arXiv subjects

J. Hakenmüller

Publications and source records attributed to J. Hakenmüller.

At least 19 recordsLinked to original sources

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 optimal sensitivity for these kinds 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 from different channels with a two-dimensional optimum filter analysis that effectively lowers the energy threshold. By combining both techniques at the same time, we obtain a baseline resolution of $2.94\pm0.05_{\text{stat}}\pm0.74_{\text{syst}}$~eV using a CaWO$_4$ based detector, achieving a promising result ahead of the first run of NUCLEUS at the experimental site.

physics.ins-det↗

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↗

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$ν$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 $\overlineσ_{\mathrm{BL}} = (2.79 \pm 0.60)\,\mathrm{eV}$ was achieved. The best-performing detector reached $σ_{\mathrm{BL}} = (2.16 \pm 0.02_{\mathrm{stat}})\,\mathrm{eV}$, 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$ν$NS measurements.

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$ν$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$ν$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$ν$NS observation. For the Physics Run, assuming complete suppression of the LEE, we project a 4.7 $σ$ observation of CE$ν$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$ν$NS and leading CE$ν$NS-based constraints on the neutrino charge radius and new mediator models.

hep-ex↗

arXiv2026

Background decomposition of the CONUS+ run 1 data

The CONUS+ experiment is measuring the coherent elastic neutrino nucleus scattering (CE$ν$NS) process using reactor anti-neutrinos as a source and four low energy threshold point-contact high-purity germanium spectrometers for their detection. It achieved the first measurement of coherent neutrino scattering at a nuclear reactor in run 1 of the experiment with a detection energy threshold of 160 eV$_{ee}$. This work presents the decomposition of the background spectra of the three detectors used in the run 1 analysis and the development of the corresponding background model with Geant4-based Monte Carlo simulations. The background model is used as the underlying input for the likelihood fit of the analysis. It is shown that reactor-correlated backgrounds are subdominant in all energy regions, specifically in the region of interest for CE$ν$NS searches below 350 eV$_{ee}$ where their contribution is one order of magnitude below the expected CE$ν$NS signal. Furthermore, cosmic ray muons and neutrons are identified as the dominant background source below 1 keV$_{ee}$ contributing approximately 75 - 90 \% of the recorded background rate. The final background model predicts an average rate of (47.5 $\pm$ 3.2) d$^{-1}$ kg$^{-1}$ in reactor on measurement in the energy region between [0.4, 1.0] keV$_{ee}$, which is in excellent agreement with the average measured value of (48.0 $\pm$ 0.6) d$^{-1}$ kg$^{-1}$. Similar agreement is found in all energy regions of both reactor on and off measurements.

physics.ins-det↗

arXiv2026

New constraints on physics within and beyond the standard model from the latest CONUS datasets

Its detections with pion-decay-at-rest, solar and recently with reactor antineutrinos by the CONUS collaboration render coherent elastic neutrino-nucleus scattering (CE$ν$NS) an established tool for investigations within and beyond the Standard Model (SM). The CONUS experiment located at the nuclear power plants in Brokdorf (Germany) and Leibstadt (Switzerland) operates Germanium semiconductor detectors in a compact shield at close distance to the reactor core. An observation with $3.7 σ$ significance is reported at the Leibstadt site, showing good agreement with its SM prediction. Physics investigations performed with the last datasets collected at the Brokdorf reactor and with the first data obtained at the Leibstadt site are summarized. By using the experimental analysis framework, the presented results contain the full systematics that underlie the experiment. Previously determined limits with neutrino-electron scattering on the neutrino magnetic moment and a neutrino millicharge are improved to $μ_ν <5.18\cdot 10^{-11}μ_\mathrm{B}$ and $q_ν<1.76\cdot 10^{-12} e_0$ (90% C.L). Further, the scale of new physics related to NSIs is improved to $Λ_{\rm NSI}$=145 GeV and limits on the coupling of light new mediators are lowered down to $4 \cdot 10^{-7}$ (90% C.L.) with the new data. Finally, the determination of the Weinberg angle with CE$ν$NS and reactor antineutrinos yields $\sin^{2}θ_W= 0.28^{+0.03}_{-0.04}$ at a momentum transfer of $\sim 10 \ \mathrm{MeV}$.

