Projet de thèse en Physique des particules
Sous la direction de Claudia Nones.
Thèses en préparation à université Paris-Saclay , dans le cadre de École doctorale Particules, hadrons, énergie et noyau : instrumentation, imagerie, cosmos et simulation (Orsay, Essonne ; 2015-....) , en partenariat avec DSM-Institut de Recherche sur les lois fondamentales de l'Univers (Irfu) (laboratoire) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 30-09-2018 .
The proposed thesis project deals with the search for a very rare hypothetical nuclear process: neutrinoless double beta decay (0n2b) . The current half-life limits on this transition are impressively long, of the order of 10**25-10**26 y . This feature makes clear why one of the hardest challenges in 0n2b decay search is to achieve very low background levels, fighting against any form of environmental radioactivity and locating the experiments underground. 0n2b plays a unique role in understanding fundamental neutrino properties (clarifying if neutrino is a Dirac particle like all the other fermions or a Majorana one, coinciding with its antimatter partner) and exploring the lepton number violation (LNV) . Lepton number is a conserved quantity in the Standard Model of elementary particles, and it implies that neutrinos and antineutrinos are different particles. 0n2b consists in the transformation of an even-even nucleus into a lighter isobar containing two more protons and accompanied by the emission of two electrons and no other particles, with a change of the total lepton number by two units: (A,Z) → (A,Z+2)+2e- . 0n2b can be induced by a plethora of hypothetical LNV mechanisms. Among them, the so-called « mass mechanism » occupies a special place, since it is mediated by the ordinary massive neutrinos which undergo flavor oscillations observed in the last twenty years. In this mechanism, the rate of the process is proportional within an uncertainty due to nuclear physics effects to the square of the effective Majorana neutrino mass m_bb, which is a parameter related to the absolute neutrino mass scale and to the mass ordering . In fact, it is a linear combination of the three neutrino masses. Present limits on m_bb are in the range 60-600 meV . In case of a special combination of of the three neutrino masses (named « inverted hierarchy ») m_bb could be higher than 20 meV, opening the way to the observation of 0n2bb. None of the current experiments has the potential to measure such a low value. This is in fact the objective of next-generation 0n2b experiments on a 5-10 year time scale. The technology for these searches has not been selected yet. A very promising approach consists in the construction of a large bolometric experiment, named CUPID [2,3], exploiting the present CUORE infrastructure. (CUORE is currently one of the most sensitive 0n2b experiments and is taking data in the Gran Sasso underground laboratory in Italy.) CUPID is supposed to probe the inverted-hierarchy hypothesis thanks to a dramatic background improvement with respect to CUORE, achieved in its baseline version by the use of luminescent bolometers [2,3], which allow rejecting the surface alpha background that currently limits the CUORE sensitivity.
Searching for double beta decay with scintillating bolometers
The present thesis project intends to explore and develop a technology that can be applied in prospect to CUPID, searching for 0n2b decay of the promising isotopes Mo-100 and Cd-116. The devices proposed to investigate this rare process will be bolometers containing Mo-100  and Cd-116 , acting at the same time as source and detector of the phenomenon. As in any classical macro-bolometer, each device will consist of a single high-purity dielectric crystal cooled down below 20 mK and thermally coupled to a temperature sensor (consisting of a neutron transmutation doped (NTD) Ge thermistor as in CUORE), which will provide a heat signal for each event. In this type of devices, the detection is mediated by the production of phonons (heat) due to nuclear energy depositions. 0n2b decay events of Mo-100 and Cd-116 will produce heat signals with a well-defined amplitude. As far the detector material is concerned, the compounds Li2MoO4  and CdWO4  are among the most promising ones in terms of energy resolution, intrinsic radiopurity and ease of crystal growth. The 0n2b signal in both cases is a peak located well above the bulk of the gamma natural radioactivity (which stops at 2615 keV), but the background is expected to be dominated by alpha surface contaminations, which produce an almost flat spectrum. The surface alpha component can be eliminated by matching the 0n2b detector with a light detector (which will be a bolometer as well) capable of detecting the scintillation light generated by nuclear events in Li2MoO4 and CdWO4. Alpha-particle light yield is about 15%-20% of beta-particle light yield at the same thermal energy. Therefore the simultaneous measurement of heat and scintillation light will allow us to reject the alpha background component, just because of the low intensity of the light they emit. Mo-100 SECTION - When this thesis project starts (October / November 2018), a pilot Mo-100 experiment based on this technology (named CUPID-Mo) will be already in data taking, located in the Modane underground laboratory. CUPID-Mo is based on an array of twenty scintillating bolometers of Li2MoO4, containing about 2.5 kg of Mo-100. One of the main activities of the PhD student will consist in performing the data analysis of CUPID-Mo (of course in the framework of the analysis team of the collaboration), but also to take care of the measurement maintenance. The objective of the analysis activity is to set limits on the 0n2b half-life of Mo-100 (which will be the best ever achieved) and on the Majorana neutrino mass m_bb. The CUPID-Mo sensitivity to this parameter is of the order of 100-200 meV, among the highest ones in the international context. Cd-116 SECTION In parallel to the analysis activity on Mo-100, the student will have the opportunity to develop new detectors based on scintillating bolometers of cadmium tungstate (CdWO4). In spite of its very favorable features, there have been only sporadic studies on 116Cd and no coherent experimental program. Under some aspects, however, 116Cd is the most promising nuclide that can be studied with scintillating bolometers. The proposed program about Cd-116 will have a remarkable impact by itself, but also on the development of CUPID, complementing nicely the Mo-100 section. The detectors to be developed will consist of a scintillating crystal of CdWO4, enriched in the isotope of interest 116Cd. This part of the thesis activity will deal with the conception, the assembly and the tests of the detectors, which will be fabricated using existing large-mass enriched 116CdWO4 crystals for a total mass of about 1.6 kg. (The concept of scintillating bolometer applied to this compound was already proved on a small mass (32 g) 116CdWO4 detector operated above ground .) The devices will be preliminary tested at surface and then in the Modane underground laboratory and/or, possibly, in the Gran Sasso underground laboratory in Italy. The data taking and analysis will also be part of the program. The student will have also the chance to participate to the elaboration of the background model - by using the simulation package GEANT4 - both for CUPID-Mo and for the Cd-based pilot experiment, a crucial step in order to evaluate the sensitivity of these two technologies when applied to the next-generation search CUPID. Preliminary estimations show that indeed both approaches have the potential to measure m_bb values as low as 20 meV.