arXiv subjects

L. Peters

Publications and source records attributed to L. Peters.

At least 19 recordsLinked to original sources

arXiv2026

Development and Commissioning of the Cryogenic Target Detectors for the Technical Run of the NUCLEUS Experiment

The NUCLEUS experiment aims to study coherent elastic neutrino-nucleus scattering (CE$\nu$NS) of reactor electron antineutrinos at the Chooz nuclear power plant in France. In this work, the cryogenic target-detector module for the NUCLEUS Technical Run was developed and commissioned at the Technical University of Munich. The module comprises four gram-scale CaWO$_4$ detectors, each equipped with two Transition Edge Sensors (TES), providing a total target mass of 6.96 g. We present the design, integration, detector characterization, and X-ray-based energy calibration of these detectors. Across six characterized detectors, a mean baseline resolution of $\overline{\sigma}_{\mathrm{BL}} = (2.79 \pm 0.60)\,\mathrm{eV}$ was achieved. The best-performing detector reached $\sigma_{\mathrm{BL}} = (2.16 \pm 0.02_{\mathrm{stat}})\,\mathrm{eV}$, surpassing the design goal by a factor of two and representing a state-of-the-art result for a cryogenic CaWO$_4$ detector. The target detector module was successfully operated simultaneously with the surrounding Cryogenic Outer Veto, which consists of six kg-scale Ge detectors for background discrimination, showing no measurable cross-talk. These results demonstrate the readiness of the target-detector system for the NUCLEUS Technical Run at Chooz and mark a key milestone in the development of cryogenic detectors for reactor-CE$\nu$NS measurements.

physics.ins-det↗

arXiv2026

Sensitivity enhancement techniques for cryogenic calorimeters in the NUCLEUS experiment

Phonon-mediated cryogenic calorimeters find application in rare event searches due to their intrinsically low energy threshold. Achieving the best sensitivity for this kind of detectors is crucial for signal identification, leading to various optimization techniques. In this work, we present two complementary methods to increase the sensitivity of cryogenic detectors read out by transition-edge sensors, developed and tested in the context of the NUCLEUS experiment. The first procedure maps the signal-to-noise ratio of the device across a wide range of operating points, to identify the configuration with maximal sensitivity to be used during data taking. The second method exploits the double readout of the detector, combining the information on different channels with a two-dimensional optimum filter analysis that effectively lowers the energy threshold. With both techniques at the same time, we obtained a baseline resolution of 2.94 $\pm$ 0.05 (stat) eV using a CaWO4 based detector, achieving a promising result in view of the first run of NUCLEUS at the experimental site.

physics.ins-det↗

arXiv2026

Prospect of the NUCLEUS Experiment at Chooz for Coherent Elastic Neutrino-Nucleus Scattering and New Physics Searches

The NUCLEUS experiment aims to measure coherent elastic neutrino-nucleus scattering (CE$\nu$NS) at unprecedentedly low nuclear recoil energies using gram-scale cryogenic calorimeters operated at the Chooz nuclear power plant in France. Access to recoil energies at the $\mathcal{O}(10~\mathrm{eV})$ scale enables CE$\nu$NS studies at extremely low momentum transfer and provides enhanced sensitivity to new physics. In this work, we present sensitivity projections for the upcoming NUCLEUS technical and physics runs, incorporating a data-driven treatment of the low-energy excess (LEE) observed during commissioning. We develop a likelihood framework that exploits reactor-power variation to disentangle signal and background in a low signal-to-background regime and to assess the impact of the dominant systematic uncertainties. For the Technical Run with a 7 g CaWO$_4$ target, we find competitive sensitivity to several scenarios beyond the Standard Model, which do not require a CE$\nu$NS observation. For the Physics Run, assuming complete suppression of the LEE, we project a 4.7 $\sigma$ observation of CE$\nu$NS with a statistical precision of about 20 % in 1 year, enabling a determination of the weak mixing angle at the lowest momentum transfer probed to date with CE$\nu$NS and leading CE$\nu$NS-based constraints on the neutrino charge radius and new mediator models.