hep-ex↗

arXiv2026

Sub-keV energy calibration of CONUS+ via 71Ge M-shell neutron activation

The CONUS+ experiment has recently reported the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) of reactor antineutrinos on germanium nuclei and is now entering a precision phase. The dominant uncertainty in the first measurement was the energy scale, which contributed 14% to the uncertainty of the prediction of the combined signal. We present a dedicated neutron activation campaign in which one of the new 2.4 kg CONUS+ germanium detectors was irradiated with a strong 241AmBe source, demonstrating that a contribution below 4% to the uncertainty of signal prediction is achievable. For the first time, the 71Ge M-shell X-ray line was clearly resolved at (158.7+-1.4) eVee, validating the CONUS+ energy reconstruction down to the detection threshold. This validation includes the understanding of the energy scale, the energy resolution, the trigger efficiency, and the correct separation of physical from noise events. These results establish the foundation for a future activation campaign at the Kernkraftwerk Leibstadt reactor site, strengthening the CONUS+ energy calibration and extending its sensitivity to precision CEvNS and beyond Standard Model physics measurements.

hep-ex↗

arXiv2026

Measurement of coherent elastic neutrino nucleus scattering on germanium by COHERENT

The COHERENT collaboration reports the most precise measurement of the coherent elastic neutrino-nucleus scattering cross section to date. This measurement was performed with COHERENT's germanium detector array, Ge-Mini, at the Spallation Neutron Source at Oak Ridge National Laboratory. A cumulative exposure of $4.68\times10^{22}$ protons on target yielded a total number of observed counts of $124^{+14}_{-12}$ and a flux-averaged cross section of $1.00 \pm 0.10 \mathrm{(statistical)} \pm 0.10 \mathrm{(systematic)}$ relative to the standard-model expectation of $5.9\times10^{-39} \mathrm{cm}^2$. The well-understood energy and timing distributions of the neutrino source allow for independent measurements of muon- and electron-neutrino scattering rates. This information is used to improve constraints on non-standard neutrino interactions mediated by heavy particles.

hep-ex↗

arXiv2026

The COHERENT Experiment: 2026 Update

The COHERENT experiment measures neutrino-induced recoils from coherent elastic neutrino-nucleus scattering (CEvNS) with multiple nuclear targets at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), USA. Several successful CEvNS measurements have been achieved in recent years with tens-of-kg detector masses, with a CsI scintillating crystal, a liquid argon single-phase detector, and high-purity germanium spectrometers. For the next phase, COHERENT aims at high-statistics detection of CEvNS events for precision tests of the standard model of particle physics, and to probe new physics beyond-the-standard model. Percent-level precision can be achieved by lowering thresholds, reducing backgrounds, and by scaling up the detector masses. It goes hand in hand with benchmarking the neutrino flux from the SNS. Further detectors will measure CEvNS in additional nuclei, including lighter target nuclei such as sodium and neon, to continue to test the expected neutron-number-squared dependence of the cross section. COHERENT can furthermore study charged-current and neutral-current inelastic neutrino-nucleus cross sections on various nuclei at neutrino energies below $\sim$50 MeV. Many of these cross sections have never been measured before, but are critical input for the interpretation of core-collapse supernova detection in large-scale neutrino experiments such as DUNE, Super-K, Hyper-K, and HALO.

hep-ex↗

arXiv2025

Evidence of Coherent Elastic Neutrino-Nucleus Scattering with COHERENT's Germanium Array

We report the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) on natural germanium, measured at the Spallation Neutron Source at Oak Ridge National Laboratory. The Ge-Mini detector of the COHERENT collaboration employs large-mass, low-noise, high-purity germanium spectrometers, enabling excellent energy resolution, and an analysis threshold of 1.5 keV electron-equivalent ionization energy. We observe a on-beam excess of 20.6 +7.1 -6.3 counts with a total exposure of 10.22 GWhkg and we reject the no-CEvNS hypothesis with 3.9 sigma significance. The result agrees with the predicted standard model of particle physics signal rate within 2 sigma.

hep-ex↗

arXiv2025

Direct observation of coherent elastic antineutrino-nucleus scattering

Neutrinos are elementary particles that interact only very weakly with matter. Neutrino experiments are, therefore, usually big, with masses in the multi-tonne range. The thresholdless interaction of coherent elastic scattering of neutrinos on atomic nuclei leads to greatly enhanced interaction rates, which allows for much smaller detectors. The study of this process gives insights into physics beyond the Standard Model of particle physics. The CONUS+ experiment was designed to first detect elastic neutrino-nucleus scattering in the fully coherent regime with low-energy neutrinos produced in nuclear reactors. For this purpose, semi-conductor detectors based on high-purity germanium crystals with extremely low-energy thresholds were developed. Here we report the first observation of a neutrino signal with a statistical significance of 3.7 sigma from the CONUS+ experiment, operated at the nuclear power plant in Leibstadt, Switzerland. In 119 days of reactor operation (395$\pm$106) neutrinos were measured compared with a predicted number from calculations assuming Standard Model physics of (347$\pm$59) events. With increased precision, there is potential for fundamental discoveries in the future. The CONUS+ results in combination with other measurements of this interaction channel might therefore mark a starting point for a new era in neutrino physics.

