MANOIR

MANOIR – Coherent scattering of dark matter and neutrinos

The MANOIR group specialises in the detection of particles that interact extremely little with matter. A cryogenic laboratory dedicated to IP2I allows the study and development of associated cryogenic detectors, sensitive to temperature rises of less than one millionth of a Kelvin. These detectors, called bolometers, are crystals installed in cryostats at very low temperatures, only 0.01 degrees above absolute zero!

Our bolometers should make it possible to discover dark matter particles, predicted in large quantities in our galaxy by Cosmology, but never detected before, forming a link between infinitely large and infinitely small! The EDELWEISS experiment, to which we are making a major contribution, is dedicated to the search for low-mass dark matter particles.

Our group is also among the leaders of the Ricochet experiment, which tackles the difficult measurement of coherent elastic scattering between reactor neutrinos and the nuclei of the crystal lattice of bolometers. This process, predicted in 1974, was not observed until 2017. The original detectors we propose will allow a very precise measurement of this scattering, and perhaps open a door to new physics!

With our bolometers, we are also trying to answer a question that has haunted physicists since the discovery of the neutrino: is it its own antiparticle (Majorana neutrino)? In this case, a particular and very rare radioactive decay should take place; the CUPID experiment, in which we are participating, tries to detect this reaction.

Thanks to its recognized expertise in cryogenic detectors, the MANOIR group’s activities are focused on research into new physics, beyond the Standard Model, through the study of the coherent scattering of dark matter particles on the one hand, and neutrinos on the other. The two associated experiments are EDELWEISS, for the direct detection of dark matter (DM) and the Ricochet project, for the first high-precision measurement of the neutrino coherent elastic scattering process (CENNS). The group’s R&D activities, both at IP2I and at the LSM (Laboratoire Souterrain de Modane), are aimed at improving the performance of the bolometers of both experiments. The group is also involved in other activities, such as the CUPID experiment, which is researching the beta double decay process without neutrino emissions, in order to prove the Majorana nature of these particles.

Activities:

• Cryogenic detectors
• The EDELWEISS experience
• The Ricochet Experience
• Research and development at IP2I
• Associated activities: CUTE, GENTIANE, LUMINEU, CUPID

Cryogenic detectors

The EDELWEISS and Ricochet experiments are interested in very different fields of physics beyond the Standard Model, both linked by the search for very low-energy nuclear recoils induced by the coherent elastic scattering of a particle on the nuclei of a target detector, with an extremely small effective cross section. Indeed, the CENNS (coherent elastic neutrino-nucleus scattering) signal expected for reactor neutrinos is equivalent to that of a DM (dark matter) particle with a mass of the order of 2.7 GeV/c² (the mass of a proton or neutron is about 1 GeV/c²). Both experiments require the development of new-generation cryogenic bolometers to extend the excellent performance obtained in terms of identification of nuclear setbacks with those of EDELWEISS, up to energies of 100 eV (1 eV = 1.6 x 10^-19 J) and below. The work on reading electronics, the development of data analysis and monitoring and acquisition tools is carried out with a strong synergy between the two projects. The fundamental difference between the two experiments, in addition to the research discipline concerned (DM or CENNS), is the choice of experimental site: an ultra-low radioactive background environment in the deepest underground laboratory in Europe (Laboratoire Souterrain de Modane or LSM) for EDELWEISS, and the close proximity of a nuclear reactor (Institut Laué Langevin or ILL) for Ricochet. Both experiments benefit from the in-depth understanding of cryogenic germanium detector technology provided by the MANOIR group.