hep-ex↗

arXiv2026

Characterization of the Low Energy Excess using a NUCLEUS Al$_{2}$O$_{3}$ detector

The NUCLEUS experiment aims to detect coherent elastic neutrino-nucleus scattering of reactor antineutrinos using low-threshold, gram-scale cryogenic calorimeters. Similar to other low-threshold experiments, NUCLEUS observes a sharp rise in the event rate below a few hundred eV, referred to as the low energy excess (LEE), whose origin remains yet unidentified. Building on results from the NUCLEUS testing and commissioning at the Technical University of Munich and from previous characterization campaigns, we present a comprehensive study of the background rate measured with a sapphire detector equipped with two transition-edge sensors under various experimental conditions. We find no evidence for a dependence of the LEE rate on the particle background level, whereas the results indicate that slower cooling-down procedures lead to lower initial LEE rates. The behavior of the LEE rate during the same cooldown is comparable across the measurements and is best described by a power law with a common exponent across datasets of $(-0.59 \pm 0.06)$, when time is expressed from the moment the detector reaches the 4 K temperature. These findings provide valuable guidance for future LEE mitigation strategies in the NUCLEUS experiment.

physics.ins-det↗

arXiv2025

Observation of two nuclear recoil peaks induced by neutron capture on Al2O3

We report the observation of two nuclear recoil peaks induced by neutron capture on aluminum in a cryogenic Al$_2$O$_3$ detector developed by the NUCLEUS collaboration for the detection of reactor neutrinos via coherent elastic neutrino-nucleus (CEvNS) process. Data collected at the Technical University of Munich in 2024 with a $^{252}$Cf source reveal a main recoil line at 1145 eV from single-$\gamma$ de-excitation of $^{28}$Al and a newly observed structure near 575 eV originating from several two-$\gamma$ cascades. The latter constitutes the first direct measurement of a nuclear recoil line induced by multi-$\gamma$ cascades. It is predicted by our simulations when the recoiling nucleus has time to stop before the emission of the next $\gamma$-ray in the cascade. These results demonstrate the potential performance of the CRAB (Calibration Recoil for Accurate Bolometry) method for in situ nuclear recoil calibration and highlight the importance of accurately modeling recoil stopping and nuclear de-excitation times in cryogenic detectors of CEvNS and dark matter interactions.

physics.ins-det↗

arXiv2025

Particle background characterization and prediction for the NUCLEUS reactor CE$\nu$NS experiment

NUCLEUS is a cryogenic detection experiment which aims to measure Coherent Elastic Neutrino-Nucleus Scattering (CE$\nu$NS) and to search for new physics at the Chooz nuclear power plant in France. This article reports on the prediction of particle-induced backgrounds, especially focusing on the sub-keV energy range, which is a poorly known region where most of the CE$\nu$NS signal from reactor antineutrinos is expected. Together with measurements of the environmental background radiations at the experimental site, extensive Monte Carlo simulations based on the Geant4 package were run both to optimize the experimental setup for background reduction and to estimate the residual rates arising from different contributions such as cosmic ray-induced radiations, environmental gammas and material radioactivity. The NUCLEUS experimental setup is predicted to achieve a total rejection power of more than two orders of magnitude, leaving a residual background component which is strongly dominated by cosmic ray-induced neutrons. In the CE$\nu$NS signal region of interest between 10 and 100 eV, a total particle background rate of $\sim$ 250 d$^{-1}$kg$^{-1}$keV$^{-1}$ is expected in the CaWO$_4$ target detectors. This corresponds to a signal-to-background ratio $\gtrsim$ 1, and therefore meets the required specifications in terms of particle background rejection for the detection of reactor antineutrinos through CE$\nu$NS.