hep-ex↗

arXiv2025

Search for the in-situ production of $^{77}$Ge in the GERDA neutrinoless double-beta decay experiment

The beta decay of $^{77}$Ge and $^{77\mathrm{m}}$Ge, both produced by neutron capture on $^{76}$Ge, is a potential background for Germanium based neutrinoless double-beta decay search experiments such as GERDA or the LEGEND experiment. In this work we present a search for $^{77}$Ge decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of $^{77}$Ge via the isomeric state of $^{77}$As (9/2$^+$, 475 keV, ${T_{1/2} = 114}\,μ$s, $^{77\mathrm{m}}$As). New digital signal processing methods were employed to select and analyze pile-up signals. No signal was observed, and an upper limit on the production rate of was set at $<0.216$ nuc/(kg$\cdot$yr) (90% CL). This corresponds to a total production rate of $^{77}$Ge and $^{77\mathrm{m}}$Ge of $<0.38$ nuc/(kg$\cdot$ yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ $^{77}$Ge and $^{77\mathrm{m}}$Ge production of (0.21$\pm$0.07) nuc/(kg$\cdot$yr), a prediction that is now further corroborated by our experimental limit. Moreover, tagging the isomeric state of $^{77\mathrm{m}}$As can be utilised to further suppress the $^{77}$Ge background. Considering the similar experimental configurations of LEGEND-1000 and GERDA, the cosmogenic background in LEGEND-1000 at LNGS is estimated to remain at a sub-dominant level.

nucl-ex↗

arXiv2024

Final CONUS results on coherent elastic neutrino-nucleus scattering at the Brokdorf reactor

The Conus experiment studies coherent elastic neutrino-nucleus scattering in four 1 kg germanium spectrometers. Low ionization energy thresholds of 210 eV were achieved. The detectors were operated inside an optimized shield at the Brokdorf nuclear power plant which provided a reactor antineutrino flux of up to 2.3$\cdot$10$^{13}$\,cm$^{-2}$s$^{-1}$. In the final phase of data collection at this site, the constraints on the neutrino interaction rate were improved by an order of magnitude as compared to the previous Conus analysis. The new limit of less than 0.34 signal events kg$^{-1}$\,d$^{-1}$ is within a factor 2 of the rate predicted by the standard model. This constraint is discussed in the context of conflicting measurements and results from another reactor neutrino experiment using similar technology.

hep-ex↗

arXiv2024

Searches for new physics below twice the electron mass with GERDA

A search for full energy depositions from bosonic keV-scale dark matter candidates of masses between 65 keV and 1021 keV has been performed with data collected during Phase II of the GERmanium Detector Array (GERDA) experiment. Our analysis includes direct dark matter absorption as well as dark Compton scattering. With a total exposure of 105.5 kg yr, no evidence for a signal above the background has been observed. The resulting exclusion limits deduced with either Bayesian or Frequentist statistics are the most stringent direct constraints in the major part of the 140-1021 keV mass range. As an example, at a mass of 150 keV the dimensionless coupling of dark photons and axion-like particles to electrons has been constrained to $α$'/$α$ < 8.7x10$^{-24}$ and g$_{ae}$ < 3.3x10$^{-12}$ at 90% credible interval (CI), respectively. Additionally, a search for peak-like signals from beyond the Standard Model decays of nucleons and electrons is performed. We find for the inclusive decay of a single neutron in $^{76}$Ge a lower lifetime limit of $τ_n$ > 1.5x10$^{24}$ yr and for a proton $τ_p$ > 1.3x10$^{24}$ yr at 90% CI. For the electron decay e$^-\rightarrowν_eγ$ a lower limit of $τ_e$ > 5.4x10$^{25}$ yr at 90% CI has been determined.

nucl-ex↗

arXiv2024

Measurement of the Electron-Neutrino Charged-Current Cross Sections on ${}^{127}$I with the COHERENT NaI$ν$E detector