The EDELWEISS experiment

Scientific background

The nature of dark matter (DM) remains one of the most important issues in particle physics. Because the mass of DM particles is unknown, and the interaction cross sections are extremely small, no single experiment is capable of covering the entire associated field of research. There is growing interest in models in which the DM particle would have escaped detection because it has a mass lower than 10 GeV/c² (this is a natural consequence, for example, of asymmetric dark matter models ADM) and interacts with nucleons via a light mediator. Emphasis is now placed on developing detectors better adapted to this new mass range, for rms sections between 10^-39 and 10^-45 cm². In the absence of background noise, the required sensitivity could be obtained with an exposure of 20 kg.yr (the equivalent of 20 kg of detectors taking data for 1 year), with detectors sensitive to energy deposits of a few eV. Such a detector would also make it possible to explore other types of DM models, where DM interacts with electrons rather than nuclei.

In this context, the EDELWEISS collaboration (France, Germany, Russia) has turned towards the use of smaller detectors with improved performance. EDELWEISS has put its priority in the development of low energy discrimination performance to reject background noise, as the first essential step of a phase called EDELWEISS-SubGeV, sensitive to the mass region of DM particles between a few hundred MeV/c² and 6 GeV/c². The detectors of the experiment are installed in a radiopure underground environment, located at the Modane Underground Laboratory (LSM), and cooled to a temperature of the order of 20 mK in a dilution cryostat. Two signals are read: the total energy (calorimetric) deposited and the ionization efficiency. Their combination separates the nuclear recoils (NR) from the electronic recoils (ER), which have ionization efficiencies that differ by a factor of ~ 3 and constitute the dominant background induced by beta and gamma radioactivity.

EDELWEISS-III
In the third phase of the experiment (EDELWEISS-III), the collaboration exploited 24 germanium crystals of 860 g each, the largest set of cryogenic germanium detectors used for DM research. For each interaction, the thermal measurement (total energy or phonon) is performed with germanium sensors of the NTD-Ge (Neutron Transmutation Doped) type and the charge signal is measured using interdigitated concentric electrodes covering all surfaces and brought to alternating potentials. This unique electrode design allows EDELWEISS FID detectors to identify interactions that have taken place near the surface of the crystals, thus defining an internal volume free from problems of incomplete charge collection associated with near-surface events (important for the search for ER-type signals).

The EDELWEISS-III experiment obtained an exposure of 8 kg.yr between June 2014 and April 2015. The sensitivity to WIMPs (Weakly Interacting Massive Particles, a particular class of weakly interacting DM particles) of 7 GeV/c² was improved by 2 orders of magnitude compared to phase 2 of the experiment. EDELWEISS-III also provided competitive results on the search for more exotic DM particles, producing electronic recoils, such as ALPs (axion-like particles), in the mass range of 0.8 to 500 keV/c².

EDELWEISS was the first experiment to combine the rejection of surface events with the energy resolution and statistical accuracy needed to accurately measure the level of tritium produced by the activation of germanium exposed to cosmic radiation. This measurement helped to better define the manufacturing constraints of the germanium detectors in the competing SuperCDMS.

The members of the MANOIR group have had many responsibilities and have been deeply involved in all aspects of EDELWEISS-III: construction, simulations, radiopurity measurements, background or detector performance studies, data acquisition, monitoring, detector calibrations and data analysis. The group carried out the reconstruction and calibration of data, and all performance studies, using statistical and simulation tools, combining multivariate analysis methods of BDT (Boosted Decision Tree) type and 2D profiled likelihood functions. The group supervised most of the data analyses, was involved in 3 of the 5 theses of the period, and had a major impact on the 6 papers produced by the EDELWEISS collaboration between 2016 and 2018.