physics.ins-det↗

arXiv2025

Commissioning of the NUCLEUS Experiment at the Technical University of Munich

The NUCLEUS experiment aims to detect coherent elastic neutrino-nucleus scattering of reactor antineutrinos on CaWO$_4$ targets in the fully coherent regime, using gram-scale cryogenic calorimeters. The experimental apparatus will be installed at the Chooz nuclear power plant in France, in the vicinity of two 4.25 GW$_{\text{th}}$ reactor cores. This work presents results from the commissioning of an essential version of the experiment at the shallow Underground Laboratory of the Technical University of Munich. For the first time, two cryogenic target detectors were tested alongside active and passive shielding systems. Over a period of two months all detector subsystems were operated with stable performance. Background measurements were conducted, providing important benchmarks for the modeling of background sources at the reactor site. Finally, we present ongoing efforts to upgrade the detector systems in preparation for a technical run at Chooz in 2026, and highlight the remaining challenges to achieving neutrino detection.

hep-ex↗

arXiv2025

Sub-keV Electron Recoil Calibration for Macroscopic Cryogenic Calorimeters using a Novel X-ray Fluorescence Source

Percent-level calibration of cryogenic macro-calorimeters with energy thresholds below 100~eV are crucial for light Dark Matter (DM) searches and reactor neutrino studies based on coherent elastic neutrino-nucleus scattering (CEvNS). This paper presents a novel calibration source based on X-ray fluorescence (XRF) of light elements. It uses a $^{55}$Fe source to irradiate a two-staged target arrangement, emitting characteristic emission lines from 677\,eV to 6.5\,keV. We demonstrate the potential of this new XRF source to calibrate a 0.75 gram CaWO$_4$ crystal of the NUCLEUS and CRAB experiments. Additionally, we introduce CryoLab, an advanced analysis tool for cryogenic detector data, featuring robust methods for data processing, calibration, and high-level analysis, implemented in MATLAB and HDF5. We also present a phenomenological model for energy resolution, which incorporates statistical contributions, systematic effects, and baseline noise, enabling a novel approach to evaluating athermal phonon collection efficiency in macro-calorimeters based on transition edge sensors (TES).

physics.ins-det↗

arXiv2025

The CRAB facility at the TU Wien TRIGA reactor: status and related physics program

The CRAB (Calibrated nuclear Recoils for Accurate Bolometry) project aims to precisely characterize the response of cryogenic detectors to sub-keV nuclear recoils of direct interest for coherent neutrino-nucleus scattering and dark matter search experiments. The CRAB method relies on the radiative capture of thermal neutrons in the target detector, resulting in a nuclear recoil with a well-defined energy. We present a new experimental setup installed at the TRIGA Mark-II reactor at Atominstitut (Vienna), providing a low intensity beam of thermal neutrons sent to the target cryogenic detector mounted inside a wet dilution refrigerator Kelvinox 100. A crown of BaF$_2$ detectors installed outside the dewar enables coincident detection of the high-energy $\gamma$ escaping the target crystal after neutron capture. After the presentation of all components of the setup we report the analysis of first commissioning data with a CaWO$_4$ detector of the \NUCLEUS experiment. They show stable operation of the cryostat and detectors on a week-scale. Due to an energy resolution currently limited to 20 eV we use neutron beam induced events at high energy, in the 10 to 100 keV range, to demonstrate the excellent agreement between the data and simulation and the accurate understanding of external background. Thanks to these data we also propose an updated decay scheme of the low-lying excited states of $^{187}$W. Finally, we present the first evidence of neutron-capture induced coincidences between $\gamma$-detectors and a cryogenic detector. These promising results pave the way for an extensive physics program with various detector materials, like CaWO$_4$, Al$_2$O$_3$, Ge and Si.

physics.ins-det↗

arXiv2025

Decoupling Pulse Tube Vibrations from a Dry Dilution Refrigerator at milli-Kelvin Temperatures

With the rising adoption of dry dilution refrigerators across scientific and industrial domains, there has been a pressing demand for highly efficient vibration decoupling systems capable of operation at cryogenic temperatures in order to achieve the low vibration levels required for operation of sensitive equipment like cryogenic detectors or quantum devices. As part of the NUCLEUS experiment, a cryogenic spring pendulum has been engineered to effectively isolate pulse tube vibrations by establishing an autonomous frame of reference for the experimental volume, while sustaining temperatures below 10 mK. Attaining attenuation of up to two orders of magnitude within the region of interest of the NUCLEUS cryogenic detectors, we achieved displacement RMS values in the order of 1 nm in the axial direction and 100 pm radially, thereby reducing vibrations below typical environmental levels. Our successful detector operation across multiple cooldown cycles demonstrated negligible sensitivity to pulse tube induced vibrations, culminating in the achievement of an ultra-low $(6.22 \pm 0.07)$ eV baseline resolution on a gram-scale CaWO$_4$ cryogenic calorimeter during continuous pulse tube operation over the course of several weeks.

