Abstract

Observations from cosmology and astronomy suggest that most of the matter in the
Universe is comprised of dark matter. In this thesis the evidence for dark matter and the
possible candidates are explored, focusing on the Weakly Interacting Massive Particles
(WIMPs) hypothesis. The various ways WIMPs can be detected are shown, with
particular regard to WIMP scattering in direct detection experiments.
Since traditional Spin Independent (SI) and Spin Dependent (SD) searches for WIMPs
have produced very tight constraints on the interaction cross section, we investigate the
WIMP scattering through a model independent Effective Field Theory (EFT) framework.
The chosen approach for this work is Chiral EFT (ChEFT), which studies the nuclear
responses of possible WIMP interactions including quantum chromo-dynamics (QCD)
effects using chiral symmetry, starting from fundamental interactions with quarks and
gluons in atomic nuclei.
This thesis presents the statistical analysis and the inference used in the XENON1T
experiment to perform a ChEFT analysis of WIMP interactions. Furthermore it shows the
work towards the upgrade of the XENON experiment, XENONnT, with particular regard
to the photomultiplier tube (PMT) testing performed at Stockholm University.
XENON1T was a dual-phase time projection chamber using a 2 tonne liquid xenon target
to detect scattering particles. WIMPs with masses above ∼ 10 GeV scattering against the
xenon nuclei would deposit enough recoil energy to create an observable event. The
XENONnT detector is a new detector that will have a target volume of ∼ 6 tonnes of
liquid xenon and is now being commissioned.
The ChEFT analysis is done considering the XENON1T data from 278.8 days of
operation and a fiducial volume of 1.3 tonne and it uses the full XENON1T combined
likelihood, constructed with background models and signal models slightly extended in
the analysis space with respect to the previous XENON1T searches, to increase the signal
acceptance for the various EFT models. Simulated data sets were used to validate the
inference and to compute the expected sensitivities for 25 different ChEFT responses.
This work shows that XENON1T can constrain the physics scale Λ of WIMP interactions
within the ChEFT framework to regions of O(102-105 GeV) for most of the channels.