EDELWEISS-LT

After the end of phase 3, the collaboration decided to redirect its efforts towards the search for coherent WIMP-core dissemination for lighter ones. This programme started in 2015 with the EDELWEISS-LT phase (for low threshold or low energy threshold). The aim is to amplify the signal to obtain a lower energy threshold. This amplification uses the LN effect (Luke-Neganov, the equivalent of the Joule effect in a low temperature semiconductor). The drift of the charges in the bolometer will produce a heat that is added to the heat created by the collision. The amplification will be all the stronger the higher the voltage applied to the detector. The associated sensitivity has been studied by Q. Arnaud during his thesis at IP2I and is published. Experimentally, an order-of-magnitude improvement in energy thresholds was obtained by the LN effect, by applying voltages up to 100 V to existing FID detectors. A new type of background noise encountered in this new energy domain has been studied at IP2I, in the framework of E. Queguiner’s thesis. At the same time, the study has improved the sensitivity of EDELWEISS to mass WIMPs between 2.8 and 5 GeV/c². It also showed the need for a high-performance ionization channel in terms of energy resolution, in order to identify background noise efficiently. These various studies led to the definition of the next phase: EDELWEISS-SubGeV.

EDELWEISS-SubGeV

The prospects for EDELWEISS-SubGeV have converged towards a development program of detectors with interdigital electrodes (FID type), with the same objectives in terms of resolution as those of Ricochet leading to the same ER/NR discrimination performances. Comparison of these potential performances showed that the mass reduction of an individual detector from 860 g to 33 g is more than compensated by the better performance of smaller detectors. An essential part of the strategy towards EDELWEISS-SubGeV lies in the strong synergy with the CryoCube R&D carried out for Ricochet, in which the group is a driving force. The difference is that for DM research, the detectors must retain the ability to operate with voltages up to 100 V, in order to benefit from LN amplification. The physics goal of this new phase of EDELWEISS is the search for NR retreats induced by coherent scattering on germanium crystals for DM particles with masses up to ~10 MeV/c², but also the search for ER retreats for DM particles with masses below 1 MeV/c² and ALPs (axion-like particles) or photons associated with a new dark sector with masses of the order of a few eV/c². Demonstration of these performances is essential to define the basic detector capable of operating at LSM in 2024, as part of an ensemble of about 1 kg of detectors optimized for the search for recoil induced by DM particles, both NR and ER. This is the goal of EDELWEISS-SubGeV whose detectors, if successful, could eventually contribute to an upgrade of the SuperCDMS experiment at SNOLAB, Canada.

RED20 et RED30

Results obtained on the surface at IP2I, following the R&D carried out on phonon sensor resolution, and published under the name EDELWEISS-Surf, demonstrated the progress made towards the SubGeV programme, both for standard NR nuclear retreat research and for research exploiting the Migdal effect. R&D has led to the definition of the first 33 g germanium detector (RED20), coupled with a Ge-NTD-type thermal sensor, capable of searching for DM particles with a mass of less than 1 GeV/c². This detector operated in the low vibration environment of the LIO cryostat, and showed an energy resolution of 17.7 eV, allowing an energy threshold of 60 eV to be applied. The EDELWEISS-Surf limit is the best surface limit, based on nuclear setbacks, for spin independent (SI) interactions above 600 MeV/c². By considering the Migdal effect, these limits could be extended to DM particles of lower mass: they are the first to cover the region between 45 and 150 MeV/c². Calculated by including Earth shielding effects, these limits led to the definition of new exclusion domains for SIMPs (Strongly Interacting Massive Particles, another category of DM particles with stronger interactions). EDELWEISS-Surf has also produced the best spin-dependent limits for interactions on neutrons between 0.5 and 1.3 GeV/c², and on protons between 0.6 and 0.8 GeV/c². The next step of the SubGeV program was to equip a 33 g germanium detector with electrodes (RED30) and install it at LSM in the EDELWEISS cryostat for a data acquisition started in January 2019. The first results have been submitted for publication (arXiv:2003.01046). By applying voltages up to 78 V, a phonon resolution of 42 eV was obtained, corresponding to a resolution of 1.60 eV_ee after Luke-Neganov amplification (LN), lower than the signal associated with a single electron. This is also the best resolution obtained on a massive semiconductor detector (>> 1 g). The associated limits on the scattering of DM particles on electrons are best above 0.5 MeV/c². This is also the first step towards the Selendis project (Marie Curie fellowship), whose goal is to use LN amplification to resolve single electron signals with massive germanium detectors.