Using an 185-kg NaI[Tl] array, COHERENT has measured the inclusive electron-neutrino charged-current cross section on ${}^{127}$I with pion decay-at-rest neutrinos produced by the Spallation Neutron Source at Oak Ridge National Laboratory. Iodine is one the heaviest targets for which low-energy ($\leq$ 50 MeV) inelastic neutrino-nucleus processes have been measured, and this is the first measurement of its inclusive cross section. After a five-year detector exposure, COHERENT reports a flux-averaged cross section for electron neutrinos of $9.2^{+2.1}_{-1.8} \times 10^{-40}$ cm$^2$. This corresponds to a value that is $\sim$41% lower than predicted using the MARLEY event generator with a measured Gamow-Teller strength distribution. In addition, the observed visible spectrum from charged-current scattering on $^{127}$I has been measured between 10 and 55 MeV, and the exclusive zero-neutron and one-or-more-neutron emission cross sections are measured to be $5.2^{+3.4}_{-3.1} \times 10^{-40}$ and $2.2^{+3.5}_{-2.2} \times 10^{-40}$ cm$^2$, respectively.

nucl-ex↗

arXiv2024

Pulse shape discrimination for the CONUS experiment in the keV and sub-keV regime

Point-contact p-type high-purity germanium detectors (PPC HPGe) are particularly suited for detection of sub-keV nuclear recoils from coherent elastic scattering of neutrinos or light dark matter particles. While these particles are expected to interact homogeneously in the entire detector volume, specific classes of external background radiation preferably deposit their energy close to the semi-active detector surface, in which diffusion processes dominate that subsequently lead to slower rising pulses compared to the ones from the fully active bulk volume. Dedicated studies of their shape are therefore highly beneficial for the understanding and the rejection of these unwanted events. This article reports about the development of a data-driven pulse shape discrimination (PSD) method for the four 1 kg size PPC HPGe detectors of the CONUS experiment in the keV and sub-keV regime down to 210 eV$_{\text{ee}}$. The impact of the electronic noise at such low energies is carefully examined. It is shown that for an acceptance of 90% of the faster signal-like pulses from the bulk volume, approx. 50% of the surface events can be rejected at the energy threshold and that their contribution is fully suppressed above 800 eV$_{\text{ee}}$. Applied to the CONUS background data, such a PSD rejection cut allows to achieve an overall (15-25)% reduction of the total background budget. The new method allows to improve the sensitivity of future CONUS analyses and to refine the corresponding background model in the sub-keV energy region.

physics.ins-det↗

arXiv2023

Full background decomposition of the CONUS experiment

The CONUS experiment is searching for coherent elastic neutrino nucleus scattering of reactor anti-neutrinos with four low energy threshold point-contact high-purity germanium spectrometers. An excellent background suppression within the region of interest below 1keV (ionization energy) is absolutely necessary to enable a signal detection. The collected data also make it possible to set limits on various models regarding beyond the standard model physics. These analyses benefit as well from the low background level of ~10d$^{-1}$kg$^{-1}$below 1keV and at higher energies. The low background level is achieved by employing a compact shell-like shield, that was adapted to the most relevant background sources at the shallow depth location of the experiment: environmental gamma-radiation and muon-induced secondaries. Overall, the compact CONUS shield including the active anti-coincidence muon-veto reduces the background by more than four orders of magnitude. The remaining background is described with validated Monte Carlo simulations which include the detector response. It is the first time that a full background decomposition in germanium operated at reactor-site has been achieved. Next to remaining muon-induced background, $^{210}$Pb within the shield and cryostat end caps, cosmogenic activation and air-borne radon are the most relevant background sources. The reactor-correlated background is negligible within the shield. The validated background model together with the parameterization of the noise are used as input to the likelihood analyses of the various physics cases.

physics.ins-det↗

arXiv2023

Accessing new physics with an undoped, cryogenic CsI CEvNS detector for COHERENT at the SNS

We consider the potential for a 10-kg undoped cryogenic CsI detector operating at the Spallation Neutron Source to measure coherent elastic neutrino-nucleus scattering and its sensitivity to discover new physics beyond the standard model. Through a combination of increased event rate, lower threshold, and good timing resolution, such a detector would significantly improve on past measurements. We considered tests of several beyond-the-standard-model scenarios such as neutrino non-standard interactions and accelerator-produced dark matter. This detector's performance was also studied for relevant questions in nuclear physics and neutrino astronomy, namely the weak charge distribution of CsI nuclei and detection of neutrinos from a core-collapse supernova.

hep-ex↗