Tesis doctoral. Tamara Pardos Yanguas (DFTUZ. Área de Física Atómica, Molecular y Nuclear)

"Advances in detector response and background modelling in the ANAIS-112 experiment for annual modulation and other rare event searches"
Doctoranda: Tamara Pardo Yanguas
Directoras: María Luisa Sarsa Sarsa, María Martínez Pérez

Numerous astronomical and cosmological observations point to the existence of dark matter, which constitutes about 27% of the Universe. Despite
extensive experimental efforts, the nature of DM remains unknown. Among the preferred dark matter candidates are axions and Weakly
Interacting Massive Particles, which can be searched for using ground-based detectors. Only the DAMA/LIBRA experiment, using NaI(Tl) detectors
at Gran Sasso National Laboratory, has reported a positive dark matter signal: an annual modulation in its detection rates compatible with the
signal that would be produced by dark matter particles distributed in the galactic halo following the most standard halo models. To independently
verify this result without relying on specific DM or halo models, using the same NaI target is essential. This is the goal of ANAIS-112, operating with
112.5 kg of NaI(Tl) scintillators since August 3, 2017, at the Canfranc Underground Laboratory (LSC). This thesis presents work conducted within
the ANAIS-112 experiment, focusing on data analysis and the development of Geant4-based simulations, with the goal of reducing systematic
effects and increasing the experimental sensitivity. Firstly, efforts have been directed towards improving the understanding of the ANAIS-112
crystals' response to nuclear recoils. Six years of ANAIS data challenge the dark matter interpretation of the DAMA/LIBRA signal. However,
systematic uncertainties, particularly related to the scintillation quenching factors (QFs) of sodium and iodine nuclei recoiling in NaI(Tl), must be
addressed to ensure reliable comparisons between ANAIS, DAMA/LIBRA, and other experiments. For that purpose, onsite neutron calibrations in
ANAIS-112 have been performed since 2021 at LSC. The calibration procedure is based on the exposure of the full detector array to Cf-252 neutron
sources. This work has compared the data from these calibrations with dedicated Geant4-based neutron simulations. Moreover, the neutron
simulations have revealed deficiencies in the Geant4 implementation of certain decay processes or cross sections in some versions. This study has
enabled the evaluation of different QF models. The simulations have proven highly sensitive to the QF used, favouring models in which the QF
increases with energy, and disfavouring the values assumed by the DAMA collaboration. In parallel, this thesis has contributed to the improvement
of the ANAIS-112 background model through a multiparametric fit of its various background components. An exhaustive review of the
contributions considered in the previous background model has been performed, which has enabled a significantly better agreement in all the
analyzed observables. Subsequently, new physics searches have been conducted using ANAIS-112 data. Specifically, a reanalysis of the annual
modulation signal and a search for solar axions have been carried out employing the experiment's six-year accumulated exposure. Finally, this
thesis also includes the work conducted within the COSINUS experiment, focused on background modelling and the analysis of the impact of
internal background components on the experiment's sensitivity.

Lugar: Sala de Grados de la Facultad de Ciencias
Día y hora: lunes 24 de noviembre, 11:30 horas