Phenomenology in direct detection

J. Billard conducts phenomenological studies that have led to publications on cosmic neutrino background noise for the direct detection of dark matter. He shows the impact of coherent interactions of solar and atmospheric neutrinos on future direct DM detection experiments. The only way around this problem will be to use directional detection of DM, i.e. to measure both the energy and direction of DM-induced recoil.

The Ricochet Experience

Scientific background

A new field of neutrino physics has been opened by the observation of a CENNS (Coherent Elastic Neutrino-Nucleus Scattering) signal by the COHERENT experiment in 2017. The coherent increase in the rate of sub-keV nuclear recoil generation per neutral current, by a factor proportional to the square of the number of nucleons (A²) of a target detector in the laboratory, paves the way for probing the neutrino sector with small-scale experiments. The Ricochet experiment (France, USA, Russia) plans to measure CENNS at low energy in order to exploit its sensitivity to new physics research. As demonstrated in a publication (see Phenomenology), after one year of data collection with an energy threshold lower than 100 eV, the Ricochet experiment should have the unique opportunity to detect the exchange of a Z’, a new additional massive mediator boson coupled to neutrinos and quarks, which could thus create interference with the standard CENNS process. More generally, Ricochet should set limits on deviations from the observed weak nuclear hypercharge. Its sensitivity to operators associated with the Lagrangian Non-Standard Interaction (NSI) describing the neutrino-nucleon interaction should be two orders of magnitude better than that of existing experiments. In addition, an abnormal NMM (Neutrino-Magnetic Moment) neutrino should distort the NR spectrum below 100 eV, which would be an unambiguous signature due to the fact that neutrinos are Majorana fermions.

The CryoCube detector and the Ricochet project

In 2018, J. Billard was awarded an ERC starting grant (called CENNS) to develop, build and install in the Ricochet experiment in 2022 the CryoCube detector, consisting of 1 kg of 33 g cryogenic bolometers, allowing the simultaneous measurement of phonon and charge signals, to discriminate event by event and identify nuclear setbacks. The MANOIR team is at the forefront of the R&D efforts associated with this project, common with the future EDELWEISS-SubGeV.

The CryoCube detector combines two cryogenic targets and techniques to take advantage of the complementary nature of the targets and the reduction of systematics: semiconductor Ge and superconducting Zn metal. The resolutions of these detectors must be excellent. It was almost achieved for phonons in 2018 (EDELWEISS-Surf) by optimizing the reading electronics of the Ge-NTD sensors with 33 g detectors (RED20 et RED30). The resolution on the load signal will be obtained by using pre-amplifiers based on HEMT (High energy mobility transistors) and a low capacitance electrode design, studied at IP2I (see R&D detectors).

The Ricochet cryostat will be installed at the site of the STEREO experiment at ILL (Grenoble), 8 m from the core of a research reactor with a thermal power of 58 MW. The objective is an installation in 2022 and a physics start-up in 2023. The group is in charge of the simulations that will enable the design of the cryostat shielding.

The preparatory work on Ricochet has led the MANOIR group to be reinforced from 2019 by 7 engineers and technicians from the IT, instrumentation, design office and electronics departments of IP2I. New employees in France, the USA and Russia will also join the Ricochet collaboration in 2019.

CENNS and phenomenology

In addition to these experimental developments, J. Billard defined the science case associated with the Ricochet experiment in a more phenomenological publication, which presents CENNS as a new probe to study physics beyond the Standard Model. By comparing the sensitivity projections of the different experiments seeking a CENNS signal, it is shown that future cryogenic CENNS experiments with energy thresholds below 100 eV will have the sensitivity to study many exotic physics scenarios.

Research and development at IP2I

The cryogenic installation LIO of the IP2I is at the heart of the synergy of the EDELWEISS and Ricochet research programs, in order to create a set of bolometer-type cryogenic detectors, with 10 eV phonon and 20 eV_ee ionization energy resolution, allowing event-by-event discrimination.

