EXCESS22@IDM

Saturday 16 Jul 2022, 09:00 → 21:25 Europe/Zurich

Technical University Vienna

Alexander Fuss, Felix Wagner (HEPHY Vienna), Florian Reindl (Vienna University of Technology (AT)), Margarita Kaznacheeva

Welcome to EXCESS22@IDM, a satellite workshop of the IDM 2022 conference!

This is the 3rd iteration of the EXCESS workshop. The workshop was initiated in 2021 to discuss sharply rising low energy event rates observed by various rare event search experiments. While the first two iterations discussed observations and interpretations of the excesses, this event will focus on:

• Updates from experiments (excess data, hypothesis tests, etc.)
• Individual contributions (excess interpretations, proposed tests, etc.)
• Outreach to entire dark matter community

While some talks will be by invitation, we also encourage everyone to submit an abstract with your individual work on the low energy excess signals.

The event is scheduled as a satellite meeting within the IDM conference, however, participation in the conference is no requirement to attend the workshop. Also talks by people who's work is not focused on dark matter are encouraged!

Location:

This workshop will be held in-person in Vienna, the location of the IDM 2022 conference: the Faculty of Electrical Engineering and Information Technology of the Technical University Vienna (TU Wien). On the workshop date, the path from the entrance of the building to the workshop location will be marked. For further information, see https://indico.cern.ch/event/922783/page/20348-venue

There is no organized lunch, but the main conference page does list suggestions for lunch places: https://indico.cern.ch/event/922783/page/20351-lunch

Accommodation:

Please consider the information at the main IDM website for Hotels: https://indico.cern.ch/event/922783/page/20355-hotels

Partner Hotels of the IDM conference are informed, such that bookings are already possible starting from July 15.

Important dates:

• 17. March: Abstract submission opens
• 16. May 30. May --> Abstract submission closed
• 2. June: Author notification 
• 9. July: Registration closes
• 16. July: Workshop date

Proceedings:

There will be workshop proceedings, published together with the IDM proceedings in SciPost.

Previous workshops:

EXCESS 2021

EXCESS 2022

09:00 → 12:00 Session Block 1: Morning talks

Convener: Felix Wagner (HEPHY Vienna)

09:00 Welcome

Speaker: Felix Wagner (HEPHY Vienna)

Welcome.pdf

09:15 Review on Low-Energy Excess Signals Observed in Cryogenic Rare Event Search Experiments

Review on Low-Energy Excess Signals Observed in Cryogenic Rare Event Search Experiments

Speaker: Jules GASCON

EXCESS-IDM2022-cryogenic-detectors.pdf

09:45 Review on Excess Signals Observed in CCD Detectors

Review on Excess Signals Observed in CCD Detectors

Speaker: Daniel Baxter (Fermi National Accelerator Laboratory)

ccds_071622.pdf

10:15 Coffee break

10:45 Ionization efficiency at sub-keV energies for crystals and noble liquids

We study the ionization and light yields produced by nuclear recoils at low
energies in pure crystals and noble liquids in the context of Lindhard's
integral equation, incorporating the effects of binding energy, improved
modeling of the electronic stopping, and electronic straggling. We
consider three different models for the electronic stopping power that
incorporate Coulomb repulsion effects at low energies, and Bohr electronic
stripping for high energies. Finally, we discuss possible new effects near threshold.

Speakers: Youssef Sarkis Mobarak (ICN-UNAM), Daniel Baxter (Fermi National Accelerator Laboratory)

Mobarak_Ionizing_efficiency.pdf

11:10 Background discrimination for low quanta events with NEWS-G

The New Experiment With Sphere-Gas (NEWS-G) employs spherical proportional counters (SPC) filled with gas to search for elastic scattering of WIMPs off light target nuclei, reaching sensitivities down to single ionisation electrons. The new-generation detector consists of a larger sphere of 140 cm diameter, equipped with a new multi-anode sensor, ”ACHINOS”, developed to ensure a sufficiently strong electric field at large radii while maintaining the capacity to achieve high gain. Due to the increased detector size, at low interaction energies the quantised nature of the ionisation process generates visible structure in the detector response. This can be exploited to identify and reject backgrounds with the goal to improve our sensitivity to WIMP recoils. In this talk, we will describe the background-rejection processing of low energy data from NEWS-G's S140, and the characterisation of its performance based on laser and Ar37 calibrations.

Speaker: Francisco Andres Vazquez de Sola

EXCESS_NEWSG_FVazquez_2022-07-16.pdf

11:35 Low-energy event excess in SuperCDMS HVeV detectors

The 1-gram silicon HVeV detectors developed within the Super Cryogenic Dark Matter Search (SuperCDMS) collaboration have achieved single-photon sensitivity and a baseline resolution of $\sigma=2.7$ eV. These cryogenic calorimeters can be operated in two configurations: (1) as phonon-assisted charge amplifiers by exploiting the Neganov-Trofimov-Luke (NTL) effect; (2) without NTL voltage (0V) by simply measuring recoil energies. The HVeV detectors observed an excess of events in the low energy region, as have several other experiments in the field. In this talk, I will present a study of the low-energy event excess through a comparison of the NTL-amplified data and the 0V data. The observed excess is consistent with a scintillation/luminescence hypothesis from the material holding the device.

