Thèse Cupid-Stage I Optimisation et Analyse du Détecteur dans le Cadre d'Une Recherche 0 de Nouvelle Génération H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Physique École doctorale : Particules, Hadrons, Énergie et Noyau : Instrumentation, Image, Cosmos et Simulation Laboratoire de recherche : Département de Physique des Particules Direction de la thèse : Benjamin SCHMIDT ORCID 0000000171185936 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-08-21T23:59:59 The CUPID experiment (CUORE Upgrade with Particle IDentification) aims to achieve unprecedented sensitivity for the detection of neutrinoless double beta decay (0nßß) using an array of 1596 lithium molybdate (Li2MoO4) crystals of ~450 kg mass. If detected this process would be a direct observation new physics in the lepton sector: in example it violates lepton number by 2 units. Dependent on the model it can provide valuable insight into the neutrino mass-scale and possibily to matter generation in the Universe through leptogenesis. The use of lithium molybdate for this study is particularly advantageous due to their scintillation properties and the high Q-value of the decay process, which lies above most environmental gamma backgrounds. The CUPID experiment employs this material as cryogenic calorimetric detectors, where the heat signal from particle interactions of O (100 microK/MeV) are registered in a sensitive thermistor at a temperature of ~10 mK. Thanks to the high Q-value Mo-100 features a particularly high sensitivity in terms of large phase space factor and nuclear transition matrix element. This will also allow for precision studies and tests of the standard model, through analyses of the shape of another process: the so-called 2 neutrino double beta decay (2nbb), which is a standard model allowed process. However, this rare process (half-life of 7x10^17yr) is not only an interesting particle/nuclear physics target, it is also expected to contribute the most important background in CUPID: the random coincidence of two events adding up in energy to the Q-value of the 0nßß search.CUPID aims to deploy its new detector array in two phases: An initial detector array with 1/3 of the mass will be deployed by 2030. In the mean time several tower scale measurement and optimization campaigns during the time of this thesis project will allow to analyze and optimize the detector performance of the CUPID detector modules. The further suppression of this so called pile-up background through detector optimization (acting on the sensor attachment of the light detector with a robotic assembly station developed at CEA) and advanced analysis techniques within this thesis will allow to enhance the sensitivity and science reach of CUPID. A further extension of the analysis techniques developed in this thesis to the processing of an array of O(1000) detectors will be tested with the existing TeO2 detecor array of CUORE. In the context of this process the developed analysis techniques will contribute to the final science analyses of CUORE, the leading experiment for 0nßß search with Te-130. Cryogenic calorimetric detectors are among the three leading technologies to search for 0nßß. They feature an energy resolutions at the O(0.1%) level, high signal containment (~80%) and analysis efficiencies (~90%) with the 0nßß candidate isotope embedded in the detector. Their versatility to test various candidate isotopes allows for the selection of high Q value isotopes with ultra-low background and favorable decay kinematics. The required cryogenic technology has advanced sufficiently to allow to deploy arrays of O(1000) detectors of several 100 g's each at ~15 mK with high-up time and low maintenance. For CUPID (CUORE Upgrade with Particle ID) these detectors will consist of 1596 scintillating Li2MoO4 detectors with Ge wafers instrumented as bolometric light detectors (LD). They will provide a continuous time-series data array with thresholds as low as O(1 keV) for the Li2MoO4 and O(10 eV) for the Ge LDs. CUPID will proceed in a staged deployment:In 2026 the cryogenic infrastructure will be upgraded, followed by a test phase of the cryostat with the existing CUORE detector array. In parallel in 2026 the Li2MoO4 crystal procurement and the construction of the detector array for CUPID will start. The first 3rd of the detectors (~80 kg of 100Mo) will be deployed by 2030. During the operation of this Stage-1 the construction for the full detector array proceeds in parallel.CEA is in charge of the robotic system for sensor attachment of LMO's and Ge LD's which will be optimized for best detector performance for validation of sensors. It is jointly managing with IJCLab the Ge LD fabrication and is contributing to the common operations and construction of CUPID at LNGS. The objective of this thesis is two-fold: (1) Optimization of TES and NTD based detectors for 0nßß. In particular through an optimization of the sensor gluing of the LD's to optimize the CUPID detector performance in terms of pile-up resolution capabilities. The so called pile-up background is the leading expected background in CUPID. An improvement hence directly implies an improvement in overall sensitivity for the 0nbb campaign; (2) Continuation of the improvement of analysis tools for pile-up rejection through the combination of multiple low/level (time series denoising, data filtering) and high level multivariate analysis performance algorithms. A test of these algorithms on tower scale Li2Mo4 detectors in the context of the finalization of CUPID's detector structure is planned. A scaling and embedding of these analysis tools into the full CUPID data production stack is expected based on the data taking with the CUORE detector array in the upgraded CUPID cryostat. Some these tools, like the denoising algorithms can further have a significant impact on the data taken with the existing TeO2 detector array in terms of the low energy physics sensitivity. The method of the proposed research will be closely embedded within the CUPID collaboration, in particular within the analysis and simulation working groups of the experiment. It will contain experimental laboratory work and CEA, IJClab and LNGS as well as data analysis and sensitivity studies.1. An optimization of the LD sensor selection and attachment procedure in terms of (doping level of the sensor - fabricated by the collaboration, glue thickness and amount) will be performed through a series of both room-temperature and cryogenic measurement campaigns. At room temperature, parameters for the robotic gluing station (dispensing time/pattern - amount of glue, thickness and coverage of glue) will be optimized to obtain high reliability attachment, failure recognition through in situ control and a repeatable dielectric isolation between Ge wafer as absorber and sensor. At cryogenic temperature the sensitivity of the detectors in (uV/keV) and pulse rise- and decay times will be studied as a function of glue deposition parameters (amount, thickness and coverage of glue layer) to optimize the combination of parameters that allows to minimize the pile-up background.2. Analysis of LD data to optimize the sensitivity of the CUIPD experiment: This high level analysis study will be embedded closely within the data reconstruction and software working groups of the CUPID collaboration. The study will use synthetic data simulations based on pulse injection, dedicated test measurements as well as tower test and full scale 1000 detector array data. A sensitivity study will be performed to quantify the impact of the analysis work on the 0nbb search in CUPID. Time permitting this analysis work can be extended to study the sensitivity of CUPIDs light detectors to secondary physics objectives like nuclear recoil or electron recoil dark matter and axion like particles.

A master degree in particle or nuclear physics is required with particular emphasis on neutrino physics and lepton number violation. The work is carried out in an international collaboration requiring English proficiency. Scientific curiosity, commitment and flexibility and communication skills will be crucial traits to flourish in an experimental collaborative environment.