The LIO cryostat

The performances required for the research of light dark matter and the CENNS study at low energy motivated the MANOIR group to turn to R&D activities in order to develop and optimize ultra-sensitive cryogenic detectors. This experimental program is carried out in a Dry Dilution Refrigerator (DDR), part of the innovative platforms of the LabEx LIO (Lyon Institute of Origins). This cryostat based on the use of a pulsed tube, manufactured by the company CryoConcept, operates without the addition of cryogenic fluids, which enables it to reach a temperature of 10 mK only 30 hours after cooling, a significant advantage for the speed required for an intensive R&D programme.

The cryostat has been operational since 2015. Two major improvements have been introduced since then to reduce the impact of mechanical vibrations induced by the pulse tube on detector performance: 1) the pulse tube head has been mechanically decoupled from the cryostat and 2) the detector has been mounted on a tower structure suspended by an elastic pendulum, made at IP2I. Vibration levels were attenuated by more than two orders of magnitude over almost the entire frequency range, reaching record levels for this type of system, making the IP2I LIO cryogenic installation a world reference in this respect.

The equipment enables the MANOIR Group to play a major role in the R&D of EDELWEISS and Ricochet. It has also been designed as an ideal platform for the development of load reading based on HEMT amplifiers. Improvements are planned for the use of both high and low impedance sensors to test both germanium semiconductors for the EDELWEISS-SubGeV and Ricochet cryogenic detectors and zinc superconductors for the Ricochet CryoCube. A new low background cryostat for Ricochet is produced by CryoConcept. Installed at the IP2I at the end of 2020 to test the CryoCube‘s detectors, it will then be transferred to the ILL for the data collection which is scheduled to start in 2022.

R&D on phonon and load sensors: towards discrimination and low energy thresholds

To guide the realization of phonon sensors with optimal energy resolution, the MANOIR Group has developed a model associated with the electrothermal response of the system including the thermal sensor, the crystal and all the thermal and electrical links between them and their environment. The model describes the steady state, the different noise components and the response to signal events and noise, both in the time and frequency domains. The physical constants of the model are derived from experimental data obtained from the IOL cryogenic facility.

This work led to the design of RED20, the first germanium detector capable of searching for DM particles with masses less than 1 GeV/c². This detector is a 33 g g germanium crystal coupled to a Ge-NTD phonon sensor. It has been installed in the low-vibration suspended tower of the LIO cryogenic plant. Experimental data showed a phonon energy resolution of 17.7 eV, allowing the use of an analysis threshold of 60 eV. This led to the world records in sensitivity published by EDELWEISS-Surf for light dark matter particles. This result also confirmed the strategy developed by the MANOIR group for EDELWEISS and Ricochet, based on 33 g g germanium detectors coupled with Ge-NTD phonon sensors. The next step was to equip a similar detector (RED30) with electrodes for a physics run in the EDELWEISS cryostat at the LSM. Further investigations of surface DM particles are planned at IP2I with other detectors using the Luke-Neganov amplification effect. Coupling this excellent resolution in phonon energy with the expected excellent resolution in ionization energy with HEMT transistor charge readings could be a unique technique for accurately measuring the ionization efficiency of nuclear retreats at energies below 100 eV, helping to eliminate the current controversy on this subject. This is a crucial issue for all experiments using semiconductor detectors to search for low-mass DM particles. Low-energy calibration requires special radioactive sources for which Dr De Jesus is responsible: neutron activation with thermal neutrons from an AmBe source, and ⁵⁵Fe sources for surface calibration, produced in collaboration with the LABRADOR service.

For the ionization pathway, the aim is to achieve an energy resolution of 20 eV_ee, which is essential for improving particle discrimination. Work is continuing on two fronts: simulations enabling electrode design and developments in electronics based on HEMTs.