physics.ins-det↗

arXiv2023

A Plastic Scintillation Muon Veto for Sub-Kelvin Temperatures

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 0.8K and observed to increase by a factor of 1.61$\pm$0.05 compared to 300K. 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.

physics.ins-det↗

arXiv2023

Study of collision and $\gamma$-cascade times following neutron-capture processes in cryogenic detectors

The emission of $\gamma$-rays after a neutron capture in a cryogenic detector can generate mono-energetic nuclear recoils in the sub-keV regime, of direct interest for the calibration of Dark Matter and Coherent Elastic Neutrino Nucleus Scattering experiments. Here we show that accurate predictions of the nuclear recoil spectra induced by neutron captures require taking into account the interplay between the development in time of the de-excitation $\gamma$-cascade of the target nucleus and that of the associated atomic collisions in matter. We present detailed simulations coupling the FIFRELIN code for the description of the $\gamma$-cascades and the IRADINA code for the modelling of the fast atomic movements in matter. Nuclear recoil spectra are predicted, and made available to the community, for concrete cases of Al$_2$O$_3$, Si, Ge and CaWO$_4$ crystals exposed to a low intensity beam of thermal neutrons. We find that timing effects cause new calibration peaks to emerge in the recoil spectra and also impact the shape of the continuous recoil distribution. We discuss how they could give access to a rich physics program, spanning the accurate study of the response of cryogenic detectors in the sub-keV range, tests of solid state physics simulations and tests of nuclear models.

physics.ins-det↗

arXiv2022

Exploring coherent elastic neutrino-nucleus scattering of reactor neutrinos with the NUCLEUS experiment

The NUCLEUS experiment aims to perform a high-precision measurement of Coherent Elastic Neutrino-Nucleus Scattering (CEvNS) at the EdF Chooz B nuclear power plant in France. CEvNS is a unique process to study neutrino properties and to search for physics beyond the Standard Model. The study of CEvNS is also important for light Dark-Matter searches. It could be a possible irreducible background for high-sensitivity Dark-Matter searches. NUCLEUS is an experiment under construction based on ultra-low threshold (20 eVnr) cryogenic calorimeters, operated at tens-of-mK temperatures.

physics.ins-det↗

arXiv2022

Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture

Coherent elastic neutrino-nucleus scattering and low-mass Dark Matter detectors rely crucially on the understanding of their response to nuclear recoils. We report the first observation of a nuclear recoil peak at around 112 eV induced by neutron capture. The measurement was performed with a CaWO$_4$ cryogenic detector from the NUCLEUS experiment exposed to a $^{252}$Cf source placed in a compact moderator. The measured spectrum is found in agreement with simulations and the expected peak structure from the single-$\gamma$ de-excitation of $^{183}$W is identified with 3 $\sigma$ significance. This result demonstrates a new method for precise, in-situ, and non-intrusive calibration of low-threshold experiments.

nucl-ex↗

arXiv2021

Magnetic phase diagram of HoFeO$_3$ by neutron diffraction

Neutron diffraction studies of HoFeO$_3$ single crystal were performed under external magnetic fields. The interplay between the external magnetic field, Dzyaloshinsky-Moria antisymmetric exchange and isotropic exchange interactions between Fe and Ho sublattice and inside Fe sublattice provides a rich phase diagram. As the result of the balance of exchange interactions inside crystal and external magnetic field we found 8 different magnetic phases, produced or suppressed by the field.

