshold of ≤20eVnr and a rise time of a few 100 μs which allows the operation above ground. The fiducialization of the detectors provides an effective discrimination of ambient γ- and surface backgrounds. Furthermore, the use of multiple targets promises a high physics potential. The NUCLEUS experiment will be located at a new experimental site at the Chooz nuclear power plant in France, providing a high anti-neutrino flux of 1.7⋅1012ν¯e/(s⋅cm2) . The commissioning of the experimental setup with a comprehensive background measurement is planned for 2022.

We report a measurement of the antineutrino rate from the fission of ^{235}U with the STEREO detector using 119 days of reactor turned on. In our analysis, we perform several detailed corrections and achieve the most precise single measurement at reactors with highly enriched ^{235}U fuel. We measure an IBD cross section per fission of σ_{f}=(6.34±0.06[stat]±0.15[sys]±0.15[model])×10^{-43}  cm^{2}/fission and observe a rate deficit of (5.2±0.8[stat]±2.3[sys]±2.3[model])% compared to the model, consistent with the deficit of the world average. Testing ^{235}U as the sole source of the deficit, we find a tension between the results of lowly and highly enriched ^{235}U fuel of 2.1 standard deviations.

We comment on the claimed observation [arXiv:arXiv:2005.05301] of sterile neutrino oscillations by the Neutrino-4 collaboration. Such a claim, which requires the existence of a new fundamental particle, demands a level of rigor commensurate with its impact. The burden lies with the Neutrino-4 collaboration to provide the information necessary to prove the validity of their claim to the community. In this note, we describe aspects of both the data and analysis method that might lead to an oscillation signature arising from a null experiment and describe additional information needed from the Neutrino-4 collaboration to support the oscillation claim. Additionally, as opposed to the assertion made by the Neutrino-4 collaboration, we also show that the method of 'coherent summation' using the $L/E$ parameter produces similar results to the methods used by the PROSPECT and the STEREO collaborations.

The STEREO experiment is a very short baseline reactor antineutrino experiment. It is designed to test the hypothesis of light sterile neutrinos being the cause of a deficit of the observed antineutrino interaction rate at short baselines with respect to the predicted rate, known as the reactor antineutrino anomaly. The STEREO experiment measures the antineutrino energy spectrum in six identical detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this article, results from 179 days of reactor turned on and 235 days of reactor turned off are reported at a high degree of detail. The current results include improvements in the modelling of detector optical properties and the gamma-cascade after neutron captures by gadolinium, the treatment of backgrounds, and the statistical method of the oscillation analysis. Using a direct comparison between antineutrino spectra of all cells, largely independent of any flux prediction, we find the data compatible with the null oscillation hypothesis. The best-fit point of the reactor antineutrino anomaly is rejected at more than 99.9% C.L.

The STEREO experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for the comparison with data, in particular in the case of the detection efficiency. Among all stable isotopes, 155Gd and 157Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8MeV. The complex level schemes of 156Gd and 158Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd (n,$ \gamma$) cascades obtained using the FIFRELIN code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the STEREO experiment. A database of ten millions of deexcitation cascades for each isotope has been generated and is now available for the user.

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