Speaker: Dr Valentina Novati (Northwestern University)

20220716_excess_SuperCDMS_Novati_final.pdf

12:00 → 13:30 Lunch break

13:30 → 16:30 Session Block 2: Afternoon talks

Convener: Alexander Fuss

13:30 Latest results on the low energy excess in CRESST-III

CRESST, similarly to many other experiments, observes an unexpected excess of events at low
energies. So far no conclusive explanation of its origin was found. The current CRESST-III data
taking campaign is dedicated to study and possibly identify the nature of this Low Energy Excess
(LEE). We are operating a variety of detector modules differing in target materials, geometries and
holding structures, designed to narrow down the list of hypotheses. In this work we show the current
status of the investigations of the ongoing CRESST-III measurements, focusing on the comparison of
time, energy and temperature dependence of the LEE in several detectors. The preliminary results
strongly exclude dark matter, radioactive backgrounds and intrinsic sources related to the crystal
bulk as a major contribution.

Speaker: Dominik Raphael Fuchs (Max Planck Society (DE))

Excess_CRESST.pdf

13:55 Heat-only background in the EDELWEISS detectors with NbSi TES sensor

The EDELWEISS collaboration has extensively studied the population of events with no ionization signal ("Heat-Only" events) in its cryogenic germanium detector equipped with Ge-NTD sensors. It recently developed and operated in an low-radioactivity underground environment a new generation of cryogenic germanium detectors equipped with NbSi thin-film sensors able to detect out-of-equilibirum phonons. The population of Heat-Only events in such a device has been measured, using the Neganov-Luke-Trofimov effect to resolve it from electron recoil events. Significant differences from the population observed in detectors equipped with Ge-NTD sensors have been observed.

Speaker: Dr Hugues LATTAUD

Excess@IDM2022.pdf

14:20 Stress Induced Background in Cryogenic Crystal Calorimeters

A number of low mass dark matter direct detection experiments have observed an excess rate of events, rising sharply below energies of around 100 eV. A similar source of background energy has been observed to shorten the coherence time of superconducting quantum bits by creating excess quasiparticles in the qubit circuit. The relaxation of stress in detector materials has been shown to cause low energy backgrounds in previous dark matter experiments, and has been proposed as a source of the current "low energy excess." By comparing detectors in high and low stress states, we have shown that stressing silicon detectors can cause excess event rates of over 80 Hz/gram below 20 eV, compared to a rate of under 0.5 Hz per gram in a low stress calorimeter. Measurements of the background rate as a function of time will be described, as well as implication for the design and operation of future cryogenic low threshold calorimeters.

Speaker: Daniel Mckinsey

EXCESS 2022 Slides Romani Roger.pdf

14:45 Low energy excess in MAGNETO R&D Data

MAGNETO is developing dark matter and neutrino detectors using magnetic quantum sensors with sub-microsecond timing resolution for phonon pulse shape discrimination in eV region. Such fast timing resolution improves separation of electron recoil background, microphonic noise, and thermal noises due to events from substrates and surrounding materials. I will discuss findings from MAGNETO R&D data regarding the low energy excess phenomenon.

kim90@llnl.gov

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development program of Lawrence Livermore National Laboratory.

Speaker: Geon-Bo Kim

Kim_EXCESS_MAGNETO_v0.pdf

14:50 Dark matter detectors and understanding of glasses

Interest in detecting coherent elastic scattering of low-energy neutrinos, dark matter particles and physics beyond the standard model brings numerous experiments involving low-temperature solid-state Detectors. The excess low-energy background was found in numerous experiments, with a wide range of detector mass, and using various readout channels: luminescence, conductivity, temperature, or phonon emission spikes. Another often present effect is glass-like relaxation processes in detector materials. We argue that in systems out of equilibrium, including glasses, processes of energy accumulation and unsteady, burst-like releases could lead to excess noise and backgrounds. Moreover, there is a long-standing contradiction in understanding the role of interactions between states bearing excess energy in the description of glasses in solid-state physics and the description of systems with energy flow in chemical physics and physics of complex systems. While these interactions consider not important in glasses, in systems with energy flow interactions bring to life important physical effects. The same problem is present in understanding material sources of noise and decoherence in quantum sensors and qubits. Joint efforts to resolve these contradictions should help to increase the sensitivity of searches for rare low-energy interactions of particles and better understand material sources of decoherence which remains one o the central problem in quantum information science.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-CONF-834593-DRAFT

Speaker: sergey pereverzev (LLNL)

10_Dark_Matter_detectors_and_understanding_of_glsses_2_Sergey.pdf

15:15 Coffee break

15:40 Energy loss due to defect creation in solid state detectors

The threshold displacement energy in solid state detector materials varies from several eV to ~100 eV. If a stable or long lived defect is created as a result of a nuclear recoil event, some part of the recoil energy is stored in the deformed lattice and is therefore not observable in a phonon detector. Thus, an accurate model of this effect is necessary for precise calibration of the recoil energy measurement in low threshold phonon detectors.
Furthermore, the sharpness of the defect creation threshold varies between materials. For a hard material such as diamond, the sharp threshold will cause a sudden onset of the energy loss effect, resulting in a prominent peak in the observed recoil spectrum just below the threshold displacement energy. We describe how this effect can be used to discriminate between nuclear and electron recoils using just the measured recoil spectrum.

Speaker: Matti Heikinheimo (University of Helsinki and Helsinki Institute of Physics)

C_impact_80eV.mp4

Eloss_Excess22IDM.pdf

16:05 Impact of crystal lattice defect quenching on coherent neutrino scattering experiments

Future experiments on coherent neutrino scattering near nuclear reactors using cryogenic solid-state phonon-mediated detectors have their global signal for the energy transferred to the target nucleus in the range of a few tens to a hundred electron volts. In this range, the creation of permanent or long-lived crystal defects can have a significant effect on the reconstruction of the true energy of the incident neutrino. I will present a study of the impact for future generations of experiments (such as Nucleus-10g) and discuss a possible envelope for the associated systematic uncertainty for future searches of physics beyond the Standard Model.

Speaker: thierry lasserre

11_CDLQ-ExcessWorkshop2022-lasserre.pdf

16:30 → 17:30 Discussion