cond-mat.other↗

arXiv2019

Dual Boson approach with instantaneous interaction

The Dual Boson approach to strongly correlated systems generally involves a dynamic (frequency-dependent) interaction in the auxiliary impurity model. In this work, we explore the consequences of forcing this interaction to be instantaneous (frequency-independent) via the use of a self-consistency condition on the instantaneous susceptibility. The result is a substantial simplification of the impurity model, especially with an eye on realistic multiband implementations, while keeping desireable properties of the Dual Boson approach, such as the charge conservation law, intact. We show and illustrate numerically that this condition enforces the absence of phase transitions in finite systems, as should be expected from general physical considerations, and respects the Mermin-Wagner theorem. In particular, the theory does not allow the metal to insulator phase transition associated with the formation of the magnetic order in the two-dimensional system. At the same time, the metal to charge ordered phase transition is allowed, as it is not associated with the spontaneous breaking of a continuous symmetry, and is accurately captured by the introduced approach.

cond-mat.str-el↗

arXiv2018

Quantum spin fluctuations and evolution of electronic structure in cuprates

Correlation effects in CuO$_2$ layers give rise to a complicated landscape of collective excitations in high-T$_{\rm c}$ cuprates. Their description requires an accurate account for electronic fluctuations at a very broad energy range and remains a challenge for the theory. Particularly, there is no conventional explanation of the experimentally observed `resonant' antiferromagnetic mode, which is often considered to be a mediator of superconductivity. Here we model spin excitations of the hole-doped cuprates in the paramagnetic regime and show that this antiferromagnetic mode is associated with electronic transitions between anti-nodal X and Y points of the quasiparticle band that is pinned to the Fermi level. We observe that upon doping of 7-12\% the electronic spectral weight redistribution leads to the formation of a very stable quasiparticle dispersion due to strong correlation effects. The reconstruction of the Fermi surface results in a flattening of the quasiparticle band at the vicinity of the nodal ${\rm M}\Gamma/2$ point, accompanied by a high density of charge carriers. Collective excitations of electrons between the nodal ${\rm M}\Gamma/2$ and ${\rm XM}/2$ points form the additional magnetic holes state in magnetic spectrum, which protects the antiferromagnetic fluctuation. Further investigation of the evolution of spin fluctuations with the temperature and doping allowed us to observe the incipience of the antiferromagnetic ordering already in the paramagnetic regime above the transition temperature. Additionally, apart from the most intensive low-energy magnetic excitations, the magnetic spectrum reveals less intensive collective spin fluctuations that correspond to electronic processes between peaks of the single-particle spectral function.

cond-mat.str-el↗

arXiv2018

On the origin of the quasi-particle peak in Cr(001) surfaces

In the spectral density of Cr(001) surfaces a sharp resonance close to the Fermi level is observed in both experiment and theory. For the physical origin of this peak two mechanisms were proposed. A single particle dz2 surface state renormalised by electron-phonon coupling and an orbital Kondo effect due to the degenerate dxz/dyz states. Despite several experimental and theoretical investigations, the origin is still under debate. In this work we address this problem by two different approaches of the dynamical mean-field theory. First, by the spin-polarized T-matrix fluctuation exchange approximation suitable for weakly and moderately correlated systems. Second, by the non-crossing approximation derived in the limit of weak hybridization (i.e. for strongly correlated systems) capturing Kondo-like processes. By using recent continuous-time quantum Monte Carlo calculations as a benchmark, we find that the high-energy features, everything except the resonance, of the spectrum is captured within the spin-polarized T-matrix fluctuation exchange approximation. More precisely the particle-particle processes provide the main contribution. For the non-crossing approximation it appears that spin-polarized calculations suffer from spurious behavior at the Fermi level. Then, we turned to non spin-polarized calculations to avoid this unphysical behavior. By employing two plausible starting hybridization functions, it is observed that the characteristics of the resonance are crucially dependent on the starting point. It appears that only one of these starting hybridizations could result in an orbital Kondo resonance in the presence of a strong magnetic field like in the Cr(001) surface. It is for a future investigation to first resolve the unphysical behavior within the spin-polarized non-crossing approximation and then check for an orbital Kondo resonance.

cond-mat.str-el↗