Illuminating Light Dark Matter with Fixed-Target Experiments

Doktorsavhandling, 2026

Astronomical and cosmological observations provide overwhelming evidence for a non-luminous matter component — dark matter (DM) — whose particle nature remains unknown. Extensive experimental searches for non-gravitational DM interactions have largely been geared for DM with masses of order GeV-TeV via nuclear recoils, yielding no discovery so far. DM in the MeV–GeV mass range, however, is theoretically motivated, reproducing the observed relic abundance while remaining invisible to nuclear recoil searches. Such light DM is better probed via electron recoils or at fixed-target experiments.
This thesis advances the theoretical and statistical toolkit for light DM searches at fixed-target experiments, spanning model building, statistical inference, global fits, and signal simulation. We survey the landscape of fixed-target searches for light DM, with a central focus on Light Dark Matter eXperiment (LDMX). We also consider the next-generation experiment SHiP, where we simulate DM signatures to assess its sensitivity to light DM.

Within the standard paradigm of minimal DM models, we present a likelihood-based statistical framework for LDMX using the full two-dimensional recoil electron distribution, spanning exclusion, discovery, parameter estimation, and model comparison in frequentist and Bayesian formulations. This framework establishes the discovery potential of LDMX, and we further quantify the direct detection exposure needed to independently confirm a potential signal.

Moving beyond this standard paradigm, we perform global fits of scalar and fermionic light DM, combining cosmological, astrophysical, and laboratory constraints. We then extend the theoretical landscape of fixed-target searches to spin-1 DM and higher electromagnetic moment dark photons, yielding rich signatures with strong complementarity to other probes. Finally, we go beyond standard nuclear modelling of dark photon production, incorporating elastic nuclear form factors and spectral functions from many-body ab initio methods in place of standard analytic treatments.