Finite element calculations have been performed with COMSOL software to produce an electrode design that represents the best compromise between low electrical capacitance (for resolution) and high fiduciary volume (for charge collection). These calculations are important for understanding the charge drift in germanium crystals at very low temperatures.

The Berkeley Cryogenics Group, with the collaboration of A. Juillard, showed on a detector the 300 g CDMS experiment that the reduction in capacitance associated with wiring was possible using HEMT-based electronics, and measured an associated ionization energy resolution of 93 eV_ee. HEMT R&D on the ionization channel started at IP2I in 2017. HEMT noise modelling showed that an energy resolution of 20 eV_ee was a reasonable target for a 33 g detector with the right set of interdigital electrodes. The HEMT transistors are produced at C2N Saclay and assembled in a SiGe cryogenic ASIC developed at CEA Saclay. With the start of the Ricochet pre-project at IP2I, electronics and computer scientists are interested in the development of 300 K electronics (analog and digital) and software for HEMT amplifiers. The goal of the HEMT program is to reach 20 eV_ee on the cryogenic LIO installation by mid-2021.

Associated activities: CUTE, GENTIANE, LUMINEU, CUPID

Members of the MANOIR group are often called upon for their expertise in other cryogenic experiments.

In the field of Monte-Carlo simulations of radioactive background, A. Cazes was involved in the first simulations of the CUTE cryogenic facility at the SNOLAB laboratory in Canada, carried out to validate the bolometers of the SuperCDMS experiment.

M. De Jesus is responsible for all radiopurity measurements performed with the HPGe (Hig-Purity Germanium) Gentian spectrometer installed at the LSM, for the EDELWEISS, LUMINEU and CUPID-Mo experiments. These measurements are then used as inputs for detailed simulations carried out to study the neutron and gamma background noise of these experiments looking for rare events.

The group’s expertise in taking data with cyrogenic bolometer arrays at the LSM has also been called upon by the LUMINEU/CUPID collaborations that are developing detectors for the search for double beta decay without neutrino emission. The CUPID-Mo phase, with 20 detectors of 300 g LiMoO₄ enriched to ¹⁰⁰Mo, and the measurement of phonon and scintillation signals, has been installed in the EDELWEISS cryostat at the LSM. The MANOIR group is involved in monitoring, detector diagnostics, data acquisition, and has carried out part of the radiopurity measurements. It also supervised the implementation of the CUPID detector geometry in the simulation code. This CUPID-Mo phase produced numerous publications, all signed by the MANOIR group. In addition, joint R&D activities are carried out with CUPID collaborators in France, both on detectors and thermal sensors, at IP2I and LSM.

Astrophysical observations can only be properly explained using a large mass of matter distributed throughout the universe. This matter has never been seen optically and therefore should not interact with light. This gave it its name: Dark Matter.

But this matter has never been detected directly! This is what the experiment EDELWEISS, installed in the Underground Laboratory of Modane, is trying to do. Dark matter is the main component of a galaxy, and therefore surrounds us. The objective of EDELWEISS is to be able to measure the effect of a collision of a dark matter particle on its detectors.

The EDELWEISS collaboration includes IP2I, IJCLab and CEA/Irfu in France, JINR in Russia and KIT in Germany. The IP2I group is the largest in number and includes the spokesman of the experiment.

The coherent elastic scattering of neutrinos is an interaction predicted a long time ago, but only recently observed. It is a neutral current interaction, and its accurate measurement will allow to observe or not any deviation from the standard model.
This is the objective of the Ricochet project, thanks to intense R&D on cryogenic detection technology, which should lead to a spectacular improvement in the performance of the latter. The experiment will contain 27 bolometers in Germanium or Zinc for a total mass of the order of a kilogram. These detectors will be capable of rejecting background noise and will be installed in a cryostat near the ILL experimental reactor to obtain a large flow of electron antineutrinos.
Ricochet is a Franco-Russian-American collaboration jointly led by IP2I and MIT and benefits from the ERC starting